Patent Publication Number: US-8122858-B2

Title: Abnormality diagnosis apparatus for cooling system of vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2008-287343 filed on Nov. 10, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an abnormality diagnosis apparatus for a cooling system of a vehicle, which includes a coolant circuit that circulates coolant between an internal combustion engine and a radiator of the vehicle and is provided with a thermostat. 
     2. Description of Related Art 
     In general, in a cooling system for cooling an internal combustion engine installed in a vehicle, a thermostat (also referred to as a thermostat valve or a thermo valve) is provided in a coolant circuit that circulates coolant between the engine and a radiator. When the coolant temperature is lower than a predetermined temperature (e.g., a temperature of the coolant at a warm-up complete state of the engine), the thermostat is closed to stop the circulation of the coolant between the engine and the radiator. In this way, the coolant temperature at the engine side is rapidly increased, and thereby the warm-up operation of the engine is facilitated. Thereafter, when the coolant temperature becomes equal to or higher than the predetermined temperature, the thermostat is opened to circulate the coolant between the engine and the radiator. In this way, the coolant temperature is adjusted within an appropriate warm-up temperature range, and thereby the overheat of the engine is limited. 
     However, when an abnormality (known as a thermostat open state abnormality) occurs in the thermostat in a warm-up incomplete temperature range of the coolant, which is lower than the predetermined temperature described above, the thermostat is left opened. When the thermostat open state abnormality occurs, the coolant of the engine, which is in the middle of the warm-up operation, is circulated to the radiator and releases the heat through the radiator. Therefore, the temperature of the coolant in the radiator cannot be rapidly increased, and thereby completion of the warm-up operation of the engine is delayed. As a result, emissions of the engine may be disadvantageously increased, and the fuel consumption may be disadvantageously increased. Therefore, when the thermostat open state abnormality occurs, such an abnormality should be sensed in an early stage, and a warning should be provided to a driver (user). 
     In order to address the above disadvantage, as recited in, for example, Japanese patent No. 3407572 (corresponding to U.S. Pat. No. 6,279,390B1), the amount of change in an accurately measured coolant temperature, which is measured with a coolant temperature sensor, is compared with a determination reference temperature to determine whether the open state abnormality of the thermostat exists within a predetermined time period upon starting of the engine. 
     Also, as recited in, for example, Japanese Patent No. 3956663 (corresponding to US 2002/0111734A1), the coolant temperature is estimated based on the amount of coolant temperature increase caused by the generation of heat from the engine and the amount of decrease in the coolant temperature caused by the release of heat from the coolant through the radiator upon application of air flow, which is generated by the forward movement of the vehicle or a radiator fan and is applied to the radiator. Then, the estimated coolant temperature and the measured coolant temperature are compared with each other to determine whether the thermostat abnormality exists. 
     In the above abnormality diagnosis techniques, the coolant temperature, which is measured with the coolant temperature sensor, or the amount of change in the measured coolant temperature is compared with the estimated coolant temperature or the determination reference temperature to determine whether the coolant temperature shows the behavior of the normal time and thereby to diagnose the abnormality of the thermostat. In order to increase the accuracy of the abnormality diagnosis, the accuracy of estimation of the coolant temperature and/or the accuracy of the determination reference temperature should be increased. In order to increase the accuracy of the estimation of the coolant temperature and/or the accuracy of the determination reference temperature, the estimation method for estimating the coolant temperature and/or the determination reference temperature should be accurately adapted by measuring the amount of heat generated from the engine and the amount of heat released from the engine under various driving conditions and the traveling conditions of the vehicle through use of the actual vehicle. This adaptation disadvantageously requires a large number of steps. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above disadvantages. According to the present invention, there is provided an abnormality diagnosis apparatus for a cooling system of a vehicle, which includes a coolant circuit that circulates coolant between an internal combustion engine and a radiator of the vehicle and is provided with a thermostat. The thermostat is closed in a predetermined warm-up incomplete temperature range of the coolant, in which warm-up of the internal combustion engine is determined to be incomplete, to stop the circulation of the coolant between the internal combustion engine and the radiator. The abnormality diagnosis apparatus includes a radiator side released heat amount information obtaining means and an abnormality diagnosis means. The radiator side released heat amount information obtaining means is for obtaining radiator side released heat amount information, which indicates an amount of heat released from the coolant through the radiator or information relevant to the amount of heat released from the coolant through the radiator. The abnormality diagnosis means is for determining whether a thermostat open state abnormality, which is an abnormality of the thermostat that disables closing of the thermostat in the warm-up incomplete temperature range of the coolant and thereby leaves the thermostat opened in the warm-up incomplete temperature range of the coolant, exists by determining whether a predetermined thermostat abnormal time correlation exists between the radiator side released heat amount information and a vehicle speed of the vehicle in the warm-up incomplete temperature range of the coolant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing an engine cooling system according to a first embodiment of the present invention; 
         FIG. 2  is a diagram showing correlation between the amount of change in the coolant temperature at abnormal time and a vehicle speed; 
         FIG. 3  is a flowchart showing a flow of an abnormality diagnosis main routine according to the first embodiment; 
         FIG. 4  is a flowchart showing a flow of a correlation determination routine according to the first embodiment; 
         FIG. 5  is a flowchart showing a flow of a coolant temperature estimation routine according to the first embodiment; 
         FIG. 6  is a flowchart showing a flow of a normality/abnormality determination routine according to the first embodiment; 
         FIG. 7  is a flowchart showing a flow of a radiator fan forceful drive routine according to the first embodiment; 
         FIG. 8  is a flowchart showing a flow of a vehicle speed correction routine according to the first embodiment; 
         FIG. 9  is a flowchart showing a flow of a correlation determination routine according to a second embodiment of the present invention; 
         FIG. 10  is a flowchart showing a flow of a correlation determination routine according to a third embodiment of the present invention; 
         FIG. 11  is a flowchart showing a flow of a normality/abnormality determination routine according to a fourth embodiment of the present invention; and 
         FIG. 12  is a flowchart showing a flow of a second abnormality diagnosis routine according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments of the present invention will be described with reference to the accompanying drawings. 
     First Embodiment 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . 
     First of all, an entire structure of an engine cooling system of the present embodiment will be briefly described with reference to  FIG. 1 . 
     A water pump  12  is provided at an inlet of a coolant passage (water jacket) of an internal combustion engine (hereinafter, simply referred to as an engine)  11 . The water pump  12  may be a mechanical water pump, which is driven by a drive force of the engine  11 , or an electric water pump, which is driven by an electric motor. A coolant circulation pipe  14  communicates between an outlet of the coolant passage of the engine  11  and an inlet of the radiator  13 , and a coolant circulation pipe  15  communicates between an outlet of the radiator  13  and an inlet of the water pump  12 . In this way, a coolant circuit  16  is formed to circulate the coolant through the coolant passage of the engine  11 , the coolant circulation pipe  14 , the radiator  13 , the coolant circulation pipe  15  and the water pump  12 . A hot coolant circuit  17  is connected to the coolant circuit  16  in parallel with the engine  11 , and a heating heater core  18  is inserted in the hot coolant circuit  17 . 
     Furthermore, a thermostat (a thermostat valve or a thermo valve)  19 , which is opened and closed in response to the coolant temperature, is inserted in a portion of the coolant circuit  16  (specifically, a connection between the coolant circulation pipe  15  and the hot coolant circuit  17  on the downstream side of the radiator  13 ). When the coolant temperature is in a predetermined warm-up incomplete temperature range, the thermostat  19  is closed to stop the circulation of the coolant between the engine  11  and the radiator  13 . The warm-up incomplete temperature range is a temperature range of the coolant, in which warm-up of the engine  11  is determined to be incomplete and which is lower than a predetermined temperature (a temperature corresponding to a warm-up complete state coolant temperature, equal to or above which the warm-up operation of the engine  11  is determined to be completed). With this closing operation of the thermostat  19 , the coolant temperature in the interior of the engine  11  is rapidly increased to promote the warm-up of the engine  11 . Thereafter, when the coolant temperature becomes equal to or higher than the predetermined temperature, the thermostat  19  is opened to circulate the coolant between the engine  11  and the radiator  13 , so that the coolant temperature is adjusted to an appropriate warm-up temperature range through heat release at the radiator  13  to limit the overheat of the engine  11 . When the heating operation for heating the interior of the passenger compartment of the vehicle is not performed, the circulation of the coolant in the hot coolant circuit  17  is kept stopped. 
     A coolant temperature sensor  20 , which measures the coolant temperature (the temperature of the coolant on the engine  11  side of the thermostat  19  in the coolant circuit  16 ), is provided at an inlet of the coolant passage of the engine  11  in the coolant circuit  16 . Furthermore, an electric radiator fan  21 , which is driven to generate a cooling air flow applied to the radiator  13  upon energization thereof, is provided at a location adjacent to the radiator  13 . 
     Furthermore, a crank angle sensor  22 , which outputs a pulse signal at every predetermined crank angle of the crankshaft, is installed to a cylinder block of the engine  11 . The crank angle and the engine rotational speed are sensed based on the output signal of the crank angle sensor  22 . Furthermore, an intake air quantity (intake air flow quantity) is sensed with an intake air flow sensor  23 , such as an air flow meter. An ambient temperature, which is the temperature of the air at the surrounding environment outside of the coolant circuit  16 , is sensed with an ambient temperature sensor  24 , and a speed (vehicle speed) of the vehicle is sensed with a vehicle speed sensor  25 . 
     Outputs of the above-described sensors are supplied to an engine control unit (ECU)  26 . The ECU  26  includes a microcomputer as its main component. When the ECU  26  executes engine control programs, which are stored in a ROM (a storage) of the ECU  26 , a fuel injection quantity of each fuel injection valve (not shown) and ignition timing of a corresponding spark plug (not shown) are controlled based on the operational state of the engine  11 . 
     In the normal operational period of the thermostat  19 , the thermostat  19  is closed when the coolant temperature is in the warm-up incomplete temperature range, which is lower than the predetermined temperature (e.g., the temperature that corresponds to the warm-up compete state temperature), so that the circulation of the coolant between the engine  11  and the radiator  13  is stopped. Thereby, the low temperature coolant (the coolant having the temperature that is generally equal to the ambient temperature) remains in the radiator  13 . Therefore, even when the vehicle speed is increased to increase the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , the amount of heat released from the coolant through the radiator  13  does not change substantially. 
     In contrast, in the state where the coolant temperature is in the warm-up incomplete temperature range, when an abnormality of the thermostat  19  occurs, the thermostat  19  may not be closed and may be left opened. This abnormality will be hereinafter referred to as a thermostat open state abnormality. When the thermostat open state abnormality occurs, the coolant in the engine  11 , which is in the middle of the warm-up operation, is circulated to the radiator  13  and releases the heat through the radiator  13 . Thus, when the vehicle speed is increased to increase the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , the amount of heat released from the coolant through the radiator  13  is increased. Furthermore, in response to the increase in the amount of heat released from the coolant through the radiator  13 , the measured coolant temperature, which is measured with the coolant temperature sensor  20  (the temperature of the coolant on the engine  11  side of the thermostat  19  in the coolant circuit  16 ), is changed. 
     Thus, when the open state abnormality of the thermostat  19  occurs in the warm-up incomplete temperature range of the coolant, a relationship between the amount of change Δthw (radiator side released heat amount information) in the actually measured coolant temperature and the vehicle speed V becomes as follows. That is, when the vehicle speed V is increased, the amount of change Δthw in the measured coolant temperature is reduced (i.e., resulting in a decrease in the increasing rate of the measured coolant temperature or an increase in the decreasing rate of the measured coolant temperature). 
     In view of the above characteristics, according to the first embodiment, respective routines, which are shown in  FIGS. 3 to 8  and will be described later, are executed by the ECU  26 . Thereby, a first abnormality diagnosis operation is executed. Specifically, it is determined whether a predetermined thermostat abnormal time correlation (see  FIG. 2 ) exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V in the state where the coolant temperature is in the warm-up incomplete temperature range, which is lower than the predetermined temperature. In this way, it is determined whether the thermostat open state abnormality (the abnormality, which causes the thermostat  19  to be left opened in the warm-up incomplete temperature range of the coolant) exists. 
     Specifically, the amount of change cf (abnormal time radiator side released heat amount information) in the abnormal time coolant temperature corresponding to the vehicle speed V is computed by using the thermostat abnormal time correlation (see  FIG. 2 ). Then, a difference (cf−Δthw) between the amount of change cf in the abnormal time coolant temperature and the amount of change Δthw in the measured coolant temperature is computed as a correlation value. Then, this correlation value is evaluated to determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. When the relationship between the amount of change Δthw in the measured coolant temperature and the vehicle speed V approaches the thermostat abnormal time correlation, the difference (cf−Δthw) between the amount of change cf in the abnormal time coolant temperature and the amount of change Δthw in the measured coolant temperature is reduced. Therefore, when the difference (cf−Δthw) between the amount of change cf in the abnormal time coolant temperature and the amount of change Δthw in the measured coolant temperature is evaluated as the correlation value, it is possible to accurately determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. 
     Furthermore, according to the first embodiment, a second abnormality diagnosis operation is executed. Specifically, in the state where the coolant temperature is in the warm-up incomplete temperature range, the coolant temperature is estimated based on the amount of increase in the coolant temperature, which is caused by the heat generation at the engine  11 , and the amount of decrease in the coolant temperature, which is caused by the heat release from, for example, the radiator  13  and the heater core  18 . Then, it is determined whether the thermostat open state abnormality exists based on the measured coolant temperature, which is measured with the coolant temperature sensor  20 , and the estimated coolant temperature. Then, when the result of the first abnormality diagnosis operation (the abnormality diagnosis operation executed by using the thermostat abnormal time correlation) is the same as, i.e., is identical to the result of the second abnormality diagnosis operation (the abnormality diagnosis operation executed by using the estimated coolant temperature), this identical abnormality diagnosis result is used as the final abnormality diagnosis result. 
     In the case where the coolant of the engine  11  is circulated to the radiator  13  during the warm-up operation of the engine  11  upon occurrence of the thermostat open state abnormality, when the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , is small due to the low vehicle speed, the amount of heat released at the radiator  13  is small. In such a case, it may not be accurately determined whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. 
     Therefore, according to the first embodiment, in the case where the vehicle speed V does not satisfy a predetermined condition during the period of executing the first abnormality diagnosis operation, the radiator fan  21  is forcefully driven, and the vehicle speed V, which is used to determine the correlation between the amount of change Δthw in the measured coolant temperature and the vehicle speed V, is corrected based on the operational state of the radiator fan  21 . 
     The abnormality diagnosis of the thermostat  19  of the first embodiment is executed when the ECU  26  executes the abnormality diagnosis routines shown in  FIGS. 3 to 8 . The procedure of each of these routines will now be described in detail. 
     An abnormality diagnosis main routine will now be described. The abnormality diagnosis main routine of  FIG. 3  is executed at predetermined intervals while the power supply to the ECU  26  is turned on. This routine serves as an abnormality diagnosis means. Upon starting this routine, at step  101 , it is determined whether a predetermined diagnosis execution condition is satisfied. For instance, the predetermined diagnosis execution condition is satisfied when the coolant temperature sensor  20  is normal, and the coolant temperature is in the warm-up incomplete temperature range, which is lower than the predetermined temperature. When it is determined that the abnormality diagnosis execution condition is not satisfied at step  101 , the present routine is terminated without further executing the abnormality diagnosis operation at and after step  102 . 
     In contrast, when it is determined that the abnormality diagnosis execution condition is satisfied at step  101 , the abnormality diagnosis operation at and after step  102  is executed in the following manner. First of all, at step  102 , a correlation determination routine of  FIG. 4  is executed to determine whether the thermostat abnormal time correlation (see  FIG. 2 ) exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. 
     Thereafter, the operation proceeds to step  103  where a coolant temperature estimation routine of  FIG. 5  is executed to estimate the coolant temperature based on the amount of increase in the coolant temperature, which is caused by the heat generation at the engine  11 , and the amount of decrease in the coolant temperature, which is caused by the heat release from, for example, the radiator  13  and the heater core  18 . 
     Thereafter, the operation proceeds to step  104 . At step  104 , it is determined whether a predetermined determination enabling condition is satisfied. For example, the predetermined determination enabling condition is satisfied when a cumulative value of the measured vehicle speeds, which have been cumulated since the time of satisfying the predetermined abnormality diagnosis execution condition, becomes larger than a corresponding predetermined value. Alternatively, the determination enabling condition may be satisfied, for example, when the number of times of executing the computation of the correlation value is larger than a corresponding predetermined number, or when an average of the measured vehicle speeds is larger than a corresponding predetermined value. 
     When it is determined that the determination enabling condition is satisfied at step  104 , it is determined that the correlation between the amount of change Δthw in the measured coolant temperature and the vehicle speed V can be accurately determined. Then, the operation proceeds to step  105 . At step  105 , a normality/abnormality determination routine shown in  FIG. 6  is executed to determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. In this way, it is determined whether the thermostat open state abnormality exists. Specifically, the first abnormality diagnosis operation is executed to determine whether the thermostat open state abnormality exists based on whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. Also, the second abnormality diagnosis operation is executed to determine whether the thermostat open state abnormality exists based on the measured coolant temperature and the estimated coolant temperature. When the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are identical to each other, this identical abnormality diagnosis result is used as the final abnormality diagnosis result. 
     A correlation determination routine shown in  FIG. 4  is a subroutine, which is executed at step  102  of the abnormality diagnosis main routine shown in  FIG. 3 . Upon starting of the present routine, at step  201 , a difference between the currently measured coolant temperature thw(i), which is measured with the coolant temperature sensor  20 , and the previously measured coolant temperature thw(i−1), which has been previously measured with the coolant temperature sensor  20  before the current time, is computed to obtain the amount of change Δthw in the measured coolant temperature per predetermined time period (e.g., per computation cycle of the present routine).
 
Δ thw=thw ( i )− thw ( i− 1)
 
     This process at step  201  serves as a radiator side released heat amount information obtaining means. 
     Thereafter, at step  202 , the amount of change cf in the abnormal time coolant temperature corresponding to the current vehicle speed V (the vehicle speed V being corrected through a vehicle speed correction routine shown in  FIG. 8 ) is computed by using the map or equation, which defines the thermostat abnormal time correlation (see  FIG. 2 ). Here, the vehicle speed V is an average vehicle speed per predetermined time period (e.g., per computation cycle of the present routine). Furthermore, the map or equation, which defines the thermostat abnormal time correlation, is formed in advance for each corresponding vehicle based on the design data and/or test data and is stored in the ROM of the ECU  26 . Alternatively, the map or equation, which defines the thermostat abnormal time correlation, may be formed and stored for each engine operational condition, and the amount of change cf in the abnormal time coolant temperature may be computed by using the map or equation, which corresponds to the current engine operational condition. 
     Thereafter, the operation proceeds to step  203 . At step  203 , the amount of change cf in the abnormal time coolant temperature is corrected based the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. In this case, for example, the amount of change cf in the abnormal time coolant temperature is corrected such that the amount of change cf in the abnormal time coolant temperature is reduced (i.e., resulting in a decrease in the increasing rate or the increase in the decreasing rate) when the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha is increased. 
     Thereafter, the operation proceeds to step  204 . At step  204 , the difference (cf−Δthw) between the amount of change cf in the abnormal time coolant temperature and the amount of change Δthw in the measured coolant temperature is obtained as a correlation value, and this correlation value is added to the previous cumulative correlation value ΣC to obtain the current cumulative correlation value ΣC.
 
Σ C=ΣC +( cf−Δthw )
 
     Thereafter, the operation proceeds to step  205  where it is determined whether the cumulative correlation value ΣC is smaller than a predetermined value K. When it is determined that the cumulative correlation value ΣC is smaller than the predetermined value K at step  205 , the operation proceeds to step  206 . At step  206 , it is determined that the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V, and thereby a correlation flag XC is set to 1. In such a case, when the determination enabling condition has been previously satisfied at step  104 , it is determined that the open state abnormality of the thermostat  19  exists in the first abnormality diagnosis operation. 
     When it is determined that the cumulative correlation value ΣC is equal to or larger than the predetermined value K at step  205 , the operation proceeds to step  207 . At step  207 , it is determined that the thermostat abnormal time correlation does not exist between the amount of change Δthw in the measured coolant temperature and the vehicle speed V, and thereby the correlation flag XC is set to 0 (zero). In such a case, when the determination enabling condition has been previously satisfied at step  104 , it is determined that the open state abnormality of the thermostat  19  does not exist in the first abnormality diagnosis operation. 
     Now, the coolant temperature estimation routine will be described. The coolant temperature estimation routine of  FIG. 5  is a subroutine executed at step  103  of the abnormality diagnosis main routine of  FIG. 3  and serves as a coolant temperature estimating means. Upon starting of this routine, at step  301 , the amount of increase ΔTup in the coolant temperature caused by the heat generation at the engine  11  is computed using a map or a mathematical equation based on the current engine operational state (e.g., the engine rotational speed, the engine load). 
     Thereafter, the operation proceeds to step  302 . At step  302 , the amount of decrease ΔTdown in the coolant temperature caused by the heat release at, for example, the radiator  13  and the heater core  18  is computed based on the current vehicle speed, the coolant temperature and the ambient temperature. 
     Then, the operation proceeds to step  303 . At step  303 , the currently estimated coolant temperature T is obtained by adding the difference (ΔTup−ΔTdown) between the amount of increase ΔTup in the coolant temperature and the amount of decrease ΔTdown in the coolant temperature to the previously estimated coolant temperature T.
 
 T=T (Δ T up−Δ T down)
 
     Now, the normality/abnormality determination routine will be described. The normality/abnormality determination routine shown in  FIG. 6  is a subroutine, which is executed at step  105  of the abnormality diagnosis main routine shown in  FIG. 3 . Upon starting of this routine, at step  401 , it is determined whether the measured coolant temperature thw is lower than the determination reference temperature A (e.g., the temperature, which is set to be between the engine start time coolant temperature and the engine warm-up complete state temperature). Then, at step  402 , it is determined whether the estimated coolant temperature T is lower than the determination reference temperature B (e.g. the temperature slightly higher than the determination reference temperature A). 
     When it is determined that the measured coolant temperature thw is equal to or higher than the determination reference temperature A at step  401 , it is determined that the measured coolant temperature thw increases in a normal manner. Thereby, in the second abnormality diagnosis operation, it is determined that the open state abnormality of the thermostat  19  does not exist. Therefore, the operation proceeds to step  403 . At step  403 , it is determined whether the result of the first abnormality diagnosis operation also indicates that the open state abnormality does not exist by checking whether the correlation flag XC is 0 (zero). 
     When it is determined that the correlation flag XC is 0 (zero), i.e., the result of the first abnormality diagnosis indicates that the open state abnormality of the thermostat  19  does not exist at step  403 , the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are identical to each other. Therefore, the operation proceeds to step  404 . At step  404 , the common result (identical result) of the first and second abnormality diagnosis operations is adapted as the final abnormality diagnosis result, and it is thereby finally determined that the open state abnormality of the thermostat  19  does not exist (the thermostat  19  being normal). Then, the present routine is terminated. 
     In contrast, when it is determined that the correlation flag XC is 1, i.e., the result of the first abnormality diagnosis operation indicates that the open state abnormality of the thermostat  19  exists at step  403 , the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are not identical to each other, i.e., are different from each other. Therefore, the operation proceeds to step  407 . At step  407 , the abnormality diagnosis operation is terminated without finally determining whether the open state abnormality of the thermostat  19  exists. 
     When it is determined that the estimated coolant temperature T is equal to or higher than the determination reference temperature B at step  402  despite the determination of that the measured coolant temperature thw is lower than the determination reference temperature A at step  401 , the measured coolant temperature thw does not increase in a normal manner. Therefore, in the second abnormality diagnosis operation, it is determined that the open state abnormality of the thermostat  19  exists, and the operation proceeds to step  405 . At step  405 , it is determined whether the correlation flag XC is 1 to determine whether the result of the first abnormality diagnosis operation indicates that the open state abnormality of the thermostat  19  exists. 
     When it is determined that the correlation flag XC is 1, i.e., the open state abnormality of the thermostat  19  exists in the first abnormality diagnosis operation at step  405 , the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are identical to each other, and the operation proceeds to step  406 . At step  406 , the common result (identical result) of the first and second abnormality diagnosis operations is adapted as the final abnormality diagnosis result, and it is thereby finally determined that the open state abnormality of the thermostat  19  exists (the thermostat  19  being abnormal). In this case, for example, an abnormality flag is set to an ON state, and a warning lamp  27 , which is provided to an instrument panel at a driver&#39;s seat side, is lit. Alternatively, a warning is provided to the driver of the vehicle by indicating a warning display on a warning display device (not shown) of the instrument panel at the driver&#39;s seat side, and this abnormality information (e.g., an abnormality code) is stored in a rewritable non-volatile memory (a rewritable memory that holds the stored data even when the power supply to the ECU  26  is turned off). Then, the present routine is terminated. 
     In contrast, when it is determined that the correlation flag XC is 0 (zero), i.e., the result of the first abnormality diagnosis operation indicates that the open state abnormality of the thermostat  19  does not exist at step  405 , the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are not identical to each other. Therefore, the operation proceeds to step  407 . At step  407 , the abnormality diagnosis operation is terminated without finally determining whether the open state abnormality of the thermostat  19  exists. 
     Now, the radiator fan forceful drive routine will be described. The radiator fan forceful drive routine shown in  FIG. 7  is executed at predetermined intervals while the power supply to the ECU  26  is turned on. Upon starting of this routine, at step  501 , it is determined whether the abnormality diagnosis execution condition, which is the same as that of step  101  of  FIG. 3 , is satisfied. When it is determined that the predetermined abnormality diagnosis execution condition is not satisfied at step  501 , the operation proceeds to step  505  where the stop state of the radiator fan  21  is maintained. 
     In contrast, when it is determined that the abnormality diagnosis execution condition is satisfied at step  501 , it is determined that the abnormality diagnosis operation of the thermostat  19  is still executed. Thereby, the operation proceeds to step  502  where the currently measured vehicle speed V, which is measured with the vehicle speed sensor  25 , is added to the previous cumulative vehicle speed value ΣV, to renew the cumulative vehicle speed value ΣV.
 
Σ V=ΣV+V  
 
     Thereafter, the operation proceeds to step  503 . At step  503 , it is determined whether the cumulative vehicle speed value ΣV has become larger than a predetermined value F within a predetermined time period since the time of starting the cumulation of the vehicle speed V during the period of executing the abnormality diagnosis operation. When it is determined that the cumulative vehicle speed value ΣV has become larger than the predetermined value F within the predetermined time period at step  503 , it is determined that the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , is small. Therefore, the operation proceeds to step  504  where the radiator fan  21  is forcefully driven. In this way, the amount of air flow, which is applied to the radiator  13 , is reliably increased with the aid of the air flow created by the radiator fan  21 , 
     In contrast, when it is determined that the cumulative vehicle speed value ΣV has not become larger that the predetermined value F within the predetermined time period at step  503 , it is determined that the vehicle speed has become sufficiently high during the period of executing the abnormality diagnosis operation, and thereby the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , is sufficiently large. Thereby, the operation proceeds to step  505  where the stop state of the radiator fan  21  is maintained. 
     Now, the vehicle speed correction routine will be described. The vehicle speed correction routine shown in  FIG. 8  is executed at predetermined intervals while the power supply to the ECU  26  is turned on. Upon starting of this routine, at step  601 , it is determined whether the radiator fan  21  is currently forcefully driven. When it is determined that the radiator fan  21  is currently forcefully driven at step  601 , the operation proceeds to step  602 . At step  602 , a correction value R is added to the vehicle speed V, which is measured with the vehicle speed sensor  25 , to correct the vehicle speed V. This corrected vehicle speed V is then used as the vehicle speed in the abnormality diagnosis operation (the routine of  FIG. 4  discussed above).
 
 V=V+R  
 
     Here, the correction value R is a value that corresponds to a required vehicle speed, which is required to generate the air flow by the forward movement of the vehicle in the amount that is equal to the amount of air flow otherwise generated by the radiator fan  21 . The correction value R is set according to the drive state of the radiator fan  21  (e.g., the rotational speed, the drive voltage). When the operational state of the radiator fan  21  at the time of the forceful drive operation of the radiator fan  21  is constant for each time, the correction value R may be a fixed constant value. 
     When it is determined that the radiator fan  21  is not currently forcefully driven at step  601 , the operation proceeds to step  603 . At step  603 , the vehicle speed V, which is measured with the vehicle speed sensor  25 , is not corrected and is directly used in the abnormality diagnosis operation. 
     According to the first embodiment, in the warm-up incomplete temperature range that is lower than the predetermined coolant temperature, the amount of change cf in the abnormal time coolant temperature corresponding to the vehicle speed V is computed through use of the thermostat abnormal time correlation. Then, the difference (cf−Δthw) between the amount of change cf in the abnormal time coolant temperature and the amount of change Δthw in the measured coolant temperature is computed as the correlation value. This correlation value (cf−Δthw) is evaluated to determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. In this way, it is possible to accurately sense the open state abnormality of the thermostat  19  through use of the thermostat abnormal time correlation. 
     According to the first embodiment, the amount of change cf in the abnormal time coolant temperature is corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. Therefore, it is possible to accurately determine the correlation between the amount of change Δthw in the measured coolant temperature and the vehicle speed V in view of the influence of the ambient temperature. 
     Furthermore, according to the first embodiment, in the case where the cumulative vehicle speed value ΣV does not exceed the predetermined value F within the predetermined time period during the abnormality diagnosis operation period, it is determined that the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , is small due to the low vehicle speed, and thereby the radiator fan  21  is forcefully driven. In this way, the amount of air flow, which is applied to the radiator  13 , is reliably increased by the air flow generated by the radiator fan  21 . Furthermore, the vehicle speed is corrected according to the operational state of the radiator fan  21 , so that the influences of the air flow generated by the radiator fan  21  can be reflected on the vehicle speed. In this way, even in the case where the amount of air flow, which is generated by the forward movement of the vehicle and is applied to the radiator  13 , is small, it is possible to accurately determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. 
     Furthermore, according to the first embodiment, the first abnormality diagnosis operation is executed to determine whether the thermostat open state abnormality exists based on whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. Also, the second abnormality diagnosis operation is executed to determine whether the thermostat open state abnormality exists based on the measured coolant temperature and the estimated coolant temperature. When the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are identical to each other, this identical abnormality diagnosis result is used as the final abnormality diagnosis result. In this way, it is possible to further improve the accuracy of the abnormality diagnosis of the thermostat  19 . 
     Furthermore, according to the first embodiment, the amount of change cf in the abnormal time coolant temperature is corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. Alternatively, the amount of change Δthw in the measured coolant temperature may be corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. 
     Second Embodiment 
     A second embodiment of the present invention will be described with reference to  FIG. 9 . In the following description, components as well as steps similar to those of the first embodiment will not be described for the sake of the simplicity, and differences, which are different from those of the first embodiment, will be mainly discussed below. 
     According to the second embodiment, the correlation determination routine of  FIG. 9  is executed by the ECU  26 , so that a ratio (Δthw/V) between the amount of change Δthw in the measured coolant temperature and the vehicle speed V is computed as a correlation value. Then, this correlation value is evaluated to determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. When the correlation between the amount of change Δthw in the measured coolant temperature and the vehicle speed V becomes closer to the thermostat abnormal time correlation, the ratio (Δthw/V) between the amount of change Δthw in the measured coolant temperature and the vehicle speed V becomes closer to a predetermined value (a ratio between the amount of change Δthw in the measured coolant temperature at the thermostat abnormal time and the vehicle speed V). Therefore, when the ratio (Δthw/V) between the amount of change Δthw in the measured coolant temperature and the vehicle speed V is evaluated as the correlation value, it is possible to accurately determine whether the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V. 
     In the correlation routine shown in  FIG. 9 , at step  701 , the amount of change Δthw in the measured coolant temperature is computed. Here, the amount of change Δthw in the measured coolant temperature may be corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. 
     Thereafter, the operation proceeds to step  702  where the ratio (Δthw/V) between the amount of change Δthw in the measured coolant temperature and the vehicle speed V is computed as a correlation value. Then, this correlation value (Δthw/V) is added to the previous cumulative correction value ΣC to updated the cumulative correlation value ΣC.
 
Σ C=ΣC +(Δ thw/V )
 
     Thereafter, the operation proceeds to step  703  where it is determined whether the cumulative correlation value ΣC is smaller than a predetermined value K. When it is determined that the cumulative correlation value ΣC is smaller than the predetermined value K at step  703 , the operation proceeds to step  704 . At step  704 , it is determined that the thermostat abnormal time correlation exists between the amount of change Δthw in the measured coolant temperature and the vehicle speed V, and thereby the correlation flag XC is set to 1. 
     When it is determined that the cumulative correlation value ΣC is equal to or larger than the predetermined value K at step  703 , the operation proceeds to step  705 . At step  705 , it is determined that the thermostat abnormal time correlation does not exist between the amount of change Δthw in the measured coolant temperature and the vehicle speed V, and thereby the correlation flag XC is set to 0 (zero). 
     Even in the second embodiment, the advantages similar to those of the first embodiment can be achieved. 
     Third Embodiment 
     A third embodiment of the present invention will be described with reference to  FIG. 10 . In the following description, components as well as steps similar to those of the first embodiment will not be described for the sake of the simplicity, and differences, which are different from those of the first embodiment, will be mainly discussed below. 
     The amount of change Δthw in the measured coolant temperature, which is measured with the coolant temperature sensor  20 , can be obtained according to the following equation (1) based on the amount of change Δthw1 in the coolant temperature caused by the application of heat from the engine  11  to the coolant, the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13 , the amount of change (−Δthw3) in the coolant temperature caused by the release of heat from the coolant through the heater core  18 , and the amount of change (−Δthw4) in the coolant temperature caused by the release of heat from the coolant through the component(s) or part(s) of the coolant circuit  16  (e.g., coolant circulation pipe) other than the radiator  13  and the heater core  18 .
 
Δ thw=Δthw 1−Δ thw 2−Δ thw 3−Δ thw 4  Equation (1)
 
     When the above equation (1) is solved for the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13 , the following equation (2) can be obtained.
 
−Δ thw 2 =Δthw−Δthw 1−(−Δ thw 3 −thw 4)   Equation (2)
 
     It is possible to obtain the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  based on the above equation (2). 
     When the open state abnormality of the thermostat  19  occurs, the following relationship is established between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V. That is, when the vehicle speed is increased, the amount of change in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is reduced (resulting in an increase in the decreasing rate). 
     Therefore, according to the third embodiment, a correlation determination routine of  FIG. 10  described below is executed by the ECU  26 . Thereby, based on the equation (2) discussed above, the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is computed as the radiator side released heat amount information. Then, it is determined whether the predetermined thermostat abnormal time correlation (see FIG.  2 ) exists between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V. In this way, it is determined whether the thermostat open state abnormality exists. 
     In the correlation determination routine of  FIG. 10 , at step  801 , the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is computed through use of the equation (2) based on the the amount of change Δthw in the measured coolant temperature, which is measured with the coolant temperature sensor  20 , the amount of change Δthw1 in the coolant temperature caused by the application of heat from the engine  11  to the coolant, the amount of change (−Δthw3) in the coolant temperature caused by the release of heat from the coolant through the heater core  18 , and the amount of change (−Δthw4) in the coolant temperature caused by the release of heat from the coolant through the component(s) or part(s) of the coolant circuit  16  (e.g., the coolant circulation pipe) other than the radiator  13  and the heater core  18 . 
     Thereafter, the operation proceeds to step  802 . At step  802 , the amount of change cf in the abnormal time coolant temperature caused by the release of heat from the coolant through the radiator  13  corresponding to the current vehicle speed V is computed through use of a map or a mathematical equation, which defines the thermostat abnormal time correlation (see  FIG. 2 ). Thereafter, the operation proceeds to step  803 . At step  803 , the amount of change cf in the abnormal time coolant temperature caused by the release of heat from the coolant through the radiator  13  is corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. 
     Thereafter, the operation proceeds to step  804 . At step  804 , the difference [cf−(−Δthw2)] between the amount of change cf in the abnormal time coolant temperature caused by the release of heat from the coolant through the radiator  13  and the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is computed as a correlation value. This correlation value [cf−(−Δthw2)] is added to the previous cumulative correlation value ΣC to update the cumulative correlation value ΣC.
 
Σ C=ΣC+[cf −(−Δ thw 2)]
 
     Thereafter, the operation proceeds to step  805  where it is determined whether the cumulative correlation value ΣC is smaller than the predetermined value K. When it is determined that the cumulative correlation value ΣC is smaller than the predetermined value K at step  805 , the operation proceeds to step  806 . At step  806 , it is determined that the thermostat abnormal time correlation exists between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V, and thereby the correlation flag XC is set to 1. 
     In contrast, when it is determined that the cumulative correlation value ΣC is equal to or larger than the predetermined value K at step  805 , the operation proceeds to step  807 . At step  807 , it is determined that the thermostat abnormal time correlation does not exist between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V, and thereby the correlation flag XC is set to 0 (zero). 
     According to the third embodiment, the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is computed based on the amount of change Δthw in the measured coolant temperature, which is measured with the coolant temperature sensor  20 , the amount of change Δthw1 in the coolant temperature caused by the application of heat from the engine  11  to the coolant, the amount of change (−Δthw3) in the coolant temperature caused by the release of heat from the coolant through the heater core  18 , and the amount of change (−Δthw4) in the coolant temperature caused by the release of heat from the coolant through the component(s) or part(s) of the coolant circuit  16  (e.g., coolant circulation pipe) other than the radiator  13  and the heater core  18 . Therefore, the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  can be accurately computed. Then, this accurately computed amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is used to determine whether the thermostat abnormal time correlation exists between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V. Therefore, the abnormality detection of the thermostat  19  can be more accurately performed. 
     In the third embodiment, the difference [cf−(−Δthw2)] between the amount of change cf in the abnormal time coolant temperature caused by the release of heat from the coolant through the radiator  13  and the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is computed as the correlation value. Then, this correlation value [cf−(−Δthw2)] is evaluated to determine whether the thermostat abnormal time correlation exists between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V. Alternatively, a ratio (−Δthw2/V) between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V may be computed as a correlation value. Then, this correlation value (−Δthw2/V) may be evaluated to determine whether the thermostat abnormal time correlation exists between the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  and the vehicle speed V. 
     Furthermore, the amount of change (−Δthw2) in the coolant temperature caused by the release of heat from the coolant through the radiator  13  may be corrected based on the difference (thw−tha) between the coolant temperature thw and the ambient temperature tha. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will be described with reference to  FIG. 11 . In the following description, components as well as steps similar to those of the first embodiment will not be described for the sake of the simplicity, and differences, which are different from those of the first embodiment, will be mainly discussed below. 
     In each of the first to third embodiments, when the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation are identical to each other, this identical abnormality diagnosis result is used as the final abnormality diagnosis result. Contrary to this, according to the fourth embodiment, a normality/abnormality determination routine of  FIG. 11  discussed later is executed by the ECU  26  to use one of the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation, which is completed earlier than the other one, as the final abnormality diagnosis result. Furthermore, in the normality/abnormality determination routine of  FIG. 11 , step  104  of the abnormality diagnosis main routine of  FIG. 3  is eliminated. 
     In the normality/abnormality determination routine of  FIG. 11 , at step  901 , it is determined wither the measured coolant temperature thw is lower than the determination reference temperature A. Then, at step  902 , it is determined whether the estimated coolant temperature T is lower than the determination reference temperature B. 
     When it is determined that the measured coolant temperature thw is equal to or higher than the determination reference temperature A at step  901 , the measured coolant temperature thw is increased in a normal manner. Therefore, it is determined that the open state abnormality of the thermostat  19  does not exist. Then, the operation proceeds to step  905  where it is finally determined that the open state abnormality of the thermostat  19  does not exist (the thermostat  19  being normal). 
     When it is determined that the estimated coolant temperature T is equal to or higher than the determination reference temperature B at step  902  after the determination of that the measured coolant temperature thw is lower than the determination reference temperature A at step  901 , the measured coolant temperature thw is not increased in a normal manner. Therefore, it is determined that the open state abnormality of the thermostat  19  exists in the second abnormality diagnosis operation. Then, the operation proceeds to step  906 , and it is finally determined that the open state abnormality of the thermostat  19  exists. 
     When it is determined that the estimated coolant temperature T is lower than the determination reference temperature B at step  902  after the determination of that the measured coolant temperature thw is lower than the determination reference temperature A at step  901 , it is determined that the second abnormality diagnosis operation has not been completed. Thereby, the operation proceeds to step  903  where it is determined whether the predetermined correlation determination condition is satisfied. For example, this may be determined by determining whether the cumulative value of the measured vehicle speeds, which have been cumulated since the time of satisfying the abnormality execution condition, has become larger than a predetermined value. The correlation determination condition may be satisfied, for example, when the number of times of executing the computation of the correlation value is larger than a corresponding predetermined number, or when an average of the measured vehicle speeds is larger than a corresponding predetermined value. 
     When it is determined that the correlation determination condition is satisfied at step  903 , it is determined that the correlation between the radiator side released heat amount information (the amount of change in the measured coolant temperature or the amount of change in the coolant temperature caused by the release of heat from the coolant through the radiator  13 ) and the vehicle speed can be accurately determined. Therefore, the operation proceeds to step  904 . At step  904 , it is determined whether the result of the first abnormality diagnosis operation indicates the existence of the open state abnormality of the thermostat  19  based on the presence of the state of the correlation flag XC=1. 
     When it is determined that the correlation flag XC is 0 (zero) at step  904 , i.e., when it is determined that the open state abnormality of the thermostat  19  does not exist in the first abnormality diagnosis operation, the operation proceeds to step  905 . At step  905 , it is finally determined that the open state abnormality of the thermostat  19  does not exist (the thermostat  19  being normal). 
     In contrast, when it is determined that the correlation flag XC is 1 at step  904 , i.e., when it is determined that the open state abnormality of the thermostat  19  exists in the first abnormality diagnosis operation, the operation proceeds to step  906 . At step  906 , it is finally determined that the open state abnormality of the thermostat  19  exists. 
     In the above described manner, the one of the result of the first abnormality diagnosis operation and the result of the second abnormality diagnosis operation, which is completed earlier than the other one, is used as the final abnormality diagnosis result. Therefore, it is possible to confirm the result of the abnormality diagnosis of the thermostat  19  in the early stage. 
     Fifth Embodiment 
     A fifth embodiment of the present invention will be described with reference to  FIG. 12 . In the following description, components as well as steps similar to those of the first embodiment will not be described for the sake of the simplicity, and differences, which are different from those of the first embodiment, will be mainly discussed below. 
     According to the fifth embodiment, the ECU  26  executes a second abnormality diagnosis routine of  FIG. 12  to change the determination condition (e.g., the determination reference value or temperature), which is used to determine whether the open state abnormality of the thermostat exists in the second abnormality diagnosis operation according to the cumulative correlation value ΣC, which is computed to determine the correlation between the radiator side released heat amount information (the amount of change in the measured coolant temperature or the amount of change in the coolant temperature caused by the release of heat from the coolant through the radiator  13 ) and the vehicle speed in the first abnormality diagnosis operation. 
     In the second abnormality diagnosis routine of  FIG. 12 , at step  1001 , the determination reference temperature A of the measured coolant temperature thw is computed through use of a map or a mathematical equation based on the cumulative correlation value ΣC, which is computed in the first abnormality diagnosis operation. The map or the mathematical equation, which is used to compute the determination reference temperature A, is set such that the determination reference temperature A is increased when the cumulative correlation value ΣC is increased, i.e., when the deviation of the correlation between the radiator side released heat amount information and the vehicle speed from the thermostat abnormal time correlation is increased. 
     Thereafter, the operation proceeds to step  1002 . At step  1002 , the determination reference temperature B of the estimated coolant temperature T is computed through use of a map or a mathematical equation based on the cumulative correlation value ΣC, which is computed in the first abnormality diagnosis operation. The map or the mathematical equation, which is used to compute the determination reference temperature B, is set such that the determination reference temperature B is increased when the cumulative correlation value ΣC is increased. 
     Thereafter, at step  1003 , it is determined whether the measured coolant temperature thw is lower than the determination reference temperature A. Then, at step  1004 , it is determined whether the estimated coolant temperature T is lower than the determination reference temperature B. 
     When it is determined that the measured coolant temperature thw is equal to or higher than the determination reference temperature A at step  1003 , the measured coolant temperature thw is increased in the normal manner. Therefore, it is determined that the open state abnormality of the thermostat  19  does not exist (the thermostat  19  being normal). Then, the operation proceeds to step  1005  where it is finally determined that the open state abnormality of the thermostat  19  does not exist (the thermostat  19  being normal). 
     In contrast, when it is determined that the estimated coolant temperature T is equal to or higher than the determination reference temperature B at step  1004  after the determination of that the measured coolant temperature thw is lower than the determination reference temperature A at step  1003 , the measured coolant temperature thw is not increased in a normal manner. Therefore, the operation proceeds to step  1006 , and it is finally determined that the open state abnormality of the thermostat  19  exists. 
     In the fifth embodiment, the determination reference temperatures, which are used to determine whether the thermostat open state abnormality exists in the second abnormality diagnosis operation, are changed based on the cumulative correlation value ΣC, which is computed to determine the correlation between the radiator side released heat amount information and the vehicle speed in the first abnormality diagnosis operation. In this way, it is possible to further improve the accuracy of the abnormality diagnosis in the second abnormality diagnosis operation by appropriately changing the determination reference temperatures of the second abnormality diagnosis operation based on the cumulative correlation value ΣC (the degree of correlation between the radiator side released heat amount information and the vehicle speed), which is computed in the first abnormality diagnosis operation. 
     In the fifth embodiment, the determination reference temperatures, which are used in the second abnormality diagnosis operation, are changed based on the cumulative correlation value ΣC. Alternatively, the measured coolant temperature thw and the estimated coolant temperature T, which are used in the second abnormality diagnosis operation, may be corrected based on the cumulative correlation value ΣC. 
     Furthermore, in each of the first to fifth embodiments, the cumulative correlation value (the cumulative value of the correlation values) is used as the determination parameter, which is used to determine the correlation between the radiator side released heat amount information (the amount of change in the measured coolant temperature or the amount of change in the coolant temperature caused by the release of heat from the coolant through the radiator  13 ) and the vehicle speed in the first abnormality diagnosis operation. However, the present invention is not limited to this. For example, an average correlation value (an average value of the correlation values) or the correlation value may be used as the determination parameter. 
     In each of the first to fifth embodiments, when the cumulative vehicle speed value ΣV has not become larger than the predetermined value F within the predetermined time period during the period of executing the abnormality diagnosis operation, the radiator fan  21  is forcefully driven. Alternatively, the radiator  21  may be always forcefully driven during the period of executing the abnormality diagnosis operation. 
     In each of the first to fifth embodiments, the amount of change in the measured coolant temperature or the amount of change in the coolant temperature caused by the release of heat from the coolant through the radiator  13  is used as the radiator side released heat amount information. However, the present invention is not limited to this. For example, the released heat amount of the radiator  13  may be used as the radiator side released heat amount information. 
     In each of the first to fifth embodiments, the first abnormality diagnosis operation (the abnormality diagnosis operation using the thermostat abnormal time correlation) and the second abnormality diagnosis operation (the abnormality diagnosis operation using the estimated coolant temperature) are executed. Alternatively, only the first abnormality diagnosis operation may be performed. 
     Furthermore, the present invention may be modified in any other appropriate manner. For example, the location of the thermostat  19 , which is provided in the coolant circuit  16 , may be modified to another location. Also, the construction of the engine cooling system may be modified into an appropriate manner. That is, the present invention may be applied to various engine cooling systems as long as the engine cooling systems have the thermostat in the coolant circuit, which circulates the coolant between the engine and the radiator. 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.