Abnormality diagnostic system for evaporative fuel-processing system of internal combustion engine for vehicles

An evaporative fuel-processing system for an internal combustion engine, comprises an evaporative emission control system in which a first control valve is arranged across an evaporative fuel-guiding passage extending between a fuel tank and a canister, a second control valve across a purging passage extending between the canister and the intake system of the engine, and a third control valve at an air inlet port of the canister, respectively. An external diagnostic device is humanly operatable for diagnosing operating conditions of the engine and the vehicle. An ECU is responsive to an output from an external diagnostic device which diagnoses operating conditions of the engine, for determining whether there is an abnormality in the evaporative emission control system, based upon an output from the tank internal pressure sensor, which is obtained when the evaporative emission control system has been brought into the predetermined negatively pressurized state, when the engine is in a predetermined operating condition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an abnormality diagnostic system for an 
evaporative fuel-processing system for internal combustion engines for 
vehicles, and more particularly to an abnormality diagnostic system which 
has a function of detecting abnormalities in an evaporative emission 
control system of the engine. 
2. Prior Art 
Conventionally, there has been widely used an evaporative fuel-processing 
system for internal combustion engines for automotive vehicles, which 
comprises an evaporative emission control system having a canister having 
an air inlet port provided therein, a first control valve arranged across 
an evaporative fuel-guiding passage extending from a fuel tank of the 
engine to the canister, and a second control valve arranged across a 
purging passage extending from the canister to an intake system of the 
engine. 
An evaporative emission control system of this kind temporarily stores 
evaporative fuel in the canister, and then purges the evaporative fuel 
into the intake system of the engine. 
Whether an evaporative emission control system of this kind is normally 
operating can be checked, for example, by bringing the evaporative 
emission control system into a predetermined negatively pressurized state, 
measuring a change in the pressure within the fuel tank (tank internal 
pressure) occurring with the lapse of time after the evaporating emission 
control system has been brought into the predetermined negatively 
pressurized state, by a tank internal pressure sensor which detects the 
tank internal pressure, and determining whether the system is normally 
operating, from the measured tank internal pressure, as proposed by 
Japanese Patent Application No. 3(1991)-262857 and corresponding U.S. Pat. 
No. 5,299,545, assigned to the assignee of the present application, for 
example. 
According to the method of the earlier application, an amount of change in 
pressure prevailing within the evaporative emission control system is 
detected by the tank internal pressure sensor, to determine an abnormality 
in the system in such a manner that if the detected pressure change amount 
is below a predetermined value, it is presumed that an amount of 
evaporative fuel leaking from the system to the outside is small and hence 
it is determined that the system is normally functioning, whereas if the 
detected pressure change amount exceeds the predetermined value, it is 
presumed that evaporative fuel is leaking in a large amount from the 
system to the outside, and hence it is determined that the system is 
malfunctioning. 
The determination of abnormality of the evaporative emission control system 
according to the method of the earlier application is carried out when 
predetermined abnormality determination-permission conditions are 
satisfied during running of the vehicle, i.e. when the engine enters a 
predetermined operating condition during running of the vehicle. 
However, the upper surface of fuel within the fuel tank largely moves or 
stirs when the vehicle is in a particular running condition such as 
acceleration, deceleration and turning. Consequently, the pressure within 
the fuel tank largely changes when the vehicle is in such a particular 
running condition. When the pressure within the fuel tank thus largely 
changes due to running of the vehicle in such a particular running 
condition, it can be erroneously determined that the system is abnormal 
even when it is normally functioning. 
Further, according to the method of the earlier application, to forcibly 
bring the interior of the evaporative emission control system into the 
predetermined negatively pressurized state, the second control valve is 
opened to communicate the interior of the system with the intake system of 
the engine via the purging passage. Then, a large amount of fuel vapor 
(evaporative fuel) is supplied into the intake system due to a gas drawing 
force created by the engine. This causes large fluctuations in the 
air-fuel ratio of a mixture supplied to the engine, and more frequent 
emission of unburnt gases through an exhaust system of the engine. 
Moreover, the abnormality determination can be frequently carried out 
whenever the abnormality determination-permission conditions become 
satisfied, resulting in spoilage of drivability and degraded exhaust 
emission characteristics of the engine. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide an abnormality diagnostic 
system for an evaporative fuel-processing system for an internal 
combustion engine for vehicles, which is capable of accurately detecting 
abnormalities in an evaporative emission control system of the engine 
without an erroneous determination, and also capable of carrying out the 
abnormality determination without spoiling the drivability and exhaust 
emission characteristics of the engine. 
To attain the object, the present invention provides an abnormality 
diagnostic system for an evaporative fuel-processing system for an 
internal combustion engine installed in a vehicle and having an intake 
system, and a fuel tank, the system comprising an evaporative emission 
control system including a canister having an air inlet port provided 
therein, an evaporative fuel-guiding passage extending between the fuel 
tank and the canister, a first control valve arranged across the 
evaporative fuel-guiding passage, a purging passage extending between the 
canister and the intake system of the engine, and a second control valve 
arranged across the purging passage, 
The abnormality diagnostic system according to the invention is 
characterized by comprising: 
tank internal pressure detecting means for detecting pressure within the 
fuel tank; 
negatively pressurizing means for bringing the evaporative emission control 
system into a predetermined negatively pressurized state; 
external diagnostic means provided externally of the engine, the external 
diagnostic means being humanly operatable for diagnosing operating 
conditions of the engine; and 
abnormality determining means responsive to an output from the external 
diagnostic means, for determining whether there is an abnormality in the 
evaporative emission control system based upon an output from the tank 
internal pressure detecting means, which output is obtained when the 
evaporative emission control system has been brought into the 
predetermined negatively pressurized state, when it is determined by the 
external diagnostic means that the engine is in a predetermined operating 
condition. 
In a preferred form of the invention, the external diagnostic means 
comprises display means capable of displaying predetermined setting values 
of a plurality of predetermined operating parameters for setting the 
predetermined operating condition, and command means for supplying a 
command signal for carrying out an abnormality diagnosis of the 
evaporative emission control system, to the abnormality determining means. 
Preferably, the abnormality determining means includes operating condition 
determining means responsive to said command signal from the command means 
of the external diagnostic means, for determining whether the engine is in 
the predetermined operating condition. 
Alternatively, the external diagnostic means comprises operating condition 
determining means for determining whether a plurality of predetermined 
operating parameters satisfy respective predetermined setting values for 
setting the predetermined operating condition, and command means 
responsive to an output from the operating condition determining means, 
for supplying a command signal for carrying out an abnormality diagnosis 
of the evaporative emission control system, to the abnormality determining 
means, when it is determined by the operating condition determining means 
that all the predetermined operating parameters satisfy the respective 
predetermined setting values. 
Preferably, in this alternative arrangement, the display means is capable 
of displaying at least one of the predetermined operating parameters which 
does not satisfy a corresponding one of the setting values. 
The external diagnostic means includes setting operation means for manually 
setting values of the predetermined operating parameters such that the 
predetermined operating condition is established. 
More preferably, the abnormality diagnostic system includes vehicle speed 
detecting means for detecting traveling speed of the vehicle, and wherein 
the abnormality determining means determines whether there is an 
abnormality in the evaporative emission control system based upon the 
output from the tank internal pressure detecting means,when the engine is 
in a predetermined operating condition, and at the same time it is 
detected by the vehicle speed detecting means that the vehicle is in a 
substantially standing condition. 
Further, the evaporative fuel-processing system may include engine 
operation detecting means for detecting whether the engine is operating, 
and a third control valve for opening and closing the air inlet port of 
the canister, and wherein the negatively pressurizing means brings the 
evaporative emission control system into the predetermined negatively 
pressurized state by controlling the first to third control valves while 
the engine is detected to be operating. As a result, the evaporative 
emission control system can be brought into the predetermined negatively 
pressurized state, merely by controlling the first to third control 
valves. 
Still further, the abnormality determining means determines abnormality of 
the evaporative emission control system, based upon a rate of change in 
the pressure within the fuel tank with the lapse of time after the 
evaporative emission control system has been brought into the 
predetermined negatively pressurized state by the negatively pressurizing 
means.

DETAILED DESCRIPTION 
The invention will now be described in detail with reference to the 
drawings showing embodiments thereof. 
Referring first to FIG. 1, there is illustrated the whole arrangement of an 
internal combustion engine installed in an automotive vehicle and an 
evaporative fuel-processing system therefor, in which is incorporated an 
abnormality diagnosistic system according to an embodiment of the 
invention. 
In the figure, reference numeral 1 designates an internal combustion engine 
(hereinafter simply referred to as "the engine") having four cylinders, 
not shown, for instance. Connected to the cylinder block of the engine 1 
is an intake pipe 2 across which is arranged a throttle body 3 
accommodating a throttle valve 3' therein. A throttle valve opening 
(.theta.TH) sensor 4 is connected to the throttle valve 3' for generating 
an electric signal indicative of the sensed throttle valve opening and 
supplying same to an electronic control unit (hereinafter referred to as 
"the ECU") 5. 
Fuel injection valves 6, only one of which is shown, are inserted into the 
interior of the intake pipe 2 at locations intermediate between the 
cylinder block of the engine 1 and the throttle valve 3' and slightly 
upstream of respective intake valves, not shown. The fuel injection valves 
6 are connected to a fuel pump 8 via a fuel supply pipe 7, and 
electrically connected to the ECU 5 to have their valve opening periods 
controlled by signals therefrom. 
A negative pressure communication passage 9 and a purging passage 10 open 
into the intake pipe 2 at respective locations downstream of the throttle 
valve 3', both of which are connected to an evaporative emission control 
system 11, referred to hereinafter. 
Further, an intake pipe absolute pressure (PBA) sensor 13 is provided in 
communication with the interior of the intake pipe 2 via a conduit 12 
opening into the intake passage 2 at a location downstream of an end of 
the purging passage 10 opening into the intake pipe 2 for supplying an 
electric signal indicative of the sensed absolute pressure within the 
intake pipe 2 to the ECU 5. 
An intake air temperature (TA) sensor 14 is inserted into the intake pipe 2 
at a location downstream of the conduit 12 for supplying an electric 
signal indicative of the sensed intake air temperature TA to the ECU 5. 
An engine coolant temperature (TW) sensor 15 formed of a thermistor or the 
like is inserted into a coolant passage filled with a coolant and formed 
in the cylinder block, for supplying an electric signal indicative of the 
sensed engine coolant temperature TW to the ECU 5. 
An engine rotational speed (NE) sensor 16 is arranged in facing relation to 
a camshaft or a crankshaft of the engine 1, neither of which is shown. The 
engine rotational speed sensor 16 generates a pulse as a TDC signal pulse 
at each of predetermined crank angles whenever the crankshaft rotates 
through 180 degrees, the pulse being supplied to the ECU 5. 
A transmission 17 is connected between wheels of a vehicle, not shown, and 
an output shaft of the engine 1, for transmitting power from the engine 1 
to the wheels. 
A vehicle speed (VSP) sensor 18 is mounted on one of the wheels, for 
supplying an electric signal indicative of the sensed vehicle speed VSP to 
the ECU 5. 
An oxygen concentration (O.sub.2) sensor 20 is inserted into an exhaust 
pipe 19 extending from the engine 1, for supplying an electric signal 
indicative of the sensed oxygen concentration to the ECU 5. 
An ignition switch (IGSW) sensor 21 detects an ON (or closed) state of an 
ignition switch IGSW, not shown, to detect that the engine 1 is in 
operation, and supplies an electric signal indicative of the ON state of 
the ignition switch IGSW to the ECU5. 
Electric devices 100 such as an air conditioner is electrically connected 
to the ECU 5, for supplying electric signals indicative of on-off states 
thereof to the ECU 5. 
A fuel tank 23 having a filler cap 22 which is removed for refueling is 
provided in the vehicle. 
The evaporative emission control system 11 is comprised of a canister 26 
containing activated carbon 24 as an adsorbent and having an air inlet 
port 25 provided in an upper wall thereof, an evaporative fuel-guiding 
passage 27 connecting between the canister 26 and the fuel tank 23, and a 
first control valve 28 arranged across the evaporative fuel-guiding 
passage 27. 
The fuel tank 23 is connected to the fuel injection valves 6 via the fuel 
pump 8 and the fuel supply pipe 7, and has a tank internal pressure (PT) 
sensor (hereinafter referred to as "the PT sensor") 29 and a fuel amount 
(FV) sensor 30, both mounted at an upper wall thereof, and a fuel 
temperature (TF) sensor 31 mounted at a lateral wall thereof. The PT 
sensor 29, the FV sensor 30, and the TF sensor 31 are electrically 
connected to the ECU 5. The PT sensor 29 senses the pressure (tank 
internal pressure) PT within the fuel tank 23 and supplies an electric 
signal indicative of the sensed tank internal pressure PT to the ECU 5. 
The FV sensor 30 senses the volumetric amount of fuel within the fuel tank 
23 and supplies an electric signal indicative of the sensed volumetric 
amount of fuel to the ECU 5. The TF sensor 31 senses the temperature of 
fuel within the fuel tank 23 and supplies an electric signal indicative of 
the sensed fuel temperature TF to the ECU 5. 
The first control valve 28 is comprised of a two-way valve 34 formed of a 
positive pressure valve 32 and a negative pressure valve 33, and a first 
electromagnetic valve 35 formed in one body with the two-way valve 34. 
More specifically, the first electromagnetic valve 35 has a rod 35a, a 
front end of which is fixed to a diaphragm 32a of the positive pressure 
valve 32. Further, the first electromagnetic valve 35 is electrically 
connected to the ECU 5 to have its operation controlled by a signal 
supplied from the ECU 5. When the first electromagnetic valve 35 is 
energized, the positive pressure valve 32 of the two-way valve 34 is 
forcedly opened to open the first control valve 28, whereas when the first 
electromagnetic valve 35 is deenergized, the valving (opening/closing) 
operation of the first control valve 28 is controlled by the two-way valve 
34 alone. 
A purge control valve (second control valve) 36 is arranged across the 
purging passage 10 extending from the canister 26, which valve has a 
solenoid, not shown, electrically connected to the ECU 5. The purge 
control valve 36 is controlled by a signal supplied from the ECU 5 to 
linearly change the opening thereof. That is, the ECU 5 supplies a desired 
amount of control current to the purge control valve 36 to control the 
opening thereof. 
A hot wire-type flowmeter (mass flowmeter) 37 is arranged in the purging 
passage 10 at a location between the canister 26 and the purge control 
valve 36. The flowmeter 37 has a platinum wire, not shown, which is heated 
by an electric current and cooled by a gas flow flowing in the purging 
passage 10 to have its electrical resistance reduced. The flowmeter 37 has 
an output characteristic variable in dependence on the concentration and 
flow rate of evaporative fuel flowing in the purging passage 10 as well as 
on the flow rate of a mixture of evaporative fuel and air being purged 
through the purging passage 10. The flowmeter 37 is electrically connected 
to the ECU 5 for supplying the same with an electric signal indicative of 
the flow rate of the mixture purged through the purging passage 10. 
A drain shut valve 38 is mounted across the negative pressure communication 
passage 9 connecting between the air inlet port 25 of the canister 26 and 
the intake pipe 2, and a second electromagnetic valve 39 is mounted across 
the negative pressure communication passage 9 at a location downstream of 
the drain shut valve 38, the drain shut valve 38 and the second 
electromagnetic valve 39 constituting a third control valve 40. 
The drain shut valve 38 has an air chamber 42 and a negative pressure 
chamber 43 defined by a diaphragm 41. Further, the air chamber 42 is 
formed of a first chamber 44 accommodating a valve element 44a, a second 
chamber 45 formed with an air introducing port 45a, and a narrowed 
communicating passage 47 connecting the second chamber 45 with the first 
chamber 44. The valve element 44a is connected via a rod 48 to the 
diaphragm 41. The negative pressure chamber 43 communicates with the 
second electromagnetic valve 39 via the communication passage 9, and has a 
spring 49 arranged therein for resiliently urging the diaphragm 41 and 
hence the valve element 44a in the direction indicated by an arrow A. 
The second electromagnetic valve 39 is constructed such that when a 
solenoid thereof is deenergized, a valve element thereof is in a seated 
position to allow air to be introduced into the negative pressure chamber 
43 via an air inlet port 50, and when the solenoid is energized, the valve 
element is in a lifted position in which the negative pressure chamber 43 
communicates with the intake pipe 2 via the communication passage 9. In 
addition, reference numeral 51 indicates a check valve. 
The ECU 5 comprises an input circuit having the functions of shaping the 
waveforms of input signals from various sensors, shifting the voltage 
levels of sensor output signals to a predetermined level, converting 
analog signals-from analog-output sensors to digital signals, and so 
forth, a central processing unit (hereinafter called "the CPU"), memory 
means storing programs executed by the CPU and for storing results of 
calculations therefrom, etc., and an output circuit which outputs driving 
signals to the fuel injection valves 6, the first and second 
electromagnetic valves 35, 39, and the purge control valve 36. 
An external diagnostic device 53 is disconnectibly connected to the ECU 5 
by means of a connection cord 52. 
As shown in detail in FIG. 2, the external diagnostic device 53 is 
comprised of a display section 56 formed, e.g. by a liquid-crystal display 
panel and having an insertion opening 55 into which a program card 54 such 
as a magnetic card, which stores setting data for setting values of a 
plurality of operating parameters related to operation of the engine, the 
vehicle, etc. for abnormality diagnoses, and a control section 57 formed 
of a keyboard, etc. The external diagnostic device 53 is disconnectibly 
connected to a connector 59 for diagnostic purposes by means of the 
connection cord 52 with connectors 58a and 58b secured to ends thereof, to 
supply command signals for commanding abnormality diagnoses including one 
for the evaporative emission control system 11. The external diagnostic 
device 53 is also adapted to supply signals indicative of values of 
operating parameters of the engine 1 which are inputted via the control 
section 57. The ECU 5 is responsive to the parameter signals from the 
external diagnostic device 53 to control the engine 1 into a predetermined 
operating condition corresponding to the parameter signals in the 
abnormality diagnostic operation for the evaporative emission control 
system 11. In FIG. 2, reference numeral 60 designates a service check 
signal (SCS) terminal. The SCS terminal 60 can be short-circuited to 
ground by a jumper wire or the like, and then a warning lamp, not shown, 
of a combination meter, not shown, will be lighted a predetermined number 
of times to enable to find out an abnormal location within the vehicle 
including the evaporative emission control system 11 as well as the engine 
1 and its related parts. More specifically, several predetermined time 
numbers are allotted to several predetermined locations within the 
vehicle, respectively. It is designed that when the SCS terminal is 
short-circuited, the warning lamp is lighted a predetermined number of 
times corresponding to the location which is determined to be abnormal, 
and then the operator will be able to locate the abnormal location. 
Next, a manner of determination of abnormality of the evaporative emission 
control system 11 according to the invention will be described in detail. 
FIG. 3 shows a manner of determining an abnormality in the evaporative 
emission control system 11, according to a first embodiment of the 
invention. 
First, at a step S1, the external diagnostic device 53 is connected to the 
ECU 5 by the connection cord 52. Then, to carry out the abnormality 
determination, the program card 54 storing setting data for setting a 
plurality of predetermined operating parameters for abnormality diagnosis 
of the system 11 is inserted into the insertion opening 55 of the external 
diagnostic deice 53, and then setting values of all the predetermined 
operating parameters are displayed on the display section 56, at a step 
S2. The predetermined operating parameters include engine coolant 
temperature TW, vehicle speed VSP, engine rotational speed NE, intake pipe 
absolute pressure PBA, and loads of the electric devices 100 including the 
air conditioner, for example, for each of which a predetermined value, 
range, or state is previously set as a setting condition for enabling the 
abnormality determination. The predetermined value, range, or state is 
displayed on the display section 56. Then, the operator carries out a 
condition-setting operation (e.g. switching over from an off state to an 
on state of the air conditioner) until all the setting conditions are set, 
i.e. all the abnormality determination-enabling conditions are satisfied, 
at a step S3. When all the setting conditions have been set, it is 
determined at a step S4 whether or not an abnormality diagnosis command 
signal has been issued from the external diagnostic device 53 to the ECU 
5. If the answer to this question is negative (NO), the process is 
immediately terminated, whereas if the answer is affirmative (YES), the 
ECU executes a monitoring (abnormality determination)-permission 
determining routine and then executes an abnormal determination routine, 
at a step S6, followed by terminating the process. 
FIG. 4 shows a manner of determining an abnormality in the evaporative 
emission control system 11, according to a second embodiment of the 
invention. 
First, at a step S11, similarly to the first embodiment, the external 
diagnostic device 53 is connected to the ECU 5 by the connection cord 52. 
Then, the external diagnostic device 53 executes the monitoring-permission 
determining routine based upon information on operating conditions from 
the ECU 5, at a step S12. If the monitoring is determined not to be 
permitted, all ones of the above-mentioned predetermined operating 
parameters, of which ranges, values, or states are not satisfied, are 
displayed on the display section 56, at a step S13. Then, the operator 
operates the control section 57 to set the setting conditions 
corresponding to the predetermined operating parameters which are not 
satisfied in range, value or state, until the latter are satisfied, at a 
step S14. Then, at a step S15, it is determined whether or not the 
monitoring has been permitted, that is, whether or not a flag FMON has 
been set to "1". If the answer to this question is negative (NO), the 
program returns to the step S12, whereas if the answer is affirmative 
(YES), the program proceeds to a step S16, where it is determined whether 
or not the abnormality diagnosis command signal has been issued from the 
external diagnostic device 53 to the ECU 5. If the answer to this question 
is negative (NO), the program is immediately terminated, whereas if the 
answer is affirmative (YES), the ECU 5 executes the abnormality 
determination routine at a step S17, followed by terminating the program. 
In the above described embodiments, the abnormality determination operation 
is executed irrespective of whether the SCS terminal 60 is 
short-circuited. However, the same operation may be executed when the SCS 
terminal 60 is short-circuited and at the same time the vehicle is under a 
failure detecting mode, whereby the abnormality determination of the 
evaporative emission control system 11 can be executed together with 
abnormality determination of other locations within the vehicle. 
FIG. 5 shows the monitoring (abnormality determination)-permission routine 
for determining whether or not monitoring of the system 11 for abnormality 
diagnosis thereof is permitted (the step S5 in FIG. 3 or the step S12 in 
FIG. 4). This routine is executed as a background processing. 
At a step S21, it is determined whether or not the engine coolant 
temperature TW detected by the TW sensor 15 falls between a predetermined 
lower limit value TWL (e.g. 50.degree. C.) and a predetermined upper limit 
value (e.g. 90.degree. C.). If the answer to this question is affirmative 
(YES), it is determined at a step S22 whether or not the intake air 
temperature TA detected by the TA sensor 14 falls between a predetermined 
lower limit value TAL (e.g. 70.degree. C.) and a predetermined higher 
limit value TAH (e.g. 90.degree. C.). If the answer to this question is 
affirmative (YES), it is determined that the engine 1 has been warmed up, 
and then the program proceeds to a step S23. 
At the step S23, it is determined whether or not the engine rotational 
speed NE detected by the NE sensor 16 falls between a predetermined lower 
limit value NEL (e.g. 2000 rpm) and a predetermined upper limit value NEH 
(e.g. 4000 rpm). If the answer to this question is affirmative (YES), it 
is determined at a step S24 whether or not the intake pipe absolute 
pressure PBA detected by the PBA sensor 13 falls between a predetermined 
lower limit value PBAL (e.g. a negative value of -350 mmHg) and a 
predetermined upper limit value PBAH (e.g. a negative value of -150 mmHg). 
If the answer to this question is affirmative (YES), it is determined at a 
step S25 whether or not the throttle valve opening .theta.TH detected by 
the .theta.TH sensor 4 falls between a predetermined lower limit value 
.theta.THL (e.g. 1.degree.) and a predetermined upper limit value 
.theta.THH (e.g. 5.degree.). If the answer to this question is affirmative 
(YES), it is determined at a step S26 whether or not the vehicle speed VSP 
detected by the VSP sensor 21 is lower than a predetermined low value VX 
(e.g. 2 km/hr). If the answer to this question is affirmative (YES), it is 
determined that the vehicle is substantially stationary or standing, and 
then the program proceeds to a step S27. At the step S27, it is determined 
whether or not the PT sensor 29, and the first to third control valves 28, 
36, and 39 are normally operating. If the answer to this question is 
affirmative (YES), the flag FMON is set to "1" at a step S28 for 
permitting monitoring of the system 11 for abnormality diagnosis, followed 
by terminating the program. On the other hand, if at least one of the 
answers to the questions of the steps S21 to S27 is negative (NO), the 
conditions for permitting monitoring are not satisfied, so that the flag 
FMON is set to "0" at a step S29, followed by terminating the program. 
Next, the manner of the abnormality determination carried out at the step 
S6 in FIG. 3 or at the step S16 in FIG. 4 will be described in detail with 
reference to FIG. 6. 
FIG. 6 shows patterns of operations of the first and second electromagnetic 
valves 35, 39 and the drain shut valve 38 and the purge control valve 36 
performed during an diagnosis of abnormality of the evaporative emission 
control system 11, and changes in the tank internal pressure PT occurring 
during the diagnosis. The operations of these valves are commanded by 
control signals from the ECU 5. 
First, during normal operation (normal purging) of the engine, as indicated 
by (i) in FIG. 6, the first electromagnetic valve 35 is energized and at 
the same time the second magnetic valve 32 is deenergized. When the 
ignition switch IGSW is closed and the engine is detected to be operating, 
by the IGSW sensor 18, the purge control valve 36 is energized to be 
opened. Then, evaporative fuel generated within the fuel tank 23 is 
allowed to flow through the evaporative fuel-guiding passage 27 into the 
canister 26 to be temporarily adsorbed by the adsorbent 24. Since the 
second electromagnetic valve 39 is deenergized as mentioned above, the 
drain shut valve 38 is open to allow fresh air to be introduced into the 
canister 26 through the air inlet port 45a so that evaporative fuel 
flowing into and stored in the canister 26 is purged together with fresh 
air through the second control valve 36 into the purging passage 10. On 
this occasion, if the fuel tank 23 is cooled due to ambient air, etc., 
negative pressure is developed within the fuel tank 23, which causes the 
negative pressure valve 33 of the two-way valve 34 to be opened so that 
part of the evaporative fuel in the canister 26 is returned through the 
two-way valve 34 into the fuel tank 23. 
When the predetermined monitoring (abnormality determination)-permission 
conditions, described before with reference to FIG. 5, are satisfied, the 
first and second electromagnetic valves 35, 39, and the purge control 
valve 36 are operated in the following manner to carry out an abnormality 
diagnosis of the evaporative emission control system 11. 
First, the tank internal pressure PT is relieved to the atmosphere, over a 
time period indicated by (ii) in FIG. 6. More specifically, the first 
electromagnetic valve 35 is held in the energized state to maintain 
communication between the fuel tank 23 and the canister 26, and at the 
same time the second electromagnetic valve 39 is held in the deenergized 
state to keep the drain shut valve 38 open. Further, the purge control 
valve 36 is held in the energized state or opened, to relieve the tank 
internal pressure PT to the atmosphere. 
Then, an amount of change in the tank internal pressure PT is measured over 
a time period indicated by (iii) in FIG. 6. 
More specifically, the second electromagnetic valve 39 is held in the 
deenergized state to keep the drain shut valve 38 open, and at the same 
time the purge control valve 36 is kept open. However, the first 
electromagnetic valve 35 is turned off into the deenergized state, to 
thereby measure an amount of change in the tank internal pressure PT 
occurring after the fuel tank 23 has ceased to be open to the atmosphere 
for the purpose of checking an amount of evaporative fuel generated in the 
fuel tank 23. 
Then, the evaporative emission control system 11 is negatively pressurized 
over a time period TR indicated by (iv) in FIG. 6. More specifically, the 
first electromagnetic valve 35 and the purge control valve 36 are held in 
the energized state, while the second electromagnetic valve 39 is turned 
on to close the drain shut valve 38, whereby the evaporative emission 
control system 11 is negatively pressurized by a gas drawing force 
developed by negative pressure in the purging passage 10 held in 
communication with the intake pipe 2. 
Then, a leak down check is carried out over a time period indicated by (v) 
in FIG. 6. 
More specifically, after the evaporative emission control system 11 is 
negatively pressurized to a predetermined degree, i.e. after the 
predetermined negatively-pressurized state of the system is established, 
the purge control valve 36 is closed, and then a change in the tank 
internal pressure PT occurring with the lapse of time thereafter is 
checked by the PT sensor 29. If the system 11 does not suffer from a 
significant leak of evaporative fuel therefrom, and hence the result of 
the leak down check shows that there is no substantial change in the tank 
internal pressure PT as indicated by the two-dot-chain line in the figure, 
it is determined that the evaporative emission control system 11 is 
normal, whereas if the system 11 suffers from a significant leak of 
evaporative fuel therefrom, and hence the result of the leak down check 
shows that there is a significant change in the tank internal pressure PT 
toward the atmospheric pressure, as indicated by the solid line, it is 
determined that the system 11 is abnormal. In this connection, if the 
evaporative emission control system 11 cannot be brought into the 
predetermined negatively pressurized state within a predetermined time 
period, the leak down check is inhibited, as hereinafter described. 
After determining whether or not the system 11 is abnormal, the system 11 
returns to the normal purging mode, as indicated by (vi) in FIG. 6. 
More specifically, while the first electromagnetic valve 35 is held in the 
energized state, the second electromagnetic valve 39 is deenergized and 
the purge control valve 36 is opened, to thereby perform normal purging of 
evaporative fuel. In this state, the tank internal pressure PT is relieved 
to the atmosphere and hence becomes substantially equal to the atmospheric 
pressure. 
Next, the manner of abnormality diagnosis of the evaporative emission 
control system 11 will be described. 
FIG. 7 shows a program for carrying out the abnormality diagnosis of the 
evaporative emission control system 11, which is executed by the CPU of 
the ECU 5. 
First, at a step S31, it is determined whether or not the monitoring 
(abnormality determination) is permitted, i.e. the flag FMON has been set 
to "1". If the answer to this question is negative (NO), the first to 
third control valves 28, 36, 40 are set to respective operative states for 
normal purging mode of the system as mentioned before, followed by 
terminating the program, whereas if the answer to this question is 
affirmative (YES), the tank internal pressure PT in the open-to-atmosphere 
condition of the system is checked at a step S32, and it is determined at 
a step S33 whether or not this check has been completed. If the answer to 
this question is negative (NO), the program is immediately terminated, 
whereas if it is affirmative (YES), the first electromagnetic valve 35 is 
turned off to check a change in the tank internal pressure PT at a step 
S34, followed by determining at a step S35 whether or not this check has 
been completed. If the answer to this question is negative (NO), the 
program is immediately terminated, whereas if it is affirmative (YES), the 
first to third control valves 28, 36, 40 are operated at a step S36 to 
bring about the negatively pressurized state of the evaporative emission 
control system 11 and the fuel tank 23. 
Simultaneously with the start of the negative pressurization at the step 
S36, a first timer tmPRG incorporated in the ECU 5 is started, and it is 
determined at a step S37 whether or not the count value thereof is larger 
than a value corresponding to a predetermined time period T1. The 
predetermined time period T1 is set to such a value as ensures that the 
system 11 is negatively pressurized to a predetermined pressure value, 
i.e. the negatively pressurized state of the system 11 is established 
within the predetermined time period T1, if the system is normal. If the 
answer to the question of the step S37 is affirmative (YES), it is 
determined that the system 11 cannot be negatively pressurized to the 
predetermined pressure value due to a hole formed in the fuel tank 23, 
etc., the program proceeds to a step S41. On the other hand, if the answer 
to the question of the step S37 is negative (NO), it is determined at a 
step S38 whether or not the negative pressurization has been completed, 
i.e. the negatively pressurized state of the system 11 is established. If 
the answer to this question is negative (NO), the program is immediately 
terminated, whereas if it is affirmative (YES), a leak down check routine, 
described in detail hereinafter, is carried out at a step S39 to check 
whether or not the system 11 is properly sealed, i.e. it is free from a 
leak of evaporative fuel therefrom in the normal operating mode thereof. 
Then, at a step S40, it is determined whether or not this check has been 
completed. 
If the answer to this question is negative (NO), the program is immediately 
terminated, whereas if the answer is affirmative (YES), the program 
proceeds to the step S41. 
At the step S41, a determination is made as to whether or not the system 11 
is in a normal condition, followed by determining at a step S42 whether 
the determination of the step S41 has been completed. If the answer to 
this question is negative (NO), the program is immediately terminated, 
whereas if it is affirmative (YES), the system 11 is set to the normal 
purging mode at a step S43, followed by terminating the program. 
Next, the above steps will be described in detail hereinbelow: 
(1) Check of Tank Internal Pressure in Open-to-Atmosphere Condition (at the 
step S32 in FIG. 7) 
FIG. 8 shows a routine for carrying out the tank internal pressure check in 
the open-to-atmosphere condition, which is executed as a background 
processing. 
First, at a step S51, the system 11 is set to the open-to-atmosphere mode, 
and at the same time, a second timer tmATMP is reset and started. More 
specifically, the first electromagnetic valve 35 is held in the energized 
state, and at the same time the second electromagnetic valve 39 is held in 
the deenergized state to keep the drain shut valve 38 open. Further, the 
purge control valve 36 is kept open. Thus, the tank internal pressure PT 
is relieved to the atmosphere (see the time period indicated by (ii) in 
FIG. 6). 
Then, at a step S52, it is determined whether or not the count value of the 
second timer tmATMP is larger than a value corresponding to a 
predetermined time period T2. The predetermined time period T2 is set to a 
predetermined value, e.g. 4 sec, which ensures that the pressure within 
the system 11 has been stabilized upon lapse thereof. If the answer to 
this question is negative (NO), the program is immediately terminated, 
while if it is affirmative (YES), the program proceeds to a step S53, 
where the tank internal pressure PATM in the open-to-atmosphere condition 
is detected by the PT sensor 29 and stored into the ECU 5, and then a 
check-over flag is set at a step S54, followed by terminating the program. 
(2) Check of A Change in Tank Internal Pressure (at the step S34 in FIG. 7) 
FIG. 9 shows a routine for checking a change in the tank internal pressure, 
which is executed as a background processing. 
First, at a step S61, the system 11 is set to a PT change-checking mode, 
and at the same time a third timer tmTP is reset and started. More 
specifically, while the purge control valve 36 and the drain shut valve 38 
are held open, the first electromagnetic valve 35 is turned off to thereby 
set the system to the PT change checking mode (see the time period 
indicated by (iii) in FIG. 6). 
Then, at a step S62, it is determined whether or not the count value of the 
third timer tmTP is larger than a value corresponding to a third 
predetermined time period T3, e.g. 10 sec. If the answer to this question 
is negative (NO), the program is immediately terminated, whereas if it is 
affirmative (YES), the tank internal pressure PCLS after the lapse of the 
predetermined time period T3 is detected and stored into the ECU 5 at a 
step S63, followed by calculation of a first rate of change PVARIA in the 
tank internal pressure at a step S64 by the use of the following equation 
(1): 
EQU PVARIA=(PCLS-PATM)/T3 (1) 
Then, the first rate of change PVARIA thus calculated is stored into the 
ECU 5 and a check-over flag is set at a step S65, followed by terminating 
the program. 
(3) Negative Pressurization (at the step S36 in FIG. 7) 
FIG. 10 shows a routine for carrying out a process of negatively 
pressurizing the system 11 to bring about the negatively pressurized state 
of the system, which is executed as a background processing. 
First, at a step S71, the system 11 is set to a negatively pressurizing 
mode. More specifically, the purge control valve 36 is kept open, and at 
the same time the first electromagnetic valve 35 is turned on, and the 
second electromagnetic valve 39 is turned on to close the drain shut valve 
38 (see the time period indicated by (iv) in FIG. 6). In this state, the 
system 11 is negatively pressurized to a predetermined value by a 
gas-drawing force created by operation of the engine 1. Then, it is 
determined at a step S72 whether or not the tank internal pressure PCHK in 
this mode of the system 11 is lower than a predetermined value P1 (e.g. 
-20 mmHg). If the answer to this question is negative (NO), the program is 
immediately terminated, whereas if it becomes affirmative (YES), a 
process-over flag is set at a step S73, followed by terminating the 
program. 
(4) Leak Down Check (at the step S39 in FIG. 7) 
FIG. 11 shows a routine for performing a leak down check of the system 11, 
which is executed as a background processing. 
First, at a step S81, the system 11 is set to a leak down check mode. More 
specifically, while the first electromagnetic valve 35 is held in the 
energized state, and at the same time the drain shut valve 38 is kept 
closed, the purge control valve 36 is closed to cut off the communication 
between the system 11 and the intake pipe 2 of the engine 1 (see the time 
period (v) in FIG. 6). 
Then, the program proceeds to a step S82, wherein it is determined whether 
or not the tank internal pressure PST at the start of the leak down check 
has been detected. In the first execution of this step S82, the answer to 
this question is negative (NO), so that the program proceeds to a step 
S83, wherein the tank internal pressure PST is detected and a fourth timer 
tmLEAK is reset and started. 
Then, it is determined at a step S84 whether or not the count value of the 
fourth timer tmLEAK is larger than a value corresponding to a fourth 
predetermined time period T4 (e.g. 10 sec). In the first execution of this 
step S84, the answer to this question is negative (NO), so that the 
program is immediately terminated. 
In the following loop, the answer to the question of the step S82 becomes 
affirmative (YES), so that the program jumps over to the step S84, wherein 
it is determined whether or not the count value of the fourth timer tmLEAK 
is larger than the value corresponding to the predetermined time period 
T4. If the answer to this question is negative (NO), the program is 
immediately terminated, whereas if it becomes affirmative (YES), the 
present tank internal pressure, i.e. the tank internal pressure PEND at 
the end of the leak down check is detected and stored into the memory 
means of the ECU 5 at a step S85, followed by calculation of a second rate 
of change PVARIB in the tank internal pressure PT at a step S86 by the use 
of the following equation (2): 
EQU PVARIB=(PEND-PST)/T4 (2) 
The second rate of change PVARIB in the tank internal pressure PT thus 
calculated is stored into the memory means of the ECU 5, and a check-over 
flag is set at a step S87, followed by terminating the program. 
(5) System Condition-Determining Process (at the step S41 in FIG. 7) 
FIG. 12 shows a routine for carrying out a process of determining a 
condition of the system 11, which is executed as a background processing. 
First, at a step S91, it is determined whether or not the count value of 
the first timer tmPRG exceeded the value corresponding to the 
predetermined value T1 during the negatively-pressurizing process. If the 
answer to this question is affirmative (YES), it is determined that the 
system 11 may suffer from a significant leak of evaporative fuel due to a 
hole formed in the fuel tank 23, etc., so that the program proceeds to a 
step S92, where it is determined whether or not the first rate of change 
PVARIA in the tank internal pressure PT is smaller than a predetermined 
value P2. If the answer to this question is affirmative (YES), which means 
that evaporative fuel was not generated in a large amount in the fuel tank 
23 so that the rate of rise in the tank internal pressure PT was low 
during the check of a change in the tank internal pressure PT at (iii) in 
FIG. 6, it is determined that the system 11 suffers from a significant 
leak of evaporative fuel from the fuel tank 23, piping connections, etc., 
determining that the evaporative emission control system 11 is abnormal 
(step S93), and then a process-over flag is set at a step S98, followed by 
terminating the program. On the other hand, if the answer to the question 
of the step S92 is negative (NO), which means that evaporative fuel was 
generated in a large amount in the fuel tank 23 to increase the tank 
internal pressure PT, which prevented the system 11 from being negatively 
pressurized in a proper manner in the negatively-pressurizing process, the 
determination of the system condition is suspended at a step S94, and then 
the process-over flag is set at the step S98, followed by terminating the 
program. 
On the other hand, if the answer to the question of the step S91 is 
negative (NO), i.e. if the system 11 was negatively pressurized to the 
predetermined value within the predetermined time period tmPRG, an 
abnormality-determining routine is carried out at a step S95, wherein it 
is determined whether or not the difference between the second rate of 
change PVARIB and the first rate of change PVARIA is larger than a 
predetermined value P3, in order to determine whether the value of the 
second rate of change PVARIB is due to a leak from the evaporative 
emission control system 11 or due to the amount of evaporative fuel 
generated within the fuel tank 23. The predetermined value P3 is set 
depending upon the negatively pressuring time period TR as shown in FIG. 
13. More specifically, it is set to a value P31 when the time period TR is 
longer than a predetermined value TR1, while it is set to a value P32 
(&gt;P31) when the former is shorter than the latter. If the answer to the 
question of the step S95 is negative (NO), it is determined that the 
system 11 is normal, followed by terminating the program, whereas if the 
answer is affirmative (YES), it is determined at a step S97 that the 
second rate of change PVARIB assumes a large value because there has been 
occurring a large leak amount from the system 11, and hence it is 
determined that the system 11 is abnormal, followed by terminating amount. 
(7) Normal Purging (at the step S43 in FIG. 7) 
FIG. 14 shows a routine for restoring the normal purging mode of the system 
11, in which the operative states of the valves are specified. 
More specifically, the first electromagnetic valve 35 is held in the 
energized state and the drain shut valve 39 and the purge control valve 36 
are opened to thereby set the system to the normal purging mode, at a step 
S111, followed by terminating the program.