Diagnostic system of an air-fuel ratio control device

A diagnostic system of an air-fuel ratio control device including an O.sub.2 sensor and an air bleed control valve actuated in response to a signal output by the O.sub.2 sensor. When the electric control current fed into the air bleed control valve reaches an upper limit or lower limit and remains at the upper limit or lower limit for a fixed time, the diagnostic system determines that the air-fuel ratio control device has malfunctioned. When the temperature of the engine cooling water is high, a diagnosis is prohibited, to prevent an incorrect diagnosis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a diagnostic system of an air-fuel ratio 
control device for use in an automobile engine. 
2. Description of the Related Art 
In an engine equipped with an air-fuel ratio control device for maintaining 
the air-fuel ratio of air-fuel mixture fed into the engine cylinders at 
the stoichiometric air-fuel ratio, if the air-fuel ratio control device 
malfunctions, the air-fuel mixture becomes lean or rich. In this case, if 
the air-fuel mixture becomes considerably lean, the output power of the 
engine is reduced and the driver becomes aware that a malfunction has 
occurred. However, if the air-fuel mixture becomes slightly lean or 
excessively rich due to the malfunction, the driver is not aware of a 
malfunction and continues to operate the engine. This results in a problem 
in that a large amount of harmful components such as CO, HC and NO.sub.X 
will be discharged from the engine. To eliminate this problem, the present 
applicant has already proposed a diagnostic system which determines 
whether or not the air-fuel ratio control device has malfunctioned by 
determining whether the air-fuel fuel mixture should become lean or rich 
on the basis of a feedback control signal (Japanese Patent Application No. 
61-243217). 
However, in an engine equipped with a carburetor, if the temperature of the 
carburetor becomes high, percolation takes place, and as a result, since 
fuel is forced into the intake passage from the carburetor, the air-fuel 
mixture becomes excessively rich. Consequently, in this case, if an 
attempt is made to determine whether or not the air-fuel ratio control 
device has malfunctioned by using the above-mentioned diagnostic system, 
although the air-fuel ratio control device has not malfunctioned, the 
determination result shows that it has malfunctioned. Namely, a problem of 
incorrect diagnosis may arise. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a diagnostic system 
capable of preventing incorrect diagnosis. 
According to the present invention, there is provided a diagnostic system 
of an air-fuel ratio control device for use in an internal combustion 
engine, the diagnostic system including: detecting means for detecting a 
temperature of the engine; diagnostic means for determining whether or not 
the air-fuel ratio control device has malfunctioned; and control means for 
controlling the diagnostic means in response to an output signal of the 
detecting means to prohibit a determination by the diagnostic means when 
the temperature of the engine exceeds a predetermined temperature. 
The present invention may be more fully understood from the description of 
preferred embodiments of the invention set forth below, together with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, reference numeral 1 designates an engine body, 2 an 
intake manifold, 3 a variable venturi type carburetor, and 4 an exhaust 
manifold. The variable venturi type carburetor 3 incudes an intake passage 
5, a suction piston 6, a fuel passage 7 which is open to the intake 
passage 5, and a throttle valve 8. The amount of fuel fed into the intake 
passage 5 from the fuel passage 7 is controlled by a needle 9 mounted on 
the suction piston 6. An air bleed passage 10 is connected to the fuel 
passage 7, and an air bleed control valve 11 is arranged in the air bleed 
passage 10. This air bleed control valve 11 is controlled on the bass of 
electric control current output from an electronic control unit 30. When 
the electric control current fed into the air bleed control valve 11 is 
increased, the amount of air fed into the fuel passage 7 from the air 
bleed passage 10 is increased, and thus the air-fuel mixture fed into the 
engine cylinders becomes lean. Conversely, when the control current fed 
into the air bleed control valve 11 is reduced, the amount of air fed into 
the fuel passage 7 from the air bleed passage 10 is reduced, and thus the 
air-fuel mixture fed into the engine cylinders becomes rich. 
The electronic control unit 30 is constructed as a digital computer and 
includes a ROM (read only memory) 32, a RAM (random access memory) 33, a 
CPU (microprocessor, etc.) 34, an input port 35, and an output port 36. 
The ROM 32, the RAM 33, the CPU 34, the input port 35, and the output port 
36 are interconnected via a bidirectional bus 31. A throttle sensor 12 
producing an output voltage which is proportional to the opening degree of 
the throttle valve 8 is attached to the throttle valve 8, an the output 
voltage of the throttle sensor 12 is input to the input port 35 via an AD 
converter 37. An O.sub.2 sensor 13 is arranged in the exhaust manifold 4, 
and the output signal of the O.sub.2 sensor 13 is input to the input port 
35 via an AD converter 38. A vacuum pressure sensor 14 producing an output 
voltage which is proportional to the level of vacuum in the intake 
manifold 2 is attached to the intake manifold 2, and the output voltage of 
the vacuum pressure sensor 14 is input to the input port 35 via an AD 
converter 39. A temperature sensor 15 producing an output voltage which is 
proportional to the temperature of cooling water of the engine is attached 
to the engine body 1, and the output voltage of the temperature sensor 15 
is input to the input port 35 via an AD converter 40. In addition, an 
engine speed sensor 20 producing output pulses having a frequency 
proportional to the engine speed is connected to the input port 35, and a 
starter switch 21 for controlling the operation of the starter motor (not 
shown) is connected to the input port 35. The output port 36 is connected, 
on one hand, to the air bleed control valve 11 via a drive circuit 41 and, 
on the other hand, to a warning lamp 22 via a drive circuit 42. 
FIG. 3 illustrates changes in the output voltage V of the O.sub.2 sensor 
13. The O.sub.2 sensor 13 produces the output voltage V of about 0.9 volt 
when the air-fuel mixture is rich, and produces the output voltage V of 
about 0.1 volt when the air-fuel mixture is lean. The output voltage V of 
the O.sub.2 sensor 13 is compared with a reference voltage Vr of about 
0.45 volt in the CPU 34. At this time, if the output voltage V of the 
O.sub.2 sensor 13 is higher than Vr, the air-fuel mixture is considered 
rich, and if the outputvoltage V of the O.sub.2 sensor 13 is lower than 
Vr, the air-fuel mixture is considered lean. 
FIG. 2 illustrates a routine for the calculation of the electric control 
current I of the air bleed control valve 11, which calculation is carried 
out on the basis of a determination of whether the air-fuel mixture is 
rich or lean. 
Referring to FIG. 2, in step 50, it is determined whether or not the 
air-fuel mixture is lean. When the air-fuel mixture is lean, the routine 
goes to step 51, and it is determined whether the air-fuel mixture has 
been changed from rich to lean after completion of the preceding 
processing cycle. When the air-fuel mixture has been changed from rich to 
lean, the routine goes to step 52, and a skip value A is subtracted from 
I. Then, the routine goes to step 53. When the air-fuel mixture has not 
been changed from rich to lean after completion of the preceding 
processing cycle, the routine goes to step 54, and an integration value 
K(K&lt;&lt;A) is subtracted from I. Then, the routine goes to step 53. 
When it is determined in step 50 that the air-fuel mixture is rich, the 
routine goes to step 55, and it is determined whether the air-fuel mixture 
has been changed from lean to rich after completion of the preceding 
processing cycle. When the air-fuel mixture has been changed from lean to 
rich, the routine goes to step 56, and the skip value A is added to I. 
Then, the routine goes to step 53. When the air-fuel mixture has not been 
changed from lean to rich after completion of the preceding processing 
cycle, the routine goes to step 57, and the integration value K is added 
to I. Then, the routine goes to step 53. In step 53, I is output to the 
output port 36. 
Consequently, as illustrated in FIG. 3, when the air-fuel mixture is 
changed from rich to lean, the value of I is abruptly reduced by the skip 
value A and then gradually reduced. Conversely, when the air-fuel mixture 
is changed from lean to rich, the value of I is abruptly increased by the 
skip value A and then gradually increased. The value of I calculated in 
each step 52, 54, 56, 57 and output to the output port 36 in step 53 in 
FIG. 2 represents a duty cycle of pulse, and the signal pulses which are 
produced at a fixed frequency and have a pulse width changed in accordance 
with the duty cycle are fed into the air bleed control valve 11. The 
opening degree of the air bleed control valve 11 is controlled in response 
to the mean value of electric current of the serial pulse and, therefore, 
I is used as the control electric current of the air bleed control valve 
11. The range of the control current I which is able to control an 
air-fuel ratio is between the minimum value MIN and the maximum value MAX 
in FIG. 3, and the control current I normally moves up and down between 
MIN and MAX while the feedback control is carried out. However, if the 
air-fuel mixture remains excessively rich the control electric current I 
is increased and reaches MAX and, if the air-fuel mixture remains 
excessively lean, the control electric current I is reduced and reaches 
MIN. Consequently, it is possible to determine whether or not the air-fuel 
ratio control device is in an abnormal state on the basis of a 
determination of whether the control electric current I becomes equal to 
MAX or MIN. 
FIGS. 4 and 5 are a flow chart for executing a diagnosis of the air-fuel 
ratio control device. The routine illustrated in FIGS. 4 and 5 is 
processed by sequential interruptions which are executed at predetermined 
intervals. 
Referring to FIGS. 4 and 5, in step 60, it is determined whether or not the 
starter switch 21 is ON. When the starter switch 21 is ON, the routine 
jumps to step 61. Conversely, when the starter switch 21 is OFF, the 
routine goes to step 62, and it is determined whether or not the starter 
switch 21 has been made OFF after completion of the preceding processing 
cycle. If the starter switch 21 has been made OFF after completion of the 
preceding processing cycle, the routine goes to step 63, and the timer is 
set, that is, the time is operated. Then, the routine goes to step 61. If 
the starter switch 21 has not been made OFF after completion of the 
preceding processing cycle, the routine goes to step 64, and it is 
determined whether or not a fixed time has elapsed after the timer was 
set. If the fixed time has not elapsed, that is, if the fixed time has not 
elapsed after the engine is started, the routine goes to step 61. 
Conversely, if the fixed time has elapsed after the engine is started, the 
routine goes to step 65. 
In step 61, it is determined whether or not the temperature T of the engine 
cooling water is higher than a predetermined temperature, for example, 
70.degree. C., on the basis of the output signal of the temperature sensor 
15. If T&gt;70.degree. C., the processing cycle is completed. Consequently, 
at this time, a diagnosis is not made. Namely, if the temperature T of the 
engine cooling water is higher than 70.degree. C. when the engine is 
started or immediately after the engine is started, the temperature of the 
carburetor 3 is considered to be very high. Consequently, at this time, 
since percolation may be generated, the diagnosis is prohibited to prevent 
an incorrect diagnosis. Conversely, if T.ltoreq.70.degree. C., the routine 
goes to step 65. Consequently, when the temperature T of the engine 
cooling water is lower than 70.degree. C. before the fixed time has 
elapsed after the engine is started, or after the fixed time has elapsed 
after the engine is started, the routine goes to step 65. 
In steps 65, 66, 67, 68, it is determined whether or not the engine is 
operating in a state wherein a diagnosis should be made, and when the 
diagnosis is made in step 69 and 70 and it is determined that the air-fuel 
ratio control device has malfunctioned, the process goes to step 71 and 
the warning lamp 22 is lit. 
Namely, in step 65, it is determined whether or not the temperature T of 
the engine cooling water is higher than 60.degree. C. and lower than 
95.degree. C. If T.ltoreq.60.degree. C., or if T.gtoreq.95.degree. C., the 
processing cycle is completed. Consequently, at this time, a diagnosis is 
not made. When T.ltoreq.60.degree. C., the air-fuel mixture is sometimes 
made rich due to the operation of the choke valve. Consequently, at this 
time, the diagnosis is prohibited to prevent an incorrect diagnosis. 
Consequently, where the fixed time has not elapsed after the engine is 
started, the diagnosis is made only when 60.degree. C.&lt;T&lt;70.degree. C. 
Conversely, where the fixed time has elapsed after the engine is started, 
the diagnosis is made only when 60.degree. C.&lt;T&lt;95.degree. C. When the 
fixed time has elapsed after the engine is started, if T.gtoreq.95.degree. 
C., percolation may occur in the carburetor 3. Consequently, in this case, 
the diagnosis is prohibited to prevent an incorrect diagnosis. As 
mentioned above, the cooling water temperature 95.degree. C. at which the 
percolation will occur when the fixed time has elapsed after the engine is 
started is higher than the cooling water temperature 70.degree. C. at 
which the percolation will occur immediately after the engine is started. 
This is because, if a vehicle is driven after the fixed time has elapsed 
after the engine is started, the carburetor 3 is cooled by air flowing 
into the engine compartment and thus the temperature of the carburetor 3 
will be reduced. 
In step 66, it is determined whether or not the opening degree .theta. of 
the throttle valve 8 exceeds 10.degree., on the basis of the output signal 
of the throttle sensor 12 and, in step 67, it is determined whether or not 
the level of vacuum P in the intake manifold 2 is larger than -80 mmHg and 
smaller than -350 mmHg, on the basis of the output signal of the vacuum 
pressure sensor 14. In addition, in step 68, it is determined whether or 
not the engine speed N is higher than 1500 r.p.m. and lower than 3000 
r.p.m., on the basis of the output signal of the engine speed sensor 20. 
As will be understood from steps 66, 67, and 68, when the amount of air 
fed into the engine cylinders is small, and thus the sensitivity of the 
air bleed operation is low, and when the engine is operating at a high 
speed and thus an air-fuel ratio necessary to ensure an output of a high 
power is required, the diagnosis is prohibited to prevent an incorrect 
diagnosis. 
In step 69, it is determined whether or not the control electric current I 
is larger than MIN and smaller than MAX. If I.ltoreq.MIN, or if 
I.gtoreq.MAX, the routine goes to step 70, and it is determined whether or 
not the state of I.ltoreq.MIN, or if I.gtoreq.MAX has continued for more 
than 10 sec. If this state has continued for more than 10 sec, it is 
determined that the air-fuel ratio control device has malfunctioned. At 
this time, the routine goes to step 71, and the warning lamp 22 is lit. 
As mentioned above, in this embodiment, when the engine is operating in a 
state wherein percolation is likely to occur in the carburetor, since the 
diagnosis is prohibited, it is possible to prevent an incorrect diagnosis 
from being made. 
FIG. 6 illustrates an alternative embodiment of the air-fuel ratio control 
device. In this embodiment, an air supply passage 23 is connected to the 
intake passage 5 downstream of the throttle valve 8, and an air control 
valve 24 is arranged in the air supply passage 23. This air control valve 
24 is controlled on the basis of a control electric current output from 
the electronic control unit 30 (FIG. 1). When the control electric current 
fed into the air control valve 24 is increased, the amount of air fed into 
the intake passage 5 from the air supply passage 23 is increased, and thus 
the air-fuel mixture fed into the engine cylinders becomes lean. 
Conversely, when the control electric current fed into the air control 
valve 24 is reduced, the amount of air fed into the intake passage 5 from 
the air supply passage 23 is reduced, and thus the air-fuel mixture fed 
into the engine cylinders becomes rich. 
Also in this embodiment, the range of the control current I which is able 
to control an air-fuel ratio is between the minimum value MIN and the 
maximum value MAX in FIG. 3, and the control current I normally moves up 
and down between MIN and MAX while the feedback control is carried out. 
However, if the air-fuel mixture remains excessively rich, the control 
electric current I is increased and reaches MAX and, if the air-fuel 
mixture remains excessively lean, the control electric current I is 
reduced and reaches MIN. Consequently, it is possible to determine whether 
or not the air-fuel ratio control device is in an abnormal state on the 
basis of a determination of whether the control electric current I has 
become equal to MAX or MIN. 
In the embodiments illustrated in FIGS. 1 and 6, when the cooling water 
temperature is high, the diagnosis is prohibited. Namely, since when the 
cooling water temperature is reduced, the temperature of the carburetor 3 
is accordingly reduced, the diagnosis is started when the cooling water 
temperature becomes lower than a temperature at which percolation will not 
occur in the carburetor 3. However, since the carburetor is not directly 
cooled by the cooling water, but cooled only by the wind flowing into the 
engine compartment, once the temperature of the carburetor becomes 
extremely high, it is not always rapidly reduced even though the 
temperature of the engine cooling water has rapidly fallen. Consequently, 
in certain engines, even if the temperature of the engine cooling water 
becomes low, percolation may occur. In this case, if the diagnosis is 
started when the temperature of the engine cooling water becomes low, an 
incorrect diagnosis may be made. 
FIGS. 7 through 12 illustrate three different embodiments of the present 
invention, all of which are capable of further preventing an incorrect 
diagnosis. 
FIGS. 7 and 8 are a flow chart of another embodiment for executing a 
diagnosis of the air-fuel ratio control device. The routine illustrated in 
FIGS. 7 and 8 is processed by sequential interruptions which are executed 
at predetermined intervals. 
Referring to FIGS. 7 and 8, in step 80, it is determined whether or not the 
starter switch 21 is ON. When the starter switch 21 is ON, the routine 
goes to step 81, and a flag is reset. Then, the routine goes to step 82. 
As hereinafter described, this flag is set when the temperature of the 
cooling water of the engine is higher than a predetermined temperature. 
Conversely, when the starter switch 21 is made OFF, the routine goes to 
step 83 from step 80, and it is determined whether the flag is set. If the 
flag is set, the processing routine is completed. Conversely, if the flag 
is reset, the routine goes to step 84, and it is determined whether or not 
the starter switch 21 has been made OFF after completion of the preceding 
processing cycle. If the starter switch 21 has been made OFF after 
completion of the preceding processing cycle, the routine goes to step 85, 
and the timer is set, that is, the timer is operated. Then, the routine 
goes to step 82. If the starter switch 21 has not been made OFF after 
completion of the preceding processing cycle, the routine goes to step 86, 
and it is determined whether or not a fixed time has elapsed after the 
timer is set. If the fixed time has not elapsed, that is, if the fixed 
time has not elapsed after the engine is started, the routine goes to step 
82. Conversely, if the fixed time has elapsed after the engine is started, 
the routine goes to step 87. 
In step 82, it is determined whether or not the temperature T of the engine 
cooling water is higher than a predetermined temperature, for example, 
70.degree. C., on the basis of the output signal of the temperature sensor 
15. If T&gt;70.degree. C., the routine goes to step 88, and the flag is set. 
Then, the processing cycle is completed. Once the flag is set, the flag 
remains set as long as the engine is operated. As can be seen from FIGS. 7 
and 8, when the flag is set, the processing cycle is instantaneously 
completed via step 83. Consequently, once the flag is set, a diagnosis is 
not made as long as the engine is operated. That is, if the temperature T 
of the engine cooling water is higher than 70.degree. C. when the engine 
is started or immediately after the engine is started, the temperature of 
the carburetor 3 is considered to be very high. Consequently, at this 
time, percolation may be generated. In addition, even if the temperature T 
of the engine cooling water becomes low thereafter, the temperature of the 
carburetor 3 does not always becomes low in accordance with a reduction in 
the temperature T of the engine cooling water. Consequently, even if the 
temperature T of the engine cooling water becomes lower than 70.degree. 
C., percolation may be generated. Consequently, when the temperature T of 
the engine cooling water is higher than 70.degree. C. when the engine is 
started or immediately after the engine is started, a diagnosis is 
thereafter prohibited to prevent an incorrect diagnosis due to the 
generation of percolation. When it is determined in step 82 that 
T.ltoreq.70.degree. C., the routine goes to step 89 and the flag is not 
set. 
In step 86, if it is determined that the fixed time has elapsed after the 
engine is started, the routine goes to step 87, and it is determined 
whether or not the temperature T of the engine cooling water is higher 
than a predetermined temperature, for example, 95.degree. C. If 
T&gt;95.degree. C., the routine goes to step 90, and the flag is set. 
Consequently, a diagnosis is prohibited as long as the engine is operated 
thereafter. That is, if the temperature T of the engine cooling water is 
higher than 95.degree. C. when the fixed time has elapsed after the engine 
is started, the temperature of the carburetor 3 is considered to be very 
high. Consequently, at this time, percolation may be generated. In 
addition, even if the temperature T of the engine cooling water becomes 
low thereafter, the temperature of the carburetor 3 does not always become 
low in accordance with a reduction in the temperature T of the engine 
cooling water. Consequently, even if the temperature T of the engine 
cooling water becomes lower than 95.degree. C., percolation may be 
generated. Consequently, when the temperature T of the engine cooling 
water is higher than 95.degree. C. after the fixed time has elapsed after 
the engine is started, the diagnosis is thereafter prohibited to prevent 
an incorrect diagnosis due to the generation of percolation. 
As mentioned above, the cooling water temperature 95.degree. C. at which 
the percolation will occur when the fixed time has elapsed after the 
engine is started is higher than the cooling water temperature 70.degree. 
C. at which the percolation will occur immediately after the engine is 
started. This is because, as already mentioned, if a vehicle is driven 
when the fixed time has elapsed after the engine is started, the 
carburetor 3 is cooled by air flowing into the engine compartment, and 
thus the temperature of the carburetor 3 will be reduced. 
When it is determined is step 87 that T.ltoreq.95.degree. C., the routine 
goes to step 89. Consequently, when the temperature T of the engine 
cooling water does not exceed 70.degree. C. before the fixed time has 
elapsed after the engine is started, or when the temperature T of the 
engine cooling water does not exceed 95.degree. C. after the fixed time 
has elapsed after the engine is started, the routine goes to step 89. 
In steps 89, 91, 92, and 93, it is determined whether or not the engine is 
operating in a state wherein a diagnosis should be made, and when the 
diagnosis is made in steps 94 and 95 and it is determined that the 
air-fuel ratio control device has malfunctioned, the process goes to step 
96 and the warning lamp 22 is lit. 
That is, in step 89, it is determined whether or not the temperature T of 
the engine cooling water is lower than 60.degree. C. If 
T.ltoreq.60.degree. C., the processing cycle is completed. Consequently, 
at this time, a diagnosis is not made. When T.ltoreq.60.degree. C., the 
air-fuel mixture may remain rich due to the operation of the choke valve. 
Consequently, at this time, the diagnosis is stopped to prevent an 
incorrect diagnosis. Consequently, where the fixed time has not elapsed 
after the engine is started, the diagnosis is made only when 60.degree. 
C.&lt;T&lt;70.degree. C. In addition, once the temperature T of the engine 
cooling water exceeds 70.degree. C., the diagnosis is not made. 
Conversely, where the fixed time has elapsed after the engine is started, 
the diagnosis is made only when 60.degree. C.&lt;T&lt;95.degree. C. In addition, 
once the temperature T of the engine cooling water exceeds 95.degree. C., 
the diagnosis is not made. 
In step 91, it is determined whether or not the opening degree 8 of the 
throttle valve 8 is smaller than 10.degree., on the basis of the output 
signal of the throttle sensor 12 and, in step 92, it is determined whether 
or not the level of vacuum P in the intake manifold 2 is larger than -80 
mmHg and smaller than -350 mmHg, on the basis of the output signal of the 
vacuum pressure sensor 14. In addition, in step 93, it is determined 
whether or not the engine speed N is higher than 1500 r.p.m. and lower 
than 3000 r.p.m., on the basis of the output signal of the engine speed 
sensor 20. 
Then, in step 94, it is determined whether or not the control electric 
current I is larger than MIN and smaller than MAX. If I.ltoreq.MIN, or if 
I.gtoreq.MAX, the routine goes to step 95, and it is determined whether or 
not the state of I.ltoreq.MIN or I.gtoreq.MAX has continued for more than 
10 sec. If the state has continued for more than 10 sec, it is determined 
that the air-fuel ratio control device has malfunctioned. At this time, 
the routine goes to step 96, and the warning lamp 22 is lit. 
As mentioned above, in this embodiment, when the engine is operating in a 
state where percolation may occur in the carburetor even if the 
temperature of the engine cooling water becomes low, since the diagnosis 
is prohibited, it is possible to prevent an incorrect diagnosis from being 
made. 
FIGS. 9 and 10 are a flow chart of a further embodiment for executing a 
diagnosis of the air-fuel ratio control device. The routine illustrated in 
FIGS. 9 and 10 is processed by sequential interruptions which are executed 
at predetermined intervals. 
Referring to FIGS. 9 and 10, in step 100, it is determined whether or not 
the starter switch 21 is ON. When the starter switch 21 is ON, the routine 
goes to step 101, and flags A and B are reset. Then, the routine goes to 
step 102. As hereinafter described, the flag A is set when the temperature 
of the engine cooling water is higher than a predetermined temperature, 
and the flag B is set when the temperature of the engine cooling water is 
higher than a predetermined temperature before a fixed time has elapsed 
after the engine is started. 
When the starter switch 21 is made OFF, the routine goes to step 103 from 
step 100, and it is determined whether the flag A is set. If the flag A is 
reset, the routine goes to step 104, and it is determined whether or not 
the starter switch 21 has been made OFF after completion of the preceding 
processing cycle. If the starter switch 21 has been made OFF after 
completion of the preceding processing cycle, the routine goes to step 
105, and the timer is set, that is, the timer is operated. Then, the 
routine goes to step 102. If the starter switch 21 has not been made OFF 
after completion of the preceding processing cycle, the routine goes to 
step 106, and it is determined whether or not a fixed time has elapsed 
after the timer is set. If the fixed time has not elapsed, that is, if the 
fixed time has not elapsed after the engine is started, the routine goes 
to step 102. Conversely, if the fixed time has elapsed after the engine is 
started, the routine goes to step 107. 
In step 102, it is determined whether or not the temperature T of the 
engine cooling water is higher than a first predetermined temperature, for 
example, 70.degree. C., on the basis of the output signal of the 
temperature sensor 15. If T&gt;70.degree. C., the routine goes to step 108, 
and the flags A and B are set. Then, the processing cycle is completed. 
Once the flag A is set, the flag A remains set until the temperature T of 
the engine cooling water falls to a certain extent. During this time, the 
diagnosis is not made. That is, if the temperature T of the engine cooling 
water is higher than the 70.degree. C. when the engine is started or 
immediately after the engine is started, the temperature of the carburetor 
3 is considered to be very high. Consequently, at this time, since 
percolation may be generated, the diagnosis is prohibited to prevent an 
incorrect diagnosis. Conversely, if T.ltoreq.70.degree. C., the routine 
goes to step 109 from step 102 and the flags A and B are not set. 
In step 106, if it is determined that the fixed time has elapsed after the 
engine is started, the routine goes to step 107, and it is determined 
whether or not the temperature T of the engine cooling water is higher 
than another first predetermined temperature, for example, 95.degree. C.. 
If T&gt;95.degree. C., the routine goes to step 110, and the flag A is set. 
Consequently, the diagnosis is prohibited. Namely, if the temperature T of 
the engine cooling water is higher than 95.degree. C. when the fixed time 
has elapsed after the engine is started, percolation may occur in the 
carburetor 3. Consequently, in this case, the diagnosis is prohibited to 
prevent an incorrect diagnosis from being made. 
When it is determined in step 107 that T.ltoreq.95.degree. C., the routine 
goes to step 109. Consequently, when the temperature T of the engine 
cooling water does not exceed 70.degree. C. before the fixed time has 
elapsed after the engine is started, or when the temperature T of the 
engine cooling water does not exceed 95.degree. C. when the fixed time has 
elapsed after the engine is started, the routine goes to step 109 and the 
flags A and B are not set. 
In steps 109, 111, 112, and 113, it is determined whether or not the engine 
is operating in a state wherein a diagnosis should be made, and when the 
diagnosis is made in steps 114 and 115, and it is determined that the 
air-fuel ratio control device has malfunctioned, the process goes to step 
116 and the warning lamp 22 is lit. 
Namely, in step 109, it is determined whether or not the temperature T of 
the engine cooling water is lower than 60.degree. C. If T&lt;60.degree. C., 
the processing cycle is completed. Consequently, at this time, a diagnosis 
is not made. Consequently, where the fixed time has not elapsed after the 
engine is started, the diagnosis is made when 60.degree. C.&lt;T&lt;70.degree. 
C. Conversely, where the fixed time has elapsed after the engine is 
started, the diagnosis is made when 60.degree. C.&lt;T&lt;95.degree. C. 
In step 111, it is determined whether or not the opening degree .theta. of 
the throttle valve 8 is smaller than 10.degree. C., on the basis of the 
output signal of the throttle sensor 12 and, in step 112, it is determined 
whether or not the level of vacuum P in the intake manifold 2 is larger 
than -80 mmHg and smaller than -350 mmHg, on the basis of the output 
signal of the vacuum pressure sensor 14. In addition, in step 113, it is 
determined whether or not the engine speed N is higher than 1500 r.p.m. 
and lower than 3000 r.p.m., on the basis of the output signal of the 
engine speed sensor 20. Then, in step 114, it is determined whether or not 
the control electric current I is larger than MIN and smaller than MAX. If 
I.ltoreq.MIN, or if I.gtoreq.MAX, the routine goes to step 115, and it is 
determined whether or not the state of I.ltoreq.MIN or I.gtoreq.MAX has 
continued for more than 10 sec. If this state has continued for more than 
10 sec, it is determined that the air-fuel ratio control device has 
malfunctioned. At this time, the routine goes to step 116, and the warning 
lamp 22 is lit. 
If the flag A is set, the routine goes to step 117 from step 103, and it is 
determined whether or not the flag B is set. When the flag B is set, that 
is, when the temperature T of the engine cooling water exceeds 70.degree. 
C. before the fixed time has elapsed after the engine is started, the 
routine goes to step 118, and it is determined whether or not the 
temperature T of the engine cooling water is lower than a predetermined 
second temperature, for example, 65.degree. C. If T.gtoreq.65.degree. C., 
the processing routine is completed. Conversely, if T&lt;65.degree. C., the 
routine goes to step 119, and the flag B is reset. After this, in step 
120, the flag A is reset, and then the processing cycle is completed. 
Consequently, if the temperature T of the engine cooling water is lower 
than 65.degree. C., the routine goes to step 109 from 107 in the next 
processing cycle, and thus a diagnosis is made. 
Even if the temperature T of the engine cooling water becomes low, the 
temperature of the carburetor 3 does not always becomes low in accordance 
with a reduction in the temperature T of the engine cooling water. 
Consequently, even if the temperature T of the engine cooling water is 
lower than 70.degree. C., percolation may occur in the carburetor 3. 
However, it has been proven that, if the temperature T of the engine 
cooling water is reduced to a certain extent, that is, is made lower than 
65.degree. C., the temperature of the carburetor 3 is reduced to a 
temperature at which percolation will not occur. Consequently, when the 
temperature T of the engine cooling water becomes lower than 65.degree. 
C., the diagnosis is started. In other words, in this embodiment, by 
prohibiting the diagnosis when the temperature T of the engine cooling 
water is higher than the second predetermined temperature 65.degree. C., 
which is lower than the first predetermined temperature 70.degree. C., an 
incorrect diagnosis is prevented. 
If it is determined in step 117 that the flag B is reset, that is, when the 
temperature T of the engine cooling water exceeds 95.degree. C. when the 
fixed time has elapsed after the engine is started, the routine goes to 
step 121, and it is determined whether or not the temperature T of the 
engine cooling water is lower than a predetermined other second 
temperature, for example, 80.degree. C. If T.gtoreq.80.degree. C., the 
processing routine is completed. Conversely, if T&lt;80.degree. C., the 
routine goes to step 120, the flag A is reset, and the processing cycle is 
completed. Consequently, if the temperature T of the engine cooling water 
is lower than 80.degree. C., the routine goes to step 109 from 107 in the 
next processing cycle, and thus a diagnosis is made. 
As mentioned above, even if the temperature T of the engine cooling water 
becomes low, the temperature of the carburetor 3 does not always becomes 
low in accordance with a reduction in the temperature T of the engine 
cooling water. Consequently, even if the temperature T of the engine 
cooling water is lower than 95.degree. C., percolation may occur in the 
carburetor 3. However, it has been proven that, if the temperature T of 
the engine cooling water is reduced to a certain extent, that is, becomes 
lower than 80.degree. C., the temperature of the carburetor 3 is reduced 
to a temperature at which percolation will not occur. Consequently, when 
the temperature T of the engine cooling water becomes lower than 
80.degree. C., the diagnosis is started. In other words, in this 
embodiment, by prohibiting the diagnosis when the temperature T of the 
engine cooling water is higher than the other second predetermined 
temperature 80.degree. C., which is lower than the other first 
predetermined temperature 95.degree. C., an incorrect diagnosis is 
prevented. As mentioned above, the cooling water temperature 80.degree. C. 
at which percolation will occur when the fixed time has elapsed after the 
engine is started is higher than the cooling water temperature 65.degree. 
C. at which percolation will occur immediately after the engine is 
started. This is because, if a vehicle is driven when the fixed time has 
elapsed after the engine is started, the carburetor 3 is cooled by air 
flowing into the engine compartment, and thus the temperature of the 
carburetor 3 will be reduced. 
In this embodiment, an incorrect diagnosis is prevented by prohibiting the 
diagnosis when the temperature of the engine cooling water becomes higher 
than a predetermined first temperature and by starting the diagnosis when 
the temperature of the engine cooling water becomes lower than a 
predetermined second temperature, which is lower than the predetermined 
first temperature. 
FIGS. 11 and 12 are a flow chart of a still further embodiment for 
executing a diagnosis of the air-fuel ratio control device. The routine 
illustrated in FIGS. 11 and 12 is processed by sequential interruptions 
which are executed at predetermined intervals. 
Referring to FIGS. 11 and 12, in step 130, it is determined whether or not 
the starter switch 21 is ON. When the starter switch 21 is ON, the routine 
goes to step 131, and a flag is reset. The routine then goes to step 132. 
As hereinafter described, this flag is set when the temperature of the 
cooling water of the engine is higher than a predetermined temperature. 
Conversely, when the starter switch 21 is made OFF, the routine goes to 
step 133 from step 130, and it is determined whether or not the starter 
switch 21 has been made OFF after completion of the preceding processing 
cycle. If the starter switch 21 has been made OFF after completion of the 
preceding processing cycle, the routine goes to step 134, and the timer I 
is set, that is, the timer I is operated. Then, the routine goes to step 
132. If the starter switch 21 has not been made OFF after completion of 
the preceding processing cycle, the routine goes to step 135, and it is 
determined whether or not a fixed time determined by the timer I has 
elapsed after the timer I is set. If the fixed time has not elapsed, that 
is, if the fixed time has not elapsed after the engine is started, the 
routine goes to step 132. Conversely, if the fixed time has elapsed after 
the engine is started, the routine goes to step 136. 
In step 132, it is determined whether or not the temperature T of the 
engine cooling water is higher than a predetermined temperature, for 
example, 70.degree. C., on the basis of the output signal of the 
temperature sensor 15. If T&gt;70.degree. C., the routine goes to step 137, 
and the flag is set. Then, the processing cycle is completed. 
As hereinafter described, during the time that the flag is set, a diagnosis 
is prohibited, and when the flag is reset, the diagnosis is started. If 
the temperature T of the engine cooling water is higher than 70.degree. C. 
when the engine is started or immediately after the engine is started, the 
temperature of the carburetor 3 is considered to be very high. 
Consequently, at this time, since percolation may be generated, the 
diagnosis is prohibited to prevent an incorrect diagnosis. 
In step 135, if it is determined that the fixed time has elapsed after the 
engine is started, the routine goes to step 136, and it is determined 
whether or not the temperature T of the engine cooling water is higher 
than a predetermined temperature, for example, 95.degree. C.. If 
T&gt;95.degree. C., the routine goes to step 138, and the flag is set. 
Consequently, the diagnosis is prohibited. If the temperature T of the 
engine cooling water is higher than 95.degree. C. when the fixed time has 
elapsed after the engine is started, percolation may occur in the 
carburetor 3. Consequently, in this case, the diagnosis is prohibited to 
prevent an incorrect diagnosis. 
If it is determined in step 132 that T.ltoreq.70.degree. C., that is, when 
the temperature T of the engine cooling water does not exceed 70.degree. 
C. before the fixed time has elapsed after the engine is started, the 
routine goes to step 139. In addition, if it is determined in step 136 
that T.ltoreq.95.degree. C., that is, when the temperature T of the engine 
cooling water does not exceed 95.degree. C. when the fixed time has 
elapsed after the engine is started, the routine goes to step 139. In step 
139, it is determined whether or not the flag is set. If the flag is 
reset, that is, when the temperature T of the engine cooling water does 
not exceed 70.degree. C. before the fixed time has elapsed, or when the 
temperature T of the engine cooling water does not exceed 95.degree. C. 
when the fixed time has elapsed after the engine is started, the routine 
goes to step 140. 
In steps 140, 141, 142, 143, it is determined whether or not the engine is 
operating in a state wherein the diagnosis should be made, and when the 
diagnosis is made in steps 144, 145 and it is determined that the air-fuel 
ratio control device has malfunctioned, the process goes to step 146 and 
the warning lamp 22 is lit. 
Namely, in step 140, it is determined whether or not the temperature T of 
the engine cooling water is lower than 60.degree. C. If T&lt;60.degree. C., 
the processing cycle is completed. Consequently, at this time, a diagnosis 
is not made. Consequently, when the fixed time has not elapsed after the 
engine is started, the diagnosis is made when 60.degree. C.&lt;T&lt;70.degree. 
C. Conversely, when the fixed time has elapsed after the engine is 
started, the diagnosis is made when 60.degree. C.&lt;T&lt;95.degree. C. 
In step 141, it is determined whether or not the opening degree .theta. of 
the throttle valve 8 is smaller than 10.degree., on the basis of the 
output signal of the throttle sensor 12 and, in step 142, it is determined 
whether or not the level of vacuum P in the intake manifold 2 is larger 
than -80 mmHg and smaller than -350 mmHg, on the basis of the output 
signal of the vacuum pressure sensor 14. In addition, in step 143, it is 
determined whether or not the engine speed N is higher than 1500 r.p.m. 
and lower than 3000 r.p.m., on the basis of the output signal of the 
engine speed sensor 20. 
In step 144, it is determined whether or not the control electric current I 
is larger than MIN and smaller than MAX. If I.ltoreq.MIN, or if 
I.gtoreq.MAX, the routine goes to step 145, and it is determined whether 
or not the state of I.ltoreq.MIN or I.gtoreq.MAX has continued for more 
than 10 sec. If this state has continued for more than 10 sec, it is 
determined that the air-fuel ratio control device has malfunctioned. At 
this time, the routine goes to step 146, and the warning lamp 22 is lit. 
Conversely, if it is determined in step 139 that the flag is set, the 
routine goes to step 147, and it is determined whether or not the routine 
has gone through step 147 for the first time. When the routine has gone 
through step 147 for the first time, the routine goes to step 148, and the 
timer II is set. Then, the processing routine is completed. 
In the next processing cycle, the routine goes from step 147 to step 149, 
and it is determined whether or not a fixed time determined by the timer 
II has elapsed after the timer II is set. If the fixed time has not 
elapsed, the processing cycle is completed. Conversely, if the fixed time 
has elapsed, the routine goes to step 150, and the flag is set. If the 
flag is set, the routine goes from step 139 to step 140 in the next 
processing cycle, and thus a diagnosis is made. 
If the temperature T of the engine cooling water exceeds 70.degree. C. 
before the fixed time has elapsed after the engine is started, the flag is 
set. Then, if the temperature T of the engine cooling water becomes lower 
than 70.degree. C., since the routine goes from step 139 to step 147, the 
diagnosis is started when the fixed time has elapsed after the temperature 
T of the engine cooling water becomes lower than 70.degree. C. In 
addition, if the temperature T of the engine cooling water exceeds 
95.degree. C. when the fixed time has elapsed after the engine is started, 
the flag is set. Then, if the temperature T of the engine cooling water 
becomes lower than 95.degree. C., since the routine goes from step 139 to 
step 147, the diagnosis is started after the fixed time has elapsed after 
the temperature T of the engine cooling water becomes lower than 
95.degree. C. Namely, even if the temperature T of the engine cooling 
water becomes low, the temperature of the carburetor 3 does not always 
becomes low in accordance with a reduction in the temperature T of the 
engine cooling water. Consequently, even if the temperature T of the 
engine cooling water becomes lower than 70.degree. C. or 90.degree. C., 
percolation may be generated. Consequently, at this time, if the diagnosis 
is made, an incorrect diagnosis may be made. Consequently, in this 
embodiment, to prevent an incorrect diagnosis, even if the temperature T 
of the engine cooling water becomes lower than 70.degree. C. or 95.degree. 
C., the diagnosis is prohibited for a fixed time until the temperature of 
the carburetor 3 has fallen. 
In this embodiment, an incorrect diagnosis is prevented by prohibiting the 
diagnosis for a fixed time until the temperature of the carburetor falls, 
even when the temperature of the engine cooling water has become lower 
than a predetermined temperature. 
While the invention has been described by reference to specific embodiments 
chosen for purposes of illustration, it should be apparent that numerous 
modifications could be made thereto by those skilled in the art without 
departing from the basic concept and scope of the invention.