(1) Fault symptoms: The chassis model of this car is E46, equipped
with an M43 engine. The fault symptoms are as follows: The engine
runs for 12 There are intermittent fluctuations from time to time.
When starting in gear or turning on the air conditioning system, the
engine speed fluctuations are more obvious.
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(2) Diagnosis and maintenance: Use the diagnostic instrument to
perform self-diagnosis and query the fault information of the engine
system. There is no fault code. A routine inspection was performed
and no problems were found. Replace spark plugs and fuel filters,
clean throttle valve. Check the idle solenoid valve and find that there
is oil in its working chamber. After cleaning it, measure its
resistance and it is 102 [standard value is (8±2)Ω]. Reinstalled the
idle solenoid valve and tried driving, but the fault persisted. Use an
oscilloscope to measure the control signal of the idle solenoid valve.
When the engine speed is 800r/ min, the signal waveform is a 12V
square wave; when the engine speed is 1200~1500r/min, the 12V
square wave gradually turns into a straight line, indicating that the
control signal is good. Check the air flow sensor and measure the
voltage at its terminal 1, there is 5V. Check the grounding condition
of terminal 5, it is normal. Terminal 2 is the signal terminal, measure
its signal voltage. When the engine speed is 1200~1500r/min, the
voltage is 2.3~2.5V; when accelerating, the voltage can increase
accordingly, which is normal. Terminal 4 of the air flow sensor is the
signal terminal of the intake air temperature sensor. Measure the
resistance between terminals 4 and 5. It is 23002 at 20℃ and 9702 at
50℃, which is normal. Continue to check the throttle position
sensor, the installation position is normal. Measure the signal
voltage, which is 0.5V when the throttle is fully closed and 4.8V
when the throttle is fully open. Moreover, the signal voltage changes
evenly during the process of opening the throttle, indicating that the
throttle position sensor is normal. Lift the vehicle and check the
oxygen sensor, which appears to be in good condition. Use a
multimeter to measure the signal voltage and find that the change is
very small. When the engine speed reaches more than 2500r/min, the
signal voltage is still 0.55V, which is inconsistent with the standard
value, indicating poor performance of the oxygen sensor. Replace
the oxygen sensor and the fault is completely eliminated.
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(3) Summary The signal voltage of the oxygen sensor should change
within the range of 0.1~0.9V, and the number of changes per minute
should be more than 20 times. When the mixture concentration is
low, the signal voltage should be lower than 0.45V; when the mixture
concentration is rich, the signal voltage should be higher than 0.45V
. The cause of the fault in this case is that the oxygen sensor is aging,
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the signal voltage is too low, and the number of changes does not
meet the requirements. The engine control module determines that
the mixture is too lean based on the wrong signal and performs
enrichment control, resulting in incomplete combustion of the
mixture and engine jitter. The particularity of this fault is that
although the oxygen sensor signal is inaccurate, the signal voltage is
still within the standard value range. Therefore, the engine control
module cannot record the relevant fault code. This requires
maintenance personnel to check item by item through conventional
methods, making maintenance difficult. There has been an increase.