【Fault description】
    The foot exits the accelerator pedal engine to turn off. Also in the moment when the car, the engine also breathe.
    【Inspection process】
    This is a manual gear. First confirm the symptoms of failure, test operation, fault symptoms immediately reproduce. Survey self-diagnosis system, the results of self-diagnostic system output code that the control system is normal. Remove the spark plug a look, all four spark plugs are blackened. Investigate fuel pressure, fuel pressure of 250-300kPa, and with the throttle opening and change, as if the fuel pressure is not the reason for the spark plug black. In order to trace the flammable mixture air-fuel ratio becomes thick reasons, the various sensors to investigate. Click to watch video teaching
    Idle vacuum sensor negative pressure -75kPa, the output signal voltage is 1.4V. The cooling water temperature sensor outputs a signal voltage of 0.4V after a complete warm-up. These two sensor output signal voltage values ​​are normal. Also investigate the oxygen sensor output signal voltage, the result is also within the reference value. The investigation carried out so far did not reveal the reason for the air-fuel ratio to become thicker. However, after the engine no load acceleration test, release the accelerator pedal, the engine speed once fell below 500r / min, and then return to the idle speed, that is 700r / min. Investigate the ignition advance angle, there is no problem.
    In order to keep the engine speed steady state, slowly release the accelerator pedal, so that although the engine speed changes little, but a little faster to speed up the operation, resulting in engine stagnation symptoms. Throttle sensor opening and closing signal is very suspicious. This is connected to the oscilloscope to check the throttle sensor signal voltage waveform, the results did not appear abnormal phenomenon.
    【Fault Analysis】
    This time, the oscilloscope probe was set on the vacuum sensor, and the signal change of PIM (intake manifold pressure) was observed while operating the throttle butterfly valve. The results did not appear to see the used signal voltage waveform. In the idle state slightly open a large throttle, the signal voltage for some time to remain constant.
    But during this period, the intake manifold has produced a great change, negative pressure from -75kPa to -53.30KPa, usually negative pressure has such a big change, the sensor output signal voltage should also have great changes. Vacuum sensor characteristics should be almost the same, so the other car on the vacuum sensor replacement up again test operation. This time the vacuum sensor output signal voltage changes normally. No load to accelerate the engine speed also fell down, repeated several times from the rapid start, the engine did not produce wheezing phenomenon.
   In the no-load acceleration, the two vacuum sensor output signal voltage waveform measurement. Abnormal vacuum sensor output signal voltage peak is 3.6V, after releasing the accelerator pedal, the signal voltage lasts 3s is still 3.6V. For this throttle butterfly valve closed, the nozzle continues to increase the amount of fuel injection, resulting in air-fuel ratio is too thick, the cylinder fire, the engine speed dropped rapidly. This is speculated that the fact that the spark plug was blackened strongly supported this inference.