A Mercedes Benz car, the user reported that the engine idle speed of the car is unstable, there is obvious shaking phenomenon; the reaction is slow when accelerating, the exhaust pipe has blasting phenomenon; the engine fault warning light on the instrument panel is on; after the three-component catalytic reactor burns red, the car has been running for a long time at high speed.
Connect the fault diagnosis instrument to check the electronic control system of the engine. The equipment shows that the mixture is too rich.
The main causes of the engine rich mixture are too little intake air, too much fuel injection and misfire of ignition system.
The maintenance personnel first measured the pressure of the fuel system, but the oil pressure was normal.
Depending on the age and symptoms of the vehicle, the mechanic suspects that there is a problem with the ignition system.
For this purpose, the maintenance personnel remove the spark plug for inspection. When checking the right spark plug, there is no abnormality, and the high-pressure jump is normal. When the left spark plug was removed for inspection, it was found that there was too much carbon between the spark plug electrodes.
By checking the ignition system, it can be judged that there is a fault in the left ignition system.
The vehicle is equipped with two independent ignition systems, i.e. left and right independent ignition mechanisms, which are directly controlled by the engine control unit.
For this reason, the maintenance personnel first switch the igniter left and right, but the fault still exists, and then switch the ignition coil. At this time, the high-pressure spark on the right side becomes weak and the high-pressure spark on the left side becomes strong, indicating that the ignition system on the left side is faulty.
After replacing the left ignition coil, the fault is eliminated.
Conclusion: the failure of the car is due to the damage of the left ignition coil, resulting in the secondary voltage drop of the ignition coil output.
After measurement, the resistance of ignition coil is the same as that of new coil.
When the ignition coil on the left fails after turn to turn breakdown, the breakdown voltage of the left oil cylinder is insufficient during operation, resulting in the unburned mixture entering the exhaust pipe. When the mixture detected by the oxygen sensor is too thick, the engine failure warning lamp is illuminated, resulting in the exhaust pipe burning the three-way catalytic reactor.