The mileage is about 149 thousand km, the chassis type is W210, and the Mercedes E200 car equipped with M111 engine. Users reflect: the engine running is unsteady and jitter is obvious, especially when the vehicle climbs up, the engine performance decreases obviously, and there is a clicking noise. Fault diagnosis: to confirm the fault symptoms, the engine is slow to react from 1000r/min acceleration, and there is a phenomenon of dyspnea. When the engine speed is over 2000r/min, the engine can slowly accelerate, and the acceleration is jitter. Analysis of the cause of the fault may be caused by too much mixture or poor ignition system. The M111 engine is a straight four cylinder engine, the ignition system uses a dual fire ignition coil, the 1 cylinder and the 4 cylinder share one ignition coil, and the 2 cylinder and the 3 cylinder share a ignition coil. The ignition coil is directly controlled by the engine control module, that is, the engine control module directly controls the ignition time. Check ignition system, spark plug and high voltage wire are normal. Connect the oscillograph to check the ignition primary voltage. The primary voltage waveform of the 1 cylinder and the 4 cylinder is different from that of the 2 cylinder and the 3 cylinder. At the idle speed, the ignition time of the ignition primary voltage of the 1 cylinder and the 4 cylinder is short. At the same time, the attenuation oscillations produced by the ignition coil and the capacitance are short, indicating that the performance of the 1-4 cylinder ignition coil is bad. Measure the ignition coil of the 1-4 cylinder, the primary winding resistance is 0.4 ohms, normal, the secondary winding resistance is infinite, indicating the circuit breaker. The engine jitter disappeared when the 1-4 cylinder ignition coil was replaced. Continue to check the problem of noise. Check the installation of muffler, silencer seal is good, no interference with other parts. Check the wheel, the wheel has no turning resistance. Tighten the bolts and make road tests. A asbestos gasket is added to the bracket between the automatic transmission and the muffler to carry out the road test. The abnormal sound is gone, so that the trouble is completely eliminated. Fault summary: ignition coil of M112 engine is easy to damage. When the ignition coil is poor, the fault is not obvious because the engine load is small at the idle condition. In the process of large load and acceleration, the ignition energy needs to be enhanced, so the fault performance is obvious.