A BMW 745Li car with a mileage of about 60 thousand km, a N62 engine and an automatic transmission, and a chassis model of E65. Users reflect: after starting the engine, idle condition jitter, engine acceleration is bad. The N62 engine is a V8 engine with independent ignition system, and each cylinder is equipped with a ignition coil. Check the ignition system, replace 8 spark plugs, the symptoms of failure did not improve. The connection diagnostic instrument is self diagnosed. The engine system has 3 fault codes. The contents are as follows: "cylinder 1 ignition identification fault", "cylinder 2 ignition identification fault", "cylinder 4 ignition identification fault", view cylinder running stability value, 1 cylinder value -4.52, 2 cylinder value 6.21, 4 cylinder value 3.50, other cylinder value. The value is not normal. Replace the 8 ignition coils with a thorough breakdown. The ignition monitoring principle of N62 engine is as follows: the engine control module follows the ignition interval. (according to the signal of the crankshaft position sensor) the crankshaft signal tooth disc is divided into 4 sector working areas (the crankshaft has a reignition), and the duration of each sector is monitored and continuously analyzed. For each point on the working characteristic curve, the maximum permissible value of its running instability is the engine. Speed, load, temperature and function index data are stored. Theoretically, the cylinder running stability is 0. If the value exceeds the limit, it is identified as cylinder misfire, and the relevant fault codes are stored. A serious ignition failure may result in the interruption of the fuel injection function of the cylinder. Maintenance experience shows that ignition coil is a vulnerable component, the main reason is that the quality is not good enough. The ignition coil in the later stage has been improved, and this kind of fault has been reduced.