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A car mileage of about 50 thousand km, equipped with M43 engine and automatic transmission, chassis type E46 318i BMW sedan. User response: after the car starts the engine, the engine jitter is too serious to accelerate and maintain idle speed. After receiving the vehicle: connect the diagnostic instrument for self diagnosis, and result the engine system has multiple fault codes. After the clear code test, the failure is still. The fault information is re searched without fault code, indicating that the fault is not directly related to the control circuit. Remove the spark plug and find that the electrode has a lot of carbon. 4 spark plugs are replaced, and the failure is still. Measure fuel pressure, start to 380kPa, normal. Unplug the wiring harness plug of the injector, connect the test light, and the pilot lamp flickers when starting, indicating that the control line is good. 4 fuel injectors were removed and found that each fuel injector had a cotton wadding dirt inside, indicating that the fuel line was contaminated. Remove the fuel filter, the internal filter core has been broken. Replace the fuel filter, clean the fuel line and the injector, after loading the test car, the failure can be reduced, but as the engine temperature increases, the jitter symptoms become more and more serious, three yuan catalytic converter burning dark red. Replace the 4 injectors, the engine can not start. The cylinder pressure is measured, resulting in a serious shortage of 2 cylinders. Remove cylinder head, remove carbon deposit, grind valve, re install and test run, remove fault thoroughly. The failure of this case is mainly caused by the lack of maintenance of the engine. It runs at low speed in the city for a long time, the fuel filter is not replaced regularly, the engine produces a lot of coke, and the injector is damaged. Finally, the engine can be completely repaired by the disintegration maintenance.