A 2007 named MG7 with a 18N4T1 engine. Due to continuous rain for several days, there was accumulated water in the car, cleaned up the water in the repair shop, and replaced the carpet inside the car. After the processing was finished, the engine could not start the machine. Fault diagnosis: at first the maintenance personnel think that there is water in the connection of the circuit, causing leakage, or the wire connector terminal rusting after the bad contact, but careful examination of the circuit, no abnormal. After further inspection, it is found that spark plugs are normal when starting, but the injector is not working. Check the power supply of injector, normal, and deduce that the engine control unit does not send fuel injection control signal to the injector. After handling the relevant tie line, the trial run is still in trouble. Because there is no special fault detector, the fault code and the engine data flow can not be read (the general fault detector can not communicate with the engine control unit of the car), and the fault diagnosis is deadlocked. After careful analysis, the engine control unit can send out the ignition control signal, but there is no fuel injection control signal: the engine control unit peripheral line fault and the engine control unit damage. Check the wiring harness of the engine control unit seriously; remove the engine control unit, open its shell, carefully check the circuit and electronic components on the circuit board, no obvious ablative marks are found, and the smell of burning is not found. The circuit board nude the engine control unit, connects the relevant wiring harness, tries to start the engine, starts about 5S each time, stops for a moment and starts again, after starting 3 times, it feels that the driver chip MAR9109PD of the fuel injector is abnormal in temperature and is obviously perm. Although it can not see the burn marks from its appearance, it can be determined inside the chip. The Department has been damaged. Find an old engine control unit, remove the same chip, and test the engine after welding. The engine still can't start the engine. I feel very strange, is it wrong? Or is the chip bad? You can smell gasoline after several times starting the engine continuously, which means that the fuel injector has been injected. There is oil and fire. Why can't the engine start the engine? The fuel pressure is measured, and the fuel injection pulse width of the starting engine is measured with the "repair spirit D2" (a portable instrument, which can check the signal of the electronic control system and the signal of the sensor) to measure the injection pulse width of the starting engine. It is about 3 ms, and the cylinder pressure is measured, and the cylinder pressure is found to be around 3 bar (1 bar=100 kPa). When the spark plug is dismantled, the spark plug is removed, a certain amount of oil is injected into each cylinder, and the engine is started on the accelerator pedal after loading, and the engine starts to start for 2 times continuously. The engine is running at the speed above 2000 r/min, and the exhaust pipe is rolling, and after a few minutes, there is a lot less smoke. Release the accelerator pedal, the engine can run idle, after a few minutes to make the engine out of fire, then start, one can start the machine, and then measure the cylinder pressure, more than 8 bar, slightly lower than normal data, but the problem is not big. At this point, troubleshooting. Fault analysis: as the drive chip of the injector is damaged, even if the CPU of the engine control unit sends out the fuel injection control signal, it does not gain power on the drive chip, and the injector will not work. After replacing the driver chip MAR9109PD of the injector, the fuel injection is normal, but because of the longer use of the car, the coke in the engine is more, after a few days, the piston ring is stuck in the ring groove, causing the cylinder seal bad, so that the cylinder pressure drops so that the engine can not start the engine. After adding extra oil, the oil film thickness is added, the cylinder pressure rises, the engine can start the engine, and the engine can run for a long time. After the engine temperature rises, the piston ring is no longer stagnant and the cylinder pressure is restored to normal. At this time, the engine is extinguished and the engine is easy to start. The engine is easy to start. The engine is easy to start. Start the machine.