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The mileage is about 190 thousand km, and the model is E70, the BMW X5 sport utility vehicle equipped with N52 engine in 2007. When the engine was overhauled, the trouble lights were found during the commissioning. Inspection and analysis: the maintenance personnel detected the engine control unit and found that there was a fault indication that the camshaft phase could not be adjusted when the cold car started. After the cleaning of the camshaft phase adjustment solenoid valve, the test run is still in trouble. Review the entire maintenance process and confirm that the installation of the timing system was very prudent. After the timing chain is loaded, the timing mark is checked 3 times, so it should not be a problem here. He suspected that the solenoid valve was dirty when it was overhauled, so that it could not be adjusted or accidentally touched the camshaft position sensor. So the replacement of the solenoid valve after commissioning, the results still prompt timing problems. The camshaft position sensor is replaced, and the problem is still unsolved. It is considered that the parts related to camshaft timing include camshaft, crankshaft and related sensors. There is no such fault before the engine overhaul. Is it the damage of the phase actuator in the maintenance process? In desperation, the valve chamber cover is removed and ready to replace the cam shaft phase actuator. Before that, the valve timing was checked and no problems were found. After removing the camshaft, it is found that the direction of the cover of the flight path camshaft is reversed. The oil pressure of the phase executor is passed through the camshaft oil passage. When the camshaft is rotated, half of the oil passage is cut off when the camshaft rotates. In this way, phase execution becomes slow. When the cold car starts, the camshaft phase needs to be adjusted quickly, so the requirement can not be satisfied. Troubleshooting: correctly install camshaft cover, trial run, confirm troubleshooting.