A FAW Mazda 6 automatic passenger car with mileage of about 25 thousand km and 2. 0L engine and manual transmission. The engine works normally in the refrigerator, without jitter, but after a few minutes in situ preheating, the engine will be jitter. As long as the engine speed is raised to about 1200r/min, the engine work will become more stable. After receiving the car: first verify the fault phenomenon, found that the above fault does exist, but the engine acceleration performance is good, automatic transmission jump gear is normal. Because of the many reasons for the engine's idle speed jitter, it was decided to check according to the principle of "easy to overcome, difficult to start" and "inside out first". Connect the Mazda special tester WDS to detect the engine electronic control system and find that there is no fault code stored. Where there was a suspicion of a vacuum leak. So the vacuum meter was connected to check the intake pipe and intake manifold, and no leakage was found. No abnormalities were found in the valve timing, spark plugs, high voltage lines and ignition coils of the vehicle. Then connect the fuel pressure gauge to measure the pressure of the fuel system within normal range. Measuring cylinder pressure of each cylinder is also within the standard range. The analysis holds that if the injector is not well atomized, the engine will be unstable at idle speed, so the 4 injectors are removed and the atomizing test is carried out in the injector cleaning machine, and the injector is also normal. Finally, the endoscope was used to check the combustion chamber. It was found that the valve and piston top were clean, and there was not much carbon deposition. Routine checks have been completed, still can not find the problem, only once again to the owners to understand the vehicle's related maintenance. The owner of the car reflected an important message: the car owner had lent the car to a friend in the previous period. In the field, the engine appeared high temperature, the cylinder mats and the related accessories were replaced, and the friend felt the trouble after returning the car back. Considering that the car had been disassembled and installed in other repair plants, it is suspected that the above fault may be caused by incorrect assembly. The engine was then reassembled to check the assembly condition and no problems were found when inspecting the flywheel and crankshaft. When decomposing the cylinder head and fixing the double camshaft with Mazda special tools, the problem was finally found. It is found that the positioning angle of the camshaft on the intake manifold side is not correct, and the difference between the camshaft angle and the standard angle is 3 to 5 degrees. Troubleshooting: re assemble the camshaft and engine after trial run according to the requirements of the maintenance manual, and find the above fault phenomenon disappear and troubleshooting. Maintenance summary: the 2. 0L engine of this mazja sedan adopts a double top camshaft structure, a special mark on the stop point position on the first cylinder, and the accuracy of the timing angle of the 2 camshaft is higher. In order to ensure the correct installation of the camshaft, a special tool must be used to fix the camshaft. Because the relative mark of the driven camshaft of the engine is completely hidden under the cylinder head cover, it is difficult to detect the valve timing. It may be due to the lack of special tools in the assembly of other repair plants, resulting in a deviation in the timing of the camshaft, resulting in these failures.