A Shanghai Volkswagen Passat B51.8L sedan, mileage of 20,000 km. The car had a more serious accident, replacing the front parts of the engine, such as the tank, the front bumper and the condenser. Maintenance personnel found in the maintenance, the air compressor on the cylinder fixed bracket bolts threaded holes due to the impact of the fracture, and in the upper part of the cylinder under the timing belt tension pulley is also under the crack, for which they welded the fracture site repair. As a result of this site without cast iron welding equipment, this part of the work commissioned by the plant to complete.
    After the completion of the repair delivery of the user, but the user car two days after the return to the factory, reported antifreeze liquid level alarm, after checking the expansion tank level below the lower limit of about 3cm. Maintenance personnel taking into account the re-filling antifreeze, the cooling system may be the existence of some gas, in the course of operation after the exhaust, the liquid level may be reduced, so the antifreeze was added, and the leak, but the leak There were no obvious leaks in the cooling system. After the delivery of the user, and advise the user to continue to observe. But after two days, this car again due to the same failure to return to the factory. Maintenance personnel carefully checked the three links, radiators, pumps, thermostats, warm water tanks and water pipes connected parts, were not found signs of leakage, check the oil did not find the oil caused by changes in the situation of oil. For the sake of safety, maintenance personnel replaced the radiator and cylinder pad again. After the repair is completed, the user is advised to advise the user to observe the liquid level and to mark the liquid level of the expansion tank for observation. Four days later, the user returned to the factory again, and reflect the car at a speed of 180kmh 500km after the antifreeze level was significantly reduced, but the traffic level in the urban area is not obvious.
    According to the user to reflect the situation, maintenance personnel carefully check and found that the top of the valve room cover like jelly like brown material, suspected to be a mixture of antifreeze and oil, but check the oil quality is no exception. So that the antifreeze itself does not enter the lubrication system, but the antifreeze vapor into the valve compartment cover attached to the valve chamber cover above. At this point, the maintenance staff for the diagnosis of the fault there are obvious differences: the view that the valve chamber antifreeze residue is the last time to replace the cylinder pad into the engine oil pan a small amount of antifreeze, in the hot car state evaporation To the inside of the valve chamber cover and condensed to form; another view that the engine waterway there is a small (or even only in the hot car will appear) to the oil oil return hole leakage phenomenon, There is no pressure, so a small amount of antifreeze into the oil, and the oil did not enter the cooling system.
    After the relevant maintenance personnel to find the author to help diagnose the fault, but this failure is indeed not easy to determine. There are signs that the antifreeze of the cooling system enters the oil, but this phenomenon is not very obvious, and before the replacement of the cylinder pad, there is indeed a small amount of antifreeze mixed with oil possible. How can we accurately diagnose the fault? At first we wanted to use a nondestructive flaw detector to detect possible points of failure, but the inspection was mainly used to check the crack on the surface of the part, which was difficult to check for cracks in the oil passage hole. After some memories, the author recalled in the university "car structure" mentioned in the hydraulic test this method, but this method is not a detailed introduction, as if used in the inspection of the manufacturing sector, in the maintenance of the actual work almost no The opportunity to use, and the repair shop is not "rich and powerful" of the factory, the lack of related equipment. But think about it, and only this method can accurately qualitative analysis of the fault, this decision to try this method. Water pressure test is the main engine all the coolant inlet and outlet closed, leaving only a pressure port. In order to facilitate the observation of the cylinder pad sealing situation, but also the piston, connecting rod and crankshaft and waterways have nothing to do and prevent the removal of parts removed. After the cylinder block and cylinder head installed, and then to the cooling system to suppress 0.4 ~ 0.5MPa and maintain, even if this is a slight leakage can also be checked out. In addition, in order to simulate the operating temperature of the engine, the engine can also be slightly heated by gas welding. In order to better "tracer", we in the closed water pressure water mixed with a small amount of detergent. After careful observation, and finally a bubble emerge, according to the air bubble to determine the crack is very small. After careful observation, although the location is very subtle, but with the help of the endoscope we finally found the crack, the crack is located in the repair site inside the engine oil return hole, the fault has finally been qualitative.
Cast iron material hardness is relatively high, but relatively brittle, in its welding process is prone to severe changes in temperature caused by stress problems. This is somewhat similar to glass, just like fine cracks on the glass. If the temperature changes drastically may cause the crack to continue to expand. In the cylinder welding, if the welding after the cooling too fast, there may be stress inside the cylinder internal stress point near the structural strength of the lower "crack" or the original cracks in the dark extension. Therefore, for the cast iron parts of the welding process requirements are very high, a little care will produce new cracks. The failure of this car is due to welding stress caused by the extension of the crack, and even produced a new crack caused by leakage. The waterway at the leaky channel is directed to the cylinder head into the valve chamber. As the crack is relatively small, the cylinder is in high temperature after a long period of high temperature operation, and the antifreeze is likely to be channeling in the form of high temperature steam The oil is finally condensed inside the valve compartment cover. Although the leakage is very small, but the long run will cause the antifreeze to slowly consume.
    After the replacement of the new engine block, the test troubleshooting.