Troubleshooting for low oil pressure in Caterpillar engines

A 966D type loader has an engine model of CAT3306. Not long ago, a problem of low engine oil pressure occurred. When the cold machine is idling, the pressure is only 50kpa. When the hot engine is idling, the pressure is almost zero (the normal oil pressure at idle of this machine: 69Mpa when using SAE10 oil; 138kpa when using SAE30 oil). When the rotational speed increases, the oil pressure can also rise accordingly. The engine was disassembled for inspection and it was found that one of the main bearing shells was scratched, while the clearances of the other large and small bearing shells were not large. There were also no problems with the oil pump gears, gear shafts and sleeves. After the repairman replaced the scratched main bearing bush and reassembled the engine, the oil pressure remained unchanged during the test run. We noticed that this engine has no pressure regulating device. There is only a hydraulic valve (Part number: 4N8385) beside the oil pump body. Could it be that there is something wrong with this valve? So, the oil pan was disassembled again and the oil pump was removed for a careful inspection. It was found that the valve core and valve seat of the hydraulic valve were lifted by alloy slag flakes that had fallen off from the bearing bush, preventing them from sealing. After the alloy slag was removed, the sealing between the valve core and the valve seat under the action of the spring force was confirmed by experiments to have good performance. After installing the oil pump onto the engine and conducting a test run, the oil pressure was normal.

This case shows that for this type of model, when ruling out low engine oil pressure or major overhauls, do not neglect to check the sealing condition of the oil pump and hydraulic valve; otherwise, it is easy to take a wrong turn.