High-pressure injection pump
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Why is there abnormal noise in the pump booster gearbox?
The most likely cause is that the check valve is not sealing properly or has some foreign object stuck in it, which increases the force with which the plunger retracts and results in a knocking sound.
The engine oil has turned black; I’m going to take it apart to check the gearbox
In any case, an disassembly inspection is definitely necessary; once it’s disassembled, everything will be clear :)
One-sentence post, half-sentence reply
The gearbox is broken; it needs to be taken apart for repair.