Kubota V3800 Engine Knocking at Idle After Fuel Contamination
Fault Codes & Symptoms
Engine knocking at idle Related symptoms: - Rough idle - Low power - White or gray smoke at idle The engine developed a knocking sound at low idle after the machine was run with contaminated diesel fuel. The knock is less noticeable at higher RPM and under load.
Confirmed cause: Broken spring in the fuel injection pump delivery valve for cylinder #1. Repair performed: Replaced the faulty delivery valve and spring assembly for cylinder #1. Bled the fuel system. Verification: Started the engine and allowed it to idle. The knocking sound was completely gone. Engine ran smoothly at all RPM ranges. Technician notes: - This failure mode is uncommon but can occur after fuel contamination, where debris or water can damage sensitive pump components. - The key diagnostic step was swapping the injector, which definitively ruled it out as the cause and focused attention on the injection pump. - Always use a line wrench when cracking injector lines to prevent rounding off the fittings.