The Yanmar injector pump delivers precisely metered and timed bursts of fuel to the fuel injectors via the injector lines. Fuel is delivered to the injector pump by a low pressure feed pump (lift pump). The injector pump draws fuel from the feed supply and distributes high pressure fuel to the injectors and the spill circuit. Air leaks and fuel restrictions reduce the amount of fuel that the lift pump delivers to the injector pump. Worn components inside the injector pump reduce the pump output and lead to poor or non-running conditions. Poor adjustments of the governor and bad fuel timing can mimic injector pump problems and should be eliminated as possibilities before tackling the injector pump.
Check the fuel level in the tank. Ensure that the fuel level is sufficient to cover the fuel pickup and prevent any air from being drawn into the system in the fuel tank. Add fuel to the tank if needed.
Check the fuel filter. Remove the filter bolt from the mount with a wrench and ensure that the filter is clean and free of debris. Replace the filter if it is clogged. Reinstall the filter to the mount and tighten the filter bolt with a wrench. Ensure that the inlet and outlet fuel hoses are tightly clamped to the fuel nipples and that the fuel filter vent is fully tightened down to prevent air from entering the system.
Remove the lift pump outlet pipe from the injector pump with a wrench. Actuate the manual lift pump lever and observe the fuel escaping from the pipe. The fuel stream should be strong with no air bubbles present. Air bubbles indicate an air leak downstream of the lift pump and a weak stream indicates a worn lift pump. Reinstall the lift pump outlet pipe to the injector pump, then tighten the fitting with a wrench.
Check the fuel control rack and governor adjustment. Refer to the service manual for year- and model-specific adjustment information.
Remove the injector pipes from the injectors with a wrench. Briefly turn the engine over using the starter motor. Observe the fuel discharge from the injector lines. The discharge should be strong and free of air bubbles. The fuel bursts coming from the pipes should be roughly equal. A significantly lower output on one or more of the pipes indicates a problem inside the injector pump. Reconnect the injector pipes, then tighten the fittings with a wrench.
If the results of steps 1 through 4 do not reveal a problem but step 5 does, then your injector pump requires service. The fuel injector pump is a fairly complicated piece of machinery that requires special tools and training to perform repairs. Remove the injector pump, then take it to a certified diesel service shop for repairs.