|
Likely, the plastic damper ring in the pump's internal governor has gotten brittle, and failed into nasty black chunks that look like coffee grounds, and larger. When one of these pieces finds it's way into the housing pressure regulating valve that is the lower part of the return fitting screwed into the pump's cover, the flow of return fuel out of the pump is restricted, and the engine won't "rev", or will stall altogether. You can disconnect the return line, and take the fitting out of the pump cover, and then see how the engine runs, taking appropriate precautions 'cause the return fuel will be spilling out. If it runs better, take a look into the fitting/valve for debris. Caution, there is a glass check ball in there, which can be broken if you get to rough digging the crud out of the fitting/valve. If there is crud in the valve, and you get it cleaned out, likely, the engine will run OK until more crud gets in there. The downside of continuing to run the machine and occasionally cleaning the valve/fitting is that eventually when enough of the governor ring has failed, there will be metal-to-metal contact of the governor parts separated by the plastic ring. A diesel pump shop will OH the pump, and install an updated governor weight carried that can't come apart and shed plastic pieces. Of course, a restriction any where else in the fuel return system between the pump and the tank will cause the same symptoms.
| 
