Diesel Fuel Pumps

[Tom Wood]

Why won’t the fuel lift pump on my diesel work with the manual knob sometimes?

Tom Wood responds:

Almost all of those little fuel lift pumps (the ones with the tiny manual thumb knob that is hidden underneath where it is unreachable) are driven by a lobe on the engine's camshaft inside the engine. Since the camshaft is generally set to turn at half the speed of the engine, and the lobe is about one-eighth of a circle in lift area, there is one chance in 16 that, when you shut off the engine that the camshaft lobe has the pump lift arm fully depressed. And when the diaphragm lift arm is already depressed as far as it can go, the little manual handle is inoperative. Probably, when you shut the engine off the last time, it quit right in the position that made your pump not work. Using the starter, bump the engine over about half a revolution. This will turn the camshaft and should free the pump lift arm.