Diesel fuel lift pump rebuild

Practically every diesel engine has a fuel lift pump. It lifts fuel from the fuel tank and primary fuel filter, then pushes it through the secondary fuel filter and into the fuel injection pump. A few engines are fitted with electrical lift pumps, which are almost always sealed and cannot be rebuilt, but must be replaced when they fail. By far the majority are mechanical pumps that are relatively easily rebuilt at reasonable cost. This article is specific to my Yanmar model 4JH2E engine, but most pumps are similar enough so this information will be useful to other makes as well.

If you have a service manual for your engine, carefully review it before starting on this project. The Yanmar service manual gives a few basic pointers, but unless you are quite an accomplished mechanic, you probably will not be able to do this job with only the service manual as your guide.

Lift pump life
In the 14 years and 5,000 engine hours that we have put on Cormorant’s engine since it was new, I have rebuilt the lift pump at roughly 2,500-hour intervals. Rather than wait for it to fail at some inconvenient time, I much prefer to practice preventative maintenance.

If your engine is hard to start, or starts easily enough, but seems to stumble for several seconds before idling smoothly, your fuel lift pump may be the problem. If you are getting diesel fuel in your engine oil (the oil level is mysteriously rising), this is a definite indication that the diaphragm in the lift pump has failed. Either obtain the parts to rebuild your pump or, if you do not possess the mechanical skills to successfully rebuild your pump, then order a new pump. Either choice will be cheaper than a tow back to home port.

Place some paper towels below the lift pump to catch any diesel fuel or engine oil that may spill. Take photos of each step to guide the reassembly process. If you don’t have a camera, then make some sketches and notes, as you are not likely to remember every tiny, but vital, detail.

Remove the two fuel hoses from the lift pump. It’s a good idea to mark which hose goes where before you remove them. A piece of masking tape is a handy marker.

Using a 10-mm open-end wrench, remove the two nuts and lock washers that hold the pump to the engine. If you feel the pump trying to push its way off of the engine, then slowly rotate the engine crankshaft a quarter to a half turn or so to relieve the pressure of the cam on the pump. This will make reinstallation much easier.

Move the pump to your clean and well-lit work area. Shake the pump and depress the plunger to remove any remaining diesel fuel and engine oil. Using the hammer and prick punch, make small marks on the three parts of the pump to aid in reassembly.

Using your Phillips-head screwdriver, remove the six screws, being careful not to lose the small lock washers. Carefully pry the pump apart. If it will not yield with reasonable force, then don a pair of leather gloves and very carefully use your utility knife to cut into the gasket and into the diaphragm to separate the three metal pieces of the pump.

Lay the cover aside and inspect the valve body for evidence of trash in the valves. Wash the valve body in clean diesel fuel, if necessary.

Depress the diaphragm and rotate it to disengage it from the pump lever.

Rotate the plunger until the lock pin is visible, then squeeze the diaphragm and plunger together a bit and using a pin punch or nail set, press out the lock pin.

Release the plunger and set it and its spring and lock pin aside.

Remove the diaphragm and its spring from the pump body.

Clean all of the parts thoroughly before reassembly.

Reassembling the lift pump
Place the new diaphragm and the old spring in the pump body and turn the diaphragm until the slot in its shaft is lined up with the notch in the plunger housing.

Place the plunger and its spring in the plunger housing, aligned for lock pin insertion, and with the drain hole in the face of the plunger towards the bottom of the pump.

Squeeze the diaphragm and plunger together with your fingers and insert the lock pin in the plunger. This is more easily said than done, so don’t be discouraged if it takes a few tries! If your fingers aren’t up to the task, a small C-clamp can be used to squeeze the diaphragm and plunger together.

Using your thumb, depress the diaphragm and turn it to engage the pump lever. Look for a small black dot on the diaphragm center that will indicate the proper position. Carefully peel back the diaphragm to confirm that the diaphragm bracket is engaged with the pump lever. The plunger lock pin will not be visible when the diaphragm is correctly positioned. Note: The pump lever will not move the diaphragm unless the plunger is depressed, so don’t be misled into thinking that the diaphragm is not engaged with the pump lever just because it doesn’t feel like it is engaged!

Place the gasket between the pump cover and the valve body, with the rough side of the valve body against the gasket, and match the two alignment marks that you made before disassembly, inserting two of the screws to hold the parts and gasket in position.

Observing the marks on the pump body, valve body and cover, carefully place these three parts together. The smooth side of the valve body goes against the diaphragm.

Carefully align the screw holes in the diaphragm with the threaded holes in the pump body and insert two of the screws for alignment. Engage only a few threads on each screw until all six are in place, and then tighten them up being careful to draw the three pieces of the pump together evenly. Do not over-torque the screws.

Reinstall the pump
Remove the old packing from the engine block using the point of the prick punch and carefully install the new packing.

Insert the pump into the engine and replace the two nuts and lock washers.

Tighten the nuts evenly, being careful not to over-torque them.

Cut off an inch of the fuel supply hose to obtain a fresh hose connection, as the old hose will have taken a set. The intake of the pump operates below atmospheric pressure, and the slightest failure of the hose to seal will result in air being drawn into the fuel. Install the discharge hose and tighten the hose clamps.

Cycle the pump lever until resistance is felt, indicating that the fuel system has been purged of air.

Start the engine and inspect for leaking fuel or oil.

Harry Hungate and his wife, Jane Lothrop, are cruising in the Mediterranean Sea, having wintered in Gaeta, Italy, aboard their Corbin 39 cutter, Cormorant.

More articles by Harry Hungate:
Upgrading Yanmar oil pressure gauge and sender

Cool your alternator

Shore power plugs outside North America

Voyaging communications report: Cormorant

By Ocean Navigator