Engine/Transmission (1994 - 1998) Fuel pressure question (again)

[jsjaastad]

Joe G. read your interesting write up you mentioned in my earlier post. I removed the lift pump and primer and hooked it up to air like you said. When I hold the piston against the spring and put it in water I get a small amout of bubbles out the weep hole. Do I need to somehow force the piston to compress the spring to do this test or would this indicate the primer pump is allowing air to enter the system? Also I blocked the output line with a cork. I assume that is to keep the water out as nothing comes out of it when air is applied to the inlet or do I need to compress the spring to move the check valves ?

Thanks again for any assistance. Tom/Idaho

 

[Joe G.]

The piston has to be in there kind of snug. I had to compress the spring a little on mine. If the piston is not in there correctly it will leak at the weep hole. If you can't get it to stop leaking at any position then something may be wrong.



The check valves are opened by fuel flow from the input to the output so if you put air pressure on the input side it will pressurize the whole system including the output steel line. They prevent fuel flow from the output to the input so if you put air pressure on the output side it won't get past the first check valve. It's a one way street in the lift pump.

 

[jsjaastad]

I managed to get a clamp to hold the piston in and retested. Small bubbles came out where the fuel temperature sensor is bolted on with the two small screws. I tightened one quite a bit and it stopped leaking air. Do you think this will fix the problem or should I get a new pump? Thanks again, Tom/Idaho

 

[Joe G.]

That ought to do it. Any small air leak will drop fuel pressure. A lot of them will not drop it so bad the engine won't run. The lift pump is very sturdy. Hardly ever fails.



It's easy to check if you have it off the engine. You will need two new O-rings. Take it apart. Make sure all three check valves allow fuel to go from input to output. There are two springs. If they are ok and the piston valve is not skinned up the lift pump is ok. It has a hard time pumping air so any air leak will mess it up. If the push buttom pump is working and not leaking it is ok. The big spring is what moves the fuel. The piston compresses the spring. It's a simple contraption.

 

[jsjaastad]

Thank you very much for your assistance. Tom/Idaho