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My fuel pressure in my 89 drops to zero in about 10 seconds after shut off and makes for hard starting after it has sat for several hours. I know the supply line check valve is working, already tested it.
So what is it that keeps the return line's fuel from emptying back into the tank? There is no check valve for it on the sending unit and the line obviously is supposed to remain pressurized but the pressure is going somewhere fast.
The FPR, injectors, fuel pump and filter have are all new and the car was doing this before any of those parts were replaced.
Clamp off the return line and see if it drops. My new FPR does that and they said it was normal. Replaced it and the other brand does the same thing but I have a HSR
Its not normal to loose fuel pressure that fast, it should hold pressure (40psi) for a long time & even when the pressure gets low the main line is filled all the way. If yours is bleeding down like that, every time you start it the pump has to refill the lines.
You could have a bad regulator, leaky injector, pin hole in the main line, bad check valve in the pump, or the connection between the pump & the line is bad. The last one is an internal fuel tank leak.
So what is it that keeps the return line's fuel from emptying back into the tank? There is no check valve for it on the sending unit and the line obviously is supposed to remain pressurized but the pressure is going somewhere fast.
The "check valve" is the FPR. There is no pressure in the return line when the engine is shut off. The pressure line is the only one that is important.
Rapid pressure drop off could be a bad FPR, bad pressure relief valve in the fuel pump or a leaking injector.
If the FPR is bad you can check for that by pinching off the return line. Then the fuel can't leak out and the pressure will stay up.
A bad pressure relief valve in the fuel pump is not a problem because the fuel pump will always be pumping and a small "leak" will not hurt anything.
A leaking injector can only be found by taking the injectors out and testing them.
I have already replaced the injectors, FPR, filter, tank gasket and fuel pump and it was doing exactly the same thing before the new parts went on. I also tested the check valve with 40lbs of air and it held steady for 15 minutes.
The FPR has no fuel present in the vacuum line and the car starts fine if it's only sat a few hours or less. When it sits overnight it takes some cranking to get fuel back to the engine.
I also find that every time I open the fuel cap there's a lot of air pressure escaping, a lot. Could this be pushing the fuel out of the line and what vents the tank to keep air pressure from building?
When you installed the new pump, did you keep and or replace the pulsator?
Fuel pump feed from one end and hard line from the other. The lines will often pull out. If it takes a long time to build pressure you could be losing part of your fuel back into the tank. And if this was not a secure fit, it would allow the fuel to drain back to the tank giving you zero pressure in seconds. If you used this part, replace it with a piece of fuel line and two hose clamps.
Thanks John but wouldn't a leak in this show up when I put 40lbs of air to the sending unit's supply side? I put the air to it and clamped off the hose and it held steady for 15 minutes. I did this from the top of the removed sending unit with a piece of spare fuel line.
I also replaced the $20 per foot submersible fuel line, of which I only needed 2", when I did the pump. And used brand new clamps everywhere.
I can't help but wonder if the air pressure in the tank has something to do with it.
07-05-2015, 08:48 AM
Fuel Return Line
