Fuel Return Question
Thread Starter
Burning Brakes
Joined: Nov 2003
Posts: 1,234
Likes: 5
From: Herriman UT
Fuel Return Question
1986 L98; I believe I have a bad fuel return check valve. Is this known as the "pulsator" in the FSM? If so, I need to replace the entire fuel pump, right? That would be odd since it was a new pump about 15k miles ago...
What driveability issues should one expect with a bad check valve?
What driveability issues should one expect with a bad check valve?
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Good question; I don't know if the check valve is inside the pump or in the pulsator and neither my FSM nor the pump instructions (did this job two days ago) clarify the issue.
For one you may not have good fuel rail prime if pressure bleeds down too fast after you turn On the ignition, though I don't know if it would be a real issue in startups.
Also you could monitor fuel pressure to see if it's at all unsteady. To do this unplug the vac line and plug it at the plenum fitting. Then run the RPMs up & down to see if pressure holds; it should be rock solid steady.
However, even a little fluctuation here may not be an observable issue while driving. Remember that fuel flow goes as the SQRT of pressure change so if there is say a 2% pressure "pulsation" there is only a 1% variation in fuel flow capability.
For one you may not have good fuel rail prime if pressure bleeds down too fast after you turn On the ignition, though I don't know if it would be a real issue in startups.
Also you could monitor fuel pressure to see if it's at all unsteady. To do this unplug the vac line and plug it at the plenum fitting. Then run the RPMs up & down to see if pressure holds; it should be rock solid steady.
However, even a little fluctuation here may not be an observable issue while driving. Remember that fuel flow goes as the SQRT of pressure change so if there is say a 2% pressure "pulsation" there is only a 1% variation in fuel flow capability.
Race Director
Joined: Jun 2004
Posts: 13,939
Likes: 705
From: WI
The pulsator is seperate from the pump, the check valve is inside the pump and would require a new pump. You could try taking it apart. If the valve is bad the car would not have much fuel pressure when starting. You could turn the key twice to get the pump to run twice before cranking if there is a long cranking time. Injectors and FPR can leak down to.
Thread Starter
Burning Brakes
Joined: Nov 2003
Posts: 1,234
Likes: 5
From: Herriman UT
Still starts fine but fuel pressure bleeds off immediately after the fuel pump shuts off. I think I've narrowed it down to the check valve as FP doesn't bleed off when I crimp the fuel supply line.
Melting Slicks
Joined: Jun 2004
Posts: 3,032
Likes: 4
The Pulsator sits on top of the pump, so I believe it could still be either of the two.
Burning Brakes
Joined: Nov 2005
Posts: 848
Likes: 1
From: 1992 LT1 6 speed Cayman Islands
However more than half the C4 have the same "problem" you have. It is not a problem at all as you can attest to as your car starts fine.
Once you turn on the key the fuel pump runs for 3 secs and this represrrurises the injectors. If the circuit for the fuel pump that turns it on for 3 secs goes out you will have to crank the car until the oil preassure pumps up and triggers the other circuit to ground the fuel pump to get fuel to the injectors.
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Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
I would do as AGENT86 suggested first. Remove the pulsator and replace it with a hose and clamps. Then recheck the fuel pressure loss. If you have verified that the leak is in the tank, make sure the fuel line connections are good. If it still leaks down after removing the pulsator then the check valve is the suspect. Question is.....is the problem bad enough purchase a new fuel pump?