Hey guys,
Just need alittle help here. I installed one of those permanent fuel pressure gauges that the mustang guys use on the fuel/injector rail, and I noticed when I shut the truck down the pressure instantly drops to 0 psi. Before it starts, when I turn the key the pressure jumps to 40 psi. Any ideas what it could be. The truck runs great though.......It did this w/ either tank selected.
My truck has dual tanks w/ the 2 high pressure in-tank pumps. 1991 F-250 w/ 5.0L

Thanks,
Jerry D.

 

It should hold pressure. There are 2 suspects: the fuel pressure regulator and a leaky injector.

Pull the vacuum hose from the FPR. If there is gasoline present in the line, you have a blown diaphragm and need to replace it. If there is no fuel present, the regulator could have some crud which is holding it off the seat (this is more likely since it builds proper pressure when running.) I do not know how to diagnose this one though.

Pull your plugs one by one and look at them. If any of them are fuel fouled, then the injector in that cylinder is leaking and needs replaced. If none of the injectors are leaking, it's probably the FPR.

Wait to see of PRKWUD has any input, he probably knows how to diagnose the crud in the regulator problem.

 

This could be a quick fix. My FPR is a Mallory adjustable, that is rebuildable. And I just replaced my plugs last week and they were all dry.....in fact shes running a tad lean.
Is there some kind of check valve that might be faulty?

 

There is a pilot operated check in each return line that closes off when the respective pump is not operating. If one of them were bad, it would cause what you're seeing but it would also cause a fuel transfer problem. The tank with the defective check valve would get the return flow from both tanks. So, is either tank getting unexpectedly full and/or peeing on the floor? Is one giving you WAY better mileage than the other?

 

No, both tanks function properly. And it runs great on both tanks.
This sucks! I should have never put this gauge on. LOL!

 

If your FPR is rebuildable, it's also cleanable. A spec of crud could keep the poppet from seating, but not interfere with the operation in the slightest. The fact that you're running a bit lean makes me think there's a regulator issue. Check the vacuum line for gas, then even if it appears OK, pull it apart and have a look. It's worth the price of a couple of gaskets and a diaphragm.

 

OK, let me first say that I do not know the specifics of this setup. (then why are you posting, everyone asks...)


BUT, is the gauge electrical or mechanical? If electrical do you have it hooked up through the key? Obviously if it's electrical and hooked up through the key then you turn off the key and it will fall to zero-- but when you turn on the kjey it begins to register again and rises right up.

Like I said I don't know the specifics of this setup, it's just what came to mind when I read the post.

 

With the key on, you say it develops pressure. What happens after the first couple seconds? If the engine isn't fired, and the key remains on, the pump shuts down after two seconds. It should react the same way it does when you shut the key off.

Either way, as suggested, check the vacuum line to the FPR for gas. If none is found, ground the fuel pump terminal in your self test connector.



Turn the key on, and find a piece of fuel line that you can clamp shut on the input side, clamp the line, and turn off the key. If the fuel pressure drops again, repeat the test except on the return hose side. If it again drops, you have a leaky injector. If the fuel pressure holds steady after clamping one of the lines, the problem is in that line. For example, if it's in the input line, you probably have either a bad check valve in the tank, or the fuel line in the tank has cracked. If you determine it's in the return line, the problem has to be the FPR.

 

Since it happens on both tanks, doesn't it have to be either an injector or the FPR? Any problem with the FDM wouldn't occur in both tanks

 

Good point.

Okay, skip testing the input line, and just check the return line.

Good call. I'm glad one of us was paying attention.

 

Ok guys did a little work on the truck today, turns out that the O ring on the FPR was rotted and broken and the diaphragm looked pretty crappy too. I ordered a rebuild kit and I should have it in a day or so. I will keep you posted on how it turns out.

 

Update:
The Mallory rebuild kit is on back order!
So ,I had to order one from Summit Racing....a $24.95 part and they charge $9.95 handling fees.
I'll keep ya posted why I install it.
Thanks again guys.

 

Update:

All is well..........I installed the rebuild kit for the FPR. and it worked.
It's holdin 42 psi as we speak.
I gotta alittle scared when I first started it up. The fuel pressure gauge was reading 80 psi! I cranked it down REAL quick!
Good to go! Thanks for all the helps guys.