Fuel pressure question
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Senior User
Joined: Oct 2006
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From: Kingwood, Texas
Fuel pressure question
I have a 96 F150, 302, 2WD, E4OD. I am having an issue with it stalling/hesitating. I have read hundreds of threads on here to try and figure this out. I checked the pressure at the rail and the front tank read 32psi for a few minutes and then would drop to 10psi. the rear tank read 4psi. I ordered a new front pump and when it arrived, it's a rear pump. I figured I'd go ahead and change it since I thought the old one was bad anyway instead of returning it. Well, the pressure at the rail is no different. I took the regulator vacuum line off and there was no gas or gas smell. I took the regulator itself off and put a vacuum tester on it and it held 40psi steady. It has a new fuel filter on it as well as plugs, cap, rotor and wires. I haven't checked yet to see if any of the fuel lines are kinked but I was wondering if it may indeed be the regulator even though it appears to be okay. Any help will be appreciated.
Fleet Owner
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From: Easton,Ks
I would not think it would be the regulator.
Check the fuel pressure with it running, at idle it should be 32-34 PSI and at WOT it should be about 40 PSI.
With it idling pull the vacuum hose off the Fuel pressure regulator and put your finger over the hole in the hose. The pressure should go to about 45 PSI with no fuel coming out of the nipple of the regulator.
If all the pressures checks out take it an auto parts store and have the pull your codes and see if they tell you anything.
Check the fuel pressure with it running, at idle it should be 32-34 PSI and at WOT it should be about 40 PSI.
With it idling pull the vacuum hose off the Fuel pressure regulator and put your finger over the hole in the hose. The pressure should go to about 45 PSI with no fuel coming out of the nipple of the regulator.
If all the pressures checks out take it an auto parts store and have the pull your codes and see if they tell you anything.
Thread Starter
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Senior User
Joined: Oct 2006
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From: Kingwood, Texas
I will try checking the pressure with the vacuum hose off. I did put my finger on the nipple and it seemed to have strong vacuum when I checked it with the engine running. Also, I do have a code reader for the truck and I rechecked this morning and there were no codes. Sorry for not posting that piece of inof earlier.
Thanks for your help, but I am stumped!
Thanks for your help, but I am stumped!
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
I will try checking the pressure with the vacuum hose off. I did put my finger on the nipple and it seemed to have strong vacuum when I checked it with the engine running. Also, I do have a code reader for the truck and I rechecked this morning and there were no codes. Sorry for not posting that piece of inof earlier.
Thanks for your help, but I am stumped!
Thanks for your help, but I am stumped!
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
I think you misread that bill.. it appears he does have a code reader.
Since changing the pump made no difference here I suggest you check power at the pump connector, if it's anything less than 12v you have an electrical problem.
Since changing the pump made no difference here I suggest you check power at the pump connector, if it's anything less than 12v you have an electrical problem.
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Senior User
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From: Kingwood, Texas
Do I disconnect the pigtail for the rear tank with the key on and the switch on the rear to check it? It may be a stupid question, but I have never checked a fuel pump for power. They've always worked, or have been mechanical!
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FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
Yes that will work but the pump only runs for a second if the engine isn't started so you'll have to be quick. On the OBD-1 trucks you can install a jumper on the test connector to make the pump run constantly.. not sure if there is something similar on the OBD-2 system.
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Fleet Owner
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From: Easton,Ks
I thought I read he did not have a reader.
No the OBD II trucks do not have a pin to ground to turn on the pumps.
But I have heard that some of the scanners can turn them on but have not worked much with the OBD II system.
No the OBD II trucks do not have a pin to ground to turn on the pumps.
But I have heard that some of the scanners can turn them on but have not worked much with the OBD II system.
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Senior User
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From: Kingwood, Texas
Thanks for all the help, it give me another strating point and if any of you think of anything else, please let me know.
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
I don't think my code reader has the capability to do that. $20 at a pawn shop! Anyway, I'm just trying to avoid taking in to the shop if I can do it myself.
Thanks for all the help, it give me another strating point and if any of you think of anything else, please let me know.
Thanks for all the help, it give me another strating point and if any of you think of anything else, please let me know.
If you have the fuel pressure then you will not have to check for power to the pumps.
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Senior User
Joined: Oct 2006
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From: Kingwood, Texas
You could get a spare fuel pump relay, cut the top off and wire tie it in the make position (energized) and plug it in. Then when you turn on the key the fuel pumps will run and you should have 45 PSI fuel pressure at the fuel rail. You will also have a code to read then but just clear it when you are done with your tests.
If you have the fuel pressure then you will not have to check for power to the pumps.
If you have the fuel pressure then you will not have to check for power to the pumps.
Once again, thanks for your help on this.
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
By the way when this relay is in the selected pump will run all the time the relay is in with the key on or off.
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Senior User
Joined: Oct 2006
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From: Kingwood, Texas
Thanks Subford, that makes sense to me now. I'll try it tonight since I have a donor relay I can cut up. This may be a another stupid question, but the fuel will just circulate back into the tank barring any clogs in the system if it has pressure, right?
Fleet Owner
Joined: Aug 2005
Posts: 23,667
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From: Easton,Ks
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