Fuel pressure readings
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Mountain Pass
Joined: Oct 2020
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Fuel pressure readings
1994 F150 351W 5.8L V8 E4OD Dual Tank
My fuel pressure has consistently tested low at idle and during power breaking. I'm getting between 26-28 PSI on both the front and rear tank at idle. With the FPR vacuum hose disconnected I'm at 30 PSI. Testing while power breaking jumps to 34-36 PSI. I've never noticed a drop in fuel pressure with any of the tests, just lower than what I have seen listed as acceptable ranges.
Do these readings seem too low? Are there other tests to run that would narrow down the source of low pressure to the pump, delivery module, fuel selector valve, etc? I'm having a stalling issue under very low throttle and I'm wondering if it is possibly due to low fuel pressure.
I'm going to have the bed off the truck this weekend to inspect the fuel system more thoroughly so if I do need to replace parts, now would be the time to do it I guess.
Current KOEO codes are:
(PO removed all smog/emissions components, blocked off egr, removed CANP, removed TAB/TAD, etc which I believe explain the codes)
334 - EGR closed voltage higher than expected
553 - TAB/TAD failure
552 - TAB/TAD failure
565 - Purge Valve failure
558 - Failure in the EGR Vacuum Regulator (EVR) solenoid circuit
My fuel pressure has consistently tested low at idle and during power breaking. I'm getting between 26-28 PSI on both the front and rear tank at idle. With the FPR vacuum hose disconnected I'm at 30 PSI. Testing while power breaking jumps to 34-36 PSI. I've never noticed a drop in fuel pressure with any of the tests, just lower than what I have seen listed as acceptable ranges.
Do these readings seem too low? Are there other tests to run that would narrow down the source of low pressure to the pump, delivery module, fuel selector valve, etc? I'm having a stalling issue under very low throttle and I'm wondering if it is possibly due to low fuel pressure.
I'm going to have the bed off the truck this weekend to inspect the fuel system more thoroughly so if I do need to replace parts, now would be the time to do it I guess.
Current KOEO codes are:
(PO removed all smog/emissions components, blocked off egr, removed CANP, removed TAB/TAD, etc which I believe explain the codes)
334 - EGR closed voltage higher than expected
553 - TAB/TAD failure
552 - TAB/TAD failure
565 - Purge Valve failure
558 - Failure in the EGR Vacuum Regulator (EVR) solenoid circuit
Last edited by bdwiggin; May 2, 2023 at 06:11 PM. Reason: Specifying Truck Year and Engine
Logistics Pro
Joined: Nov 2013
Posts: 3,928
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From: Mojave, Ca
Are you sure your fuel pressure gauge is accurate? Maybe try another gauge and see if they're roughly the same. Your "FPR vacuum disconnected" and your power braking measurements should be the same. What's you're idling battery voltage at?
Weak fuel pumps won't cause a stalling problem at low power settings.
Stumbling or stalling at low rpm could be a vacuum leak, TPS, ECT, or a few other things that others will suggest.
Weak fuel pumps won't cause a stalling problem at low power settings.
Stumbling or stalling at low rpm could be a vacuum leak, TPS, ECT, or a few other things that others will suggest.
Thread Starter
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Mountain Pass
Joined: Oct 2020
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I have tested with two separate gauges and the results were within a couple PSI of each other. I've never seen above 30 PSI with either of them. Even if the lower pressure isn't causing the stalling, do these numbers seem low enough to be of concern?
Fleet Mechanic
Joined: Aug 2015
Posts: 1,834
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From: Candy Mountain
Changing the fuel filter wouldn't be a bad idea if you don't know how long it has been in service.
Maybe a bad pressure regulator or incorrect regulator.
Have you checked your battery and alternator?
Never hurts to clean your ground connections.
Maybe a bad pressure regulator or incorrect regulator.
Have you checked your battery and alternator?
Never hurts to clean your ground connections.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
What year is your truck and what engine does it have? Anything '90 or newer doesn't have a tank selector valve.
Yes those pressures are low, with vacuum removed from the regulator you should have 40psi min(V8's) or 55psi(I6).
Is this a dual tank truck? If so do they both work?
Yes those pressures are low, with vacuum removed from the regulator you should have 40psi min(V8's) or 55psi(I6).
Is this a dual tank truck? If so do they both work?
Thread Starter
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Mountain Pass
Joined: Oct 2020
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Likes: 6
FPR was replaced last year with motorcraft part - I'd have to look up the part number but it was pointed out to me on this forum which one to get.
Battery is new this year - previous owner put in a new alternator but I haven't necessarily tested it.
Haven't cleaned ground connections so I will take a look at those.
Thread Starter
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Mountain Pass
Joined: Oct 2020
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Likes: 6
What year is your truck and what engine does it have? Anything '90 or newer doesn't have a tank selector valve.
Yes those pressures are low, with vacuum removed from the regulator you should have 40psi min(V8's) or 55psi(I6).
Is this a dual tank truck? If so do they both work?
Yes those pressures are low, with vacuum removed from the regulator you should have 40psi min(V8's) or 55psi(I6).
Is this a dual tank truck? If so do they both work?
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Logistics Pro
Joined: Nov 2013
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From: Mojave, Ca
The fact that your fuel pressure doesn't dip when power braking means that your pumps are fine. All signs point to a weak fuel pressure regulator or bad fuel pressure gauges.
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Thread Starter
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Mountain Pass
Joined: Oct 2020
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I will say that with the original FPR - I was getting the same readings as the current replacement FPR. Guess it could be a gauge problem.. first test was done with a standard pressure gauge from amazon. I ended up installing a mechanical gauge directly on the fuel rail so I could keep tabs on it frequently and in different situations.
Logistics Pro
Joined: Nov 2013
Posts: 3,928
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From: Mojave, Ca
I will say that with the original FPR - I was getting the same readings as the current replacement FPR. Guess it could be a gauge problem.. first test was done with a standard pressure gauge from amazon. I ended up installing a mechanical gauge directly on the fuel rail so I could keep tabs on it frequently and in different situations.
Thread Starter
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Mountain Pass
Joined: Oct 2020
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Not that I can tell. The fuel gauge appears to work on both tanks and they seem to hold true based on fuel consumption by tank. I can take both tanks down to E with the other staying full. Are there other indicators that fuel would be transferring between tanks?
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
But does the empty tank remain empty after switching to the other tank?
The common fuel transfer problem will overfill a full tank pushing fuel out the filler neck onto the ground, and there is no indication that is happening except perhaps a raw fuel smell after stopping.
The common fuel transfer problem will overfill a full tank pushing fuel out the filler neck onto the ground, and there is no indication that is happening except perhaps a raw fuel smell after stopping.
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Mountain Pass
Joined: Oct 2020
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I cannot say that I’ve paid enough attention to that specifically. If the fuel is actually being transferred between tanks - what does that mean?
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
Half of the fuel volume the pump moves just goes into the other tank unrestricted so the pump won't be able to build correct pressure at the engine. It's as if there was a giant leak in the fuel line.