Low fuel pressure from rear fuel pump despite replacement
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Trailering
Joined: Mar 2025
Posts: 21
Likes: 6
Low fuel pressure from rear fuel pump despite replacement
Fixing up a 1993 F150 with the 5.0 and 2wd for the kid to drive to school. It was someone else's project truck that I had to "unproject". In the process, I have replaced the rear tank, pump, front pump, fuel filter, fuel regulator, dash cluster, and EGR. She runs and drives, but when you run off the rear tank, the engine is a different tone, and I put a pressure gauge on the fuel rail and only get 25 psi off that pump at idle. The front pump puts out 36. If I pull the vacuum hose from the FPR, I get the rear pump to bump up to 36. I have tried three pumps in the rear of different brands, thinking the cheap one I started with was bad. The lines are free, the power to the pumps is equal voltage, and the fuses look in good shape in the fuse box. I put a vacuum gauge on the hose and got the 17. That is not the strongest vacuum, but it's consistent.
What am I missing?
What am I missing?
Fleet Mechanic
Joined: Oct 2025
Posts: 1,707
Likes: 372
Did you replace the whole module or just the pump inside? Do you have the external check valve on the line? It attaches at the tank and the line attaches to it. Mine on the front tank were rusted open, rear tank was rusted shut..
Last edited by 90project5.0; May 19, 2026 at 10:50 AM.
Cargo Master
Joined: Mar 2007
Posts: 2,793
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Don't trust me or my thinking but sometimes we get fixated on something and ignore everything else.
I don't run factory pressures on my truck and I do not recall what mine are. It's one of those things I set and forget. If I recall, I did replace the front pump before I went on a cross country trip because the fuel pressures were not consistent between the two pumps. When I got done, they still were not, but much more similar than with the original front pump.
If you spend all your time/money on assuming that the rear is not working right. Then you replace all the parts for the rear, maybe it's because the front one [the one you are using for comparison and assume is working properly] is the problem. I'm not saying that is what is wrong here, but I've have focused on problems in the past only to find out I was looking at the wrong thing.
I don't run factory pressures on my truck and I do not recall what mine are. It's one of those things I set and forget. If I recall, I did replace the front pump before I went on a cross country trip because the fuel pressures were not consistent between the two pumps. When I got done, they still were not, but much more similar than with the original front pump.
If you spend all your time/money on assuming that the rear is not working right. Then you replace all the parts for the rear, maybe it's because the front one [the one you are using for comparison and assume is working properly] is the problem. I'm not saying that is what is wrong here, but I've have focused on problems in the past only to find out I was looking at the wrong thing.
Lead Driver
Joined: Sep 2002
Posts: 9,498
Likes: 2,020
From: North Texas
Both pumps could be crap and/or the gauge used to diag. Let us say that a physician stated you will die in 30 days, would one not prefer at least a second opinion?
Bottom of the barrel brands are pretty lame, I lost a fresh Carter pump I tossed in my 05MY Civic in under one year. It runs, but cannot unleash all 115hp of 1.7L fury! It just bogs and rolls over. Pisses me off due to the labor, so unsure what brand to acquire now to make it boogie.
Bottom of the barrel brands are pretty lame, I lost a fresh Carter pump I tossed in my 05MY Civic in under one year. It runs, but cannot unleash all 115hp of 1.7L fury! It just bogs and rolls over. Pisses me off due to the labor, so unsure what brand to acquire now to make it boogie.
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Trailering
Joined: Mar 2025
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Update.... I need to confirm this, but on a hunch, I removed the front fuel pump (the box is off right now, so it's easy access) and hooked it up back to the fuel system, but sitting outside of the tank. I ran the truck off the REAR fuel pump, and then fuel leaked out of the Front fuel pump. I have an external check valve hooked up to both pumps but not on the return line. Thoughts?
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Cargo Master
Joined: Mar 2007
Posts: 2,793
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The valves should open when the pump is running. That way, fuel can go back to the right tank. This is done in the fuel pump assembly. I'm not certain how you can do that externally.
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Trailering
Joined: Mar 2025
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Update...Front pump
I bought an Oreilly's pump for a front tank and then pulled the other Spectra that I got from Rock Auto. The Rock Auto pump's shuttle valve was either nonexistent or malfunctioning. I put in the Oreilly pump and now have no transfer between tanks but... I lost 10 lbs of pressure and now both pumps produce 25 psi. She runs and drives fine, so I am lost.
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