Fuel Pump Cutting Out
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Mountain Pass
Joined: Apr 2019
Posts: 128
Likes: 18
Fuel Pump Cutting Out
1990 F150 4.9
Front fuel pump cut out on way back home, made it back on rear tank. Front pump is fairly new less than a year old. When back home turned key to on and no priming noise from (front) pump. I let the truck sit a couple hours then went back to look at it, now front pump was priming and working (started truck). Shut truck off and checked voltage at pump and was seeing 8v, checked selector switch and was seeing around 4v. These numbers seem low to me yeah? Wondering if I'm getting resistance somewhere, maybe somethings getting too warm? Mostly looking for ideas, thanks for any help.
Front fuel pump cut out on way back home, made it back on rear tank. Front pump is fairly new less than a year old. When back home turned key to on and no priming noise from (front) pump. I let the truck sit a couple hours then went back to look at it, now front pump was priming and working (started truck). Shut truck off and checked voltage at pump and was seeing 8v, checked selector switch and was seeing around 4v. These numbers seem low to me yeah? Wondering if I'm getting resistance somewhere, maybe somethings getting too warm? Mostly looking for ideas, thanks for any help.
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Mountain Pass
Joined: Apr 2019
Posts: 128
Likes: 18
I'll mention too this pumps been giving me some trouble on warm starts. Will start fine cold but if I'm running around town and start it back up truck will sputter and buck like its not getting fuel. I've gotten in the habit of starting on rear tank while warm then switching to front tank at 2000 rpms or so; there may be a little hiccup but it usually runs fine after that.
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
Those voltage readings are from the fuel pump circuit in the computer (PCM). The fuel pump relay is energized for 1-2 seconds when cycling the key from Off to Run. To keep the fuel pump relay energized to test the circuit jumper Pins 2 & 6 on the diagnostic plug under the hood, then turn the key to the Run position.
Cargo Master
Joined: Mar 2010
Posts: 3,328
Likes: 292
From: Laramie, WY
I replaced a pump maybe 10 years ago and used the included crimp connectors that go on the wires in the tank. After a few years those connections started to fail. I pulled the fuel pump out, removed the crimp connectors, soldered the wires, and reinstalled the pump. Has been working well since then. Not saying that's what you've got going on but the symptoms were similar, figured it's worth mentioning. Ultimately it was low voltage to the pump.
Hotshot
Joined: Jul 2020
Posts: 10,400
Likes: 2,606
From: Near Salt Lake City
Always replace the pump module assembly (with the new sender assembly and the wiring) if you can.
After about 120k miles on the OEM pump, the wires get burnt inside the insulation is what I have seen
That way when you wire in just a new pump, you run the risk of a faulty connection
Usually, the wiring is good for 2 pumps at least, maybe 250k
GMs are worse, the connector melts on those Suburbans and Yukons etc
After about 120k miles on the OEM pump, the wires get burnt inside the insulation is what I have seen
That way when you wire in just a new pump, you run the risk of a faulty connection
Usually, the wiring is good for 2 pumps at least, maybe 250k
GMs are worse, the connector melts on those Suburbans and Yukons etc
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Mountain Pass
Joined: Apr 2019
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Checked voltage with the diagnostic pin jumper (thanks rla2005), found 12v at selector switch and 12v at the pump connector. I'm pretty sure its a faulty pump at this point. I've gotten ahold of the parts store about a replacement. I'll let you guys know how it works out.
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Mountain Pass
Joined: Apr 2019
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GoinBoarding, I checked the connections on the pump a couple months ago when it was first acting up, they were still intact but i re-did them with some heat shrink tubing to be safe. Still had trouble with it but I appreciate the idea thank you.
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Mountain Pass
Joined: Apr 2019
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Likes: 18
ManicMechanic, I haven't heard that before about the wires in the hanger/sender, I'll poke around with the ohmmeter the next time i have it out and see if there's any resistance. Thanks for the tip.
Cargo Master
Joined: Mar 2010
Posts: 3,328
Likes: 292
From: Laramie, WY
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Mountain Pass
Joined: Apr 2019
Posts: 128
Likes: 18
Hey guys, got the fuel pump replaced today. The truck started up fine, idled right up to temp; I picked up the rpms a few times and everything felt smooth. Truck started right up again after shutdown. Manicmechanic I checked the hanger wires for resistance and they both tested good .1ohms if I remember right, also wiggled them around some. I think the hangers good for another pump or two. I'm planning on taking the truck to town on Sunday, will see how it does with the longer trip.
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Mountain Pass
Joined: Apr 2019
Posts: 128
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Made it to town and back with no trouble at all, truck also started up warm without any issue. It's nice to take off without the truck lurching and bucking. I'm convinced the last pump I got was rated for too low of a psi. New pump is a MasterPro (Oriley's) MPA23001 for anyone whos interested/confused by the number of choices out there for pumps. Thanks again for the help everyone.
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