Fuel pump only works unsinstalled
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5th Gear
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Fuel pump only works unsinstalled
Appreciate any insight on this. 1998 F150 4x4 4.6L. Bought it a couple months ago. Previous owner stated the fuel pump, tank, filter and lines were replaced. I can visually confirm all of these are new with exception of the pump because anything sitting in gas looks clean. Since I bought it I replaced the multi function switch which requires the key cylinder to be removed. Ran good for a couple hundred miles since purchase. Now the Fuel pump intermittently primes when key is turned to on position (mostly not at all). Fuses and relay test good. Inertia switch ok. I have 12 volts to the fuel pump connector when key is turned to on position. I've removed the fuel pump twice now and bench tested with a battery. Every time I put power to the pump with the bench test battery it works. After I reinstall the pump it won't come on, or intermittently comes on after sitting for a few days or more. Could this be a ground issue somewhere or something else? Thanks
Lead Driver
Joined: Nov 2020
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From: Sportsman's Paradise
Appreciate any insight on this. 1998 F150 4x4 4.6L. Bought it a couple months ago. Previous owner stated the fuel pump, tank, filter and lines were replaced. I can visually confirm all of these are new with exception of the pump because anything sitting in gas looks clean. Since I bought it I replaced the multi function switch which requires the key cylinder to be removed. Ran good for a couple hundred miles since purchase. Now the Fuel pump intermittently primes when key is turned to on position (mostly not at all). Fuses and relay test good. Inertia switch ok. I have 12 volts to the fuel pump connector when key is turned to on position. I've removed the fuel pump twice now and bench tested with a battery. Every time I put power to the pump with the bench test battery it works. After I reinstall the pump it won't come on, or intermittently comes on after sitting for a few days or more. Could this be a ground issue somewhere or something else? Thanks
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5th Gear
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Thanks RLXXI, but I also bench tested the pump using the plug connector leads and it worked fine. Starting to wonder if it could be a crank shaft position sensor, only because I now noticed the CEL does not come on when i turn ignition to on position. THoughts?
Lead Driver
Joined: Nov 2020
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From: Sportsman's Paradise
Did you run codes to see if any are stored in the system? Just because the light is out doesn't necessarily mean anything other than the bulb might be burned out from 27 years in use.
Fleet Mechanic
Joined: Jun 2023
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From: WA
You have some good clues but the connections between them aren't quite right. The CEL/SES light being off and the crank position sensor aren't related. The CEL light turning on when the key is turned on is actually a bulb check/system check process. The fact that it doesn't turn on is probably your most important clue. Overall, it looks like you have intermittent loss of power.
As RXLXI suggests, a code reader will tell you a lot. They're pretty cheap these days.
Why did you replace the multi-function switch?
As RXLXI suggests, a code reader will tell you a lot. They're pretty cheap these days.
Why did you replace the multi-function switch?
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5th Gear
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Thanks guys!
1. I just replaced the cranks sensor and it didn't help.
1.2 I know I'll get crap for this but the while removing the original crank sensor, the 8mm bolt snapped. Instead of completely removing the AC pump in order to drill out the broken bolt I was able to tightly secure the new crank sensor with zip ties.
2. I demised the crank sensor was the problem because the truck intermitently starts and when it does start I see the CEL light comes on while cranking and when it doesn't start there is no CEL and no tach moving.
3. I changed the multi function switch because the high beams and turns were not functioning. The new MFS fixes that and yes it does have tilt steering.
Appreciate anymore ideas!
1. I just replaced the cranks sensor and it didn't help.
1.2 I know I'll get crap for this but the while removing the original crank sensor, the 8mm bolt snapped. Instead of completely removing the AC pump in order to drill out the broken bolt I was able to tightly secure the new crank sensor with zip ties.
2. I demised the crank sensor was the problem because the truck intermitently starts and when it does start I see the CEL light comes on while cranking and when it doesn't start there is no CEL and no tach moving.
3. I changed the multi function switch because the high beams and turns were not functioning. The new MFS fixes that and yes it does have tilt steering.
Appreciate anymore ideas!
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Fleet Mechanic
Joined: Jun 2023
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From: WA
Since the problem is intermittent it's probably not a blown fuse problem. If it was mine I'd turn the key On and if the CEL light was not on I'd start shaking things until it comes on. Try to figure out what exactly is different when it comes on. Temperature, wheel position, whatever.
i would also check/wiggle/bump any fuse or relay associated with the PCM. I don't know for sure but the CEL is turned on by the PCM if it sets a code. So it would make sense that if the PCM doesn't have power the CEL would not either.
Here's an online service manual link. Look at the Fuses section it describes all of the fuses and relays.
https://www.fordservicecontent.com/F.../98f12og1e.pdf
p.s. if it is a PCM power problem I would be worried about the engine dying while driving.
p.s. 2 It might be that a bad CEL bulb shuts down the PCM, as a safety feature. No bulb circuit, no start. You might focus on the bulb itself.
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