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Hey guys, I have a 92 F250 351W straight drive pickup with regular cab and 8ft bed. The problem i'm having is the fuel pump will give a quick buzz when you cycle the key but will not perform it's usual 3 second 'ish purge, but it is pushing fuel all the way to the engine when it does this and if you cycle the key 3 or 4 times then fire the engine it will run normal for 30 seconds or so until it chokes out and quits. It has both a rear tank and a side tank and both act exactly the same. I'm curious if anyone has any ideas on this problem, here is what i have done so far: New fuel pressure regulator, new pump relay, new tank selector switch, new fuel filter, and a used fuel pump. I've also checked the wiring from under the dash all the way to the rear tank and I dont see any issues with it, tho my wiring abilities aren't great anyway. I also noticed some kind of electrical box under the rear bumper that the wires run to and I'm curious if anyone can tell me what this is. Thanks for any help!
I'm not usually one to throw parts at a vehicle and I'm sort of spitballing here, but what about trying a new fuel pump diode? It's a white plug looking thing in the "Power Distribution Box" under the hood where your fuel pump relay lives. I can send a picture of mine if you aren't sure what it looks like. All I know about it is that my fuel pump won't run without it - maybe yours is going bad?
What makes me think of this is that you mention that BOTH tanks have the similar issue. If not that, it's possible the ECM is starting to go bad - I know they can cause fuel pump issues as well. Or the "used" fuel pump might not be the best either?
Have you connected a fuel pressure gauge and watched it during that 30 sec run? If not, I would get one on there so you know for sure what is going on.
Have you connected a fuel pressure gauge and watched it during that 30 sec run? If not, I would get one on there so you know for sure what is going on.
This...
If there is good pressure, your problem lies elsewhere than the pump(s)
If no pressure. Move to the pumps and test voltage at the pumps during your short run time. You may need to enlist some help for this. If power is getting to the pump but no pressure is at the rails, you either have a failed pump, or a line partially clogged up.
Have you checked out your filter??
If no power is getting to the pump with the engine running, work your way back testing the new switch you installed, then power going into and out of the fuse for the fuel and on back till you reach the computer. If no power is found back to that point, well...
Note that I'm not completely familiar with the fuel circuit on these trucks and the above is just a guideline. Check your manual for schematics that show where you can test. Anywhere there is an electrical connection, you can test voltage.
And don't forget to test the fuse for tone. It may look good, but there may be a break you can't see. It's happened to me.
The problem i'm having is the fuel pump will give a quick buzz when you cycle the key but will not perform it's usual 3 second 'ish purge, but it is pushing fuel all the way to the engine when it does this
The fuel pump should be energized for 1-2 seconds when cycling the key from Off to Run. This is built in programming of the computer. From your description it looks like the pump is now running continuously with the key in the Run position?
I'm not usually one to throw parts at a vehicle and I'm sort of spitballing here, but what about trying a new fuel pump diode? It's a white plug looking thing in the "Power Distribution Box" under the hood where your fuel pump relay lives. I can send a picture of mine if you aren't sure what it looks like. All I know about it is that my fuel pump won't run without it - maybe yours is going bad?
What makes me think of this is that you mention that BOTH tanks have the similar issue. If not that, it's possible the ECM is starting to go bad - I know they can cause fuel pump issues as well. Or the "used" fuel pump might not be the best either?
Originally Posted by technobabble
Have you connected a fuel pressure gauge and watched it during that 30 sec run? If not, I would get one on there so you know for sure what is going on.
I was thinking the same thing about the computer but it's hard to find one so far and when i ask about the diode nobody knows what i'm talking about. I'm going to keep searching for a computer and also see if i cant find a diode and replace which ever one i find first and see how that does, or both if no luck. I also have a buddy coming over this weekend with a code reader and a pressure gauge so i'll get those checked out and post with any info on that. Thanks for the ideas guys, gives me something to go off of. I'll be in touch.
There is no fuel pump diode, it's a series protection diode inline with the EEC relay coil power feed. The fact that the truck does try to run and the fuel pump(s) run (via the load side of the EEC relay) means that protection diode is good. That is a bunny hole trying to pursue that avenue.
I asked previously " From your description it looks like the pump is now running continuously with the key in the Run position?" with no response. That is the key question here IMHO.
the fuel pump will give a quick buzz when you cycle the key but will not perform it's usual 3 second 'ish purge, but it is pushing fuel all the way to the engine when it does this and if you cycle the key 3 or 4 times then fire the engine it will run normal for 30 seconds or so until it chokes out and quits.
From this, is sounds like the pump just comes on for less than a second.
OP, is this correct, and is this a change to how it was operating previously? Did the fuel pump run longer during the initial prime before?
What happens is the ECM will "prime the pump" when you turn the key to on, after you start the vehicle, the ECM will use engine speed (signaled from somewhere, cam, coil ect.) to determine how to run the pump. It's possible the ECM is not getting an engine speed signal. Maybe someone can chime in with where these trucks get that from.
I would definitely start by following the testing procedures listed above. You have already put a lot of parts toward this problem with no solution. You need to find the problem to fix it. I understand "accuracy through volume" but it is rarely economical.
There is no fuel pump diode, it's a series protection diode inline with the EEC relay coil power feed. The fact that the truck does try to run and the fuel pump(s) run (via the load side of the EEC relay) means that protection diode is good. That is a bunny hole trying to pursue that avenue.
I asked previously " From your description it looks like the pump is now running continuously with the key in the Run position?" with no response. That is the key question here IMHO.
Oh well, it was worth a shot. So Randy, is that diode just extra protection for the fuel pump relay circuit then?
There is no fuel pump diode, it's a series protection diode inline with the EEC relay coil power feed. The fact that the truck does try to run and the fuel pump(s) run (via the load side of the EEC relay) means that protection diode is good. That is a bunny hole trying to pursue that avenue.
I asked previously " From your description it looks like the pump is now running continuously with the key in the Run position?" with no response. That is the key question here IMHO.
Neither fuel pump run continuously, only a very short buzz when the key is first turned to the on position. It seems that the pumps do not run at all after starting the engine.
Ok guys so I tested the voltage at the fuel pumps and they are only getting 6.7 volts at any given time, so not enough juice to run i assume. Also pulled the computer out and took a look at it and one of the electrolitic capacitors was corroded on the bottom and has a lead broken, (it's a 47 microfarad 25 volt one, if im not mistaken, that sits right next to a yellow one) Does anyone think this could be the cause of the low voltage to the pumps, and/or the pumps not running when engine is fired? Also if anyone has a link to a good wiring diagram please send so i can get a good idea of how it's all run. I figure ill go ahead and get a capacitor Monday and try re-soldering it.
Have you connected a fuel pressure gauge and watched it during that 30 sec run? If not, I would get one on there so you know for sure what is going on.
I have not tried a pressure gauge while running, but I had a buddy push the needle down on the fuel rail and fuel was just barely rolling out of it. He was saying that it should be spraying pretty good if we had anywhere near the correct pressure.
You need to replace all three capacitors and it is most like contributing to your issue. So at least we know for sure now the fuel pump relay is getting energized for the 1-2 second period when cycling the key from the Off to Run position.
The reason for your fuel pump voltage sitting at ~6.7 VDC is the fuel pump relay is not energized, you are measuring the fuel pump monitor feedback. In order to get full voltage is to jumper Pin 2 & 6 on the data link connector then turn the key to the Run position. This will keep the fuel pump relay energized.
Since the relay is energized and dropped out as expecedt I think you are going down the wrong path. You need to repair your computer, then see if the computer will keep the fuel pump relay energized with the engine running. Which means it is seeing the PIP signal from inside the distributor. This signal also triggers the computer (via the TFI) to fire the coil.
You need to replace all three capacitors and it is most like contributing to your issue. So at least we know for sure now the fuel pump relay is getting energized for the 1-2 second period when cycling the key from the Off to Run position.
The reason for your fuel pump voltage sitting at ~6.7 VDC is the fuel pump relay is not energized, you are measuring the fuel pump monitor feedback. In order to get full voltage is to jumper Pin 2 & 6 on the data link connector then turn the key to the Run position. This will keep the fuel pump relay energized.
Since the relay is energized and dropped out as expecedt I think you are going down the wrong path. You need to repair your computer, then see if the computer will keep the fuel pump relay energized with the engine running. Which means it is seeing the PIP signal from inside the distributor. This signal also triggers the computer (via the TFI) to fire the coil.
Fuel Pump diagrams
PCM and Fuel Pump Relays
Fuel Tank Selector Diagram:
Hope everyone had a good Christmas. Thanks for the diagram, this helped a lot!. So I've done a little more work lately and here's what I've come up with. I replaced the EEC relay with no luck, and checked a bunch of grounds and hot connections around the engine and frame, and I replaced 2 of the 3 capacitors in the computer but didn't have the third one so I tried it and no change in the pumps. I'm going to try resoldering the 2 when i go to replace the third capacitor soon for the heck of it and see. so i figured out how to get the truck to throw me some codes without a reader and here's what I got from that with what I think is accurate descriptions:
CODE / WHEN / DESCRIPTION
114 - KOEO, KOER : ACT out of self test range 0.3 - 3.7
116 - KOEO, KOER : ECT out of self test range 0.3 - 3.7
542 - KOEO, CM : Fuel pump circuit failure
543 - KOEO, CM : When PCM commanded the fuel pump on voltage was not detected on FPM
556 - KOEO, CM : Fuel pump circuit failure
From what I've been able to research, the FPM is something to do with the computer (or is the computer?) What do ya'll make of these codes?
