[FIXED] 2000 3.8 Mustang new fuel pump running constantly at key on

I had been getting longer and longer crank times on my '00 V6 Mustang, and after hooking up a pressure gauge to the rail, noticed an immediate drop-off in pressure. Thinking that the check valve in the Airtex pump I had put in 2.5 years ago was on its way out, I ordered a Delphi unit. The pressure loss ended up being a stuck open injector. I replaced them all, and pressure now holds. Not directly important, but worth noting that that issue is fixed. Anyway, I put the new pump in, went to prime it, and it whirred loudly and continuously instead of just priming for 2 seconds. With the pressure gauge on, at key on, pressure shoots up to about 95 PSI, much higher than the last pump. Spec pressure should be 30-40 PSI. These Mustangs are returnless, with an FPDM, so I'm not sure if that is the problem, but it was working when I had the old pump in and was draining the tank before taking it out, but 2 hours later with the new pump in, the pump was running continuously.

When I put the new pump in and jacked the tank back up, I heard something that sounded like plastic cracking. I didn't think a lot of it until trying to run the pump. Then I thought maybe I had broken a connector, and that was causing the pump to keep running. Since then, i've dropped the tank again, all of the connectors look fine. It may have just been the plastic piece where the right side of the tank seats against the underside of the trunk.

I'm not sure what to check first, I don't have a Verus or an Autel or anything like that, just a multimeter and an Actron scanner, and as I said in my intro post, for some reason I just have a hard time understanding electrical systems. Any help would be appreciated. Thanks!

if the tank is still down can you swap the old pump in and see if it still runs continuosly??

you dont have to swap the whole pump in just plug it in and see. also any codes??

As your multimeter got a duty cycle scale ? If not check the voltage at the pump if is near battery voltage you are likely looking at a fuel pump module or pressure sensor related issue.

Not to worry, these FPDM systems aren't that bad. Here's a shot of the wiring diagram for our reference.



It may look complex, but your list of possibilities is actually pretty short. One is a failed FPDM, stuck full fielding the pump. Two would be an issue with the Fuel Pump Command signal from the PCM, causing the FPDM to think the PCM wants a ton of pressure (unlikely). Third would be short to ground on the fuel pump control wire.

I think the best place to start might be with some multimeter measurements at pins 3, 10, 7 and 1 at the FPDM. Probably located in the trunk. Voltage and duty cycle measurements at those pins are what we're looking for. Do these with the FPDM plugged in, and the fuel pump running.

While you're there, you can do some quick resistance measurements across the fuel pump circuit itself. With the key off and the FPDM disconnected, test pins 3 and 10 for resistance to ground. There should be none. Or, use an incandescent test light connected to B+ on the same pins. Neither wire should light.

Thanks to everyone who has replied. To answer Andy's question, my multimeter doesn't have a duty cycle feature, it's a pretty basic MM. For the FPDM voltage, I've read in other posts on sites about how if voltage is over 10.5V, the FPDM is bad. I think that was for another Ford, maybe an F-150. Could that still apply here? Is it possible that I burned out the FPDM when I drained the tank before swapping the pump? Can I test the FPDM with the pump just plugged in or will I need to put it in the tank, lift the tank, and hook everything else back up? Sorry for all the questions.

Brocktoon wrote: For the FPDM voltage, I've read in other posts on sites about how if voltage is over 10.5V, the FPDM is bad. I think that was for another Ford, maybe an F-150. Could that still apply here?

I've not heard of that number before... :huh: Do you know which circuit the other posts are referring to? At least one of the fuel pump wires is gonna have battery voltage on it by design, so I'm not sure how valid that specification is.

Is it possible that I burned out the FPDM when I drained the tank before swapping the pump?

It's possible. How did you set the old fuel pump up to drain the tank?

Can I test the FPDM with the pump just plugged in or will I need to put it in the tank, lift the tank, and hook everything else back up? Sorry for all the questions.

No worries! I actually missed the part where you said you've got the tank back out of it. :blush:

You can try just plugging the pump in without the tank, but I worry that you'll damage it by running it dry for too long... Do you have an incandescent test light? You could connect that across the fuel pump pins and observe the brightness for testing.

I've not heard of that number before... :huh: Do you know which circuit the other posts are referring to? At least one of the fuel pump wires is gonna have battery voltage on it by design, so I'm not sure how valid that specification is.

I could be off about the 10.5V number. I think it was in relation between FPDM power and FPDM harness ground. Mostly for 4.6 Mustangs and F-150s pulling a P1233 code and no pressure. Pretty easy to find info on a fuel pump with low pressure. One with high pressure? Eh, not so much....

It's possible. How did you set the old fuel pump up to drain the tank?

I just ran the engine with my pressure gauge hooked up and held the pressure release button and drained it into a can. There isn't an easily accessible pump relay that I could just jump, and I figured it would be better than cycling key on and off for an hour or two.

You can try just plugging the pump in without the tank, but I worry that you'll damage it by running it dry for too long... Do you have an incandescent test light? You could connect that across the fuel pump pins and observe the brightness for testing.

Yep, I do have a test light, and I can put the pump in the tank and at least set it on a spare pair of jack stands if I have to, that's not a problem.

Brocktoon wrote: I could be off about the 10.5V number. I think it was in relation between FPDM power and FPDM harness ground. Mostly for 4.6 Mustangs and F-150s pulling a P1233 code and no pressure. Pretty easy to find info on a fuel pump with low pressure. One with high pressure? Eh, not so much....

Ahhhhh I get it. Not a bad test, in the context of low fuel pressure and the P1233. Thanks for the clarification!

I just ran the engine with my pressure gauge hooked up and held the pressure release button and drained it into a can. There isn't an easily accessible pump relay that I could just jump, and I figured it would be better than cycling key on and off for an hour or two.

Gotcha! Nah, I doubt this caused it.

Yep, I do have a test light, and I can put the pump in the tank and at least set it on a spare pair of jack stands if I have to, that's not a problem.

Sweet, we can totally use that. For now, try this: CAREFULLY connect your test light across pins 3 and 10 at the FPDM, with the fuel pump still disconnected. Take care not to short the connections to ground, or to each other. Not trying to be insulting! But there is the possibility of damaging the FPDM (if it isn't toast already) if wires get crossed.

I set this same test up on my Escape, which uses the same design of FPDM. The light should behave like this for its two second prime:



If the light stays lit brightly past the prime, then the only thing left is a short to ground on the fuel pump control wire. Since the pump and tank are out of it at this point, this is unlikely, but a good visual is probably worthwhile. If you see no problems, then the FPDM is done.

If the light behaves like the one above, then either the fuel pump control wire was shorted to ground when the tank was installed, or something is wrong with the pump itself.

Let me know what you find!

Sweet, we can totally use that. For now, try this: CAREFULLY connect your test light across pins 3 and 10 at the FPDM, with the fuel pump still disconnected. Take care not to short the connections to ground, or to each other. Not trying to be insulting! But there is the possibility of damaging the FPDM (if it isn't toast already) if wires get crossed.

Just double checking: I'm back probing pins 3 and 10, each with their own probe, laying the test light on them both and hooked light to ground?

Brocktoon wrote: Just double checking: I'm back probing pins 3 and 10, each with their own probe, laying the test light on them both and hooked light to ground?

No problem! Not quite... Sorry, I didn't describe that very well. :blush: I Painted over the diagram:



Make sense? Put another way, the test light is there to take the place of the pump as the load in the circuit.

Ah, ok, I think I got it now. Test light probe on pin 10, and light clip on pin 3.

Brocktoon wrote: Ah, ok, I think I got it now. Test light probe on pin 10, and light clip on pin 3.

This is a much simpler explanation. :blush: :lol:

Ok, I'm back.

After a couple of false starts, some bad words, trying to find thin enough pins, and yanking the alligator clips off my cheapo jumper wires to hold everything in place, it seems that the problem is NOT my FPDM. My test light lit up nice and bright for 2 seconds and then went off.

OK, I'm good with that test!

Now I think you're ready to connect the new fuel pump to its harness and see if it runs on after the key on prime. No tank required. If it does, there's something wrong with the pump. If it does now, then I'd suspect some kind of wiring problem that got moved when the tank was dropped.

Pump sounds good too! At least as good as a Delphi can sound out of the tank. Two seconds and then off.

Alrighty, not a pump issue, then!

If you don't see any evidence that the pump harness got pinched on installation, then I think you're OK to put the tank back in. Or, install the tank and pump enough to run the engine with the pressure gauge on it, anyway.

We know the FPDM isn't stuck full fielding the pump, and we know the pump isn't incorrectly wired.

could of been the fpdm. intermittent messing up. next time or if you can get it to run constantly leave your test light connected and see if it lights the light

So I just put the pump in the tank, managed to get the tank lifted went to turn the car on....

and it's buzzing constantly again. And as an added insult, the needle in the gas gauge didn't move. Probably messed up the sending unit from taking it in, and out...in and out...in and out.

I really don't know what I'm doing wrong. I can't even take a video because my smartphone won't stay powered on.

Story of my life, pretty much....

hook your testlight up again see if the fuel pump driver module is keeping it on. maybe try tapping the driver module see if it quits.