Fuel pressure falls to zero at idle and stalls.

[Tim32]

Hey guys, I did a smoke test this week chasing vacuum leaks and found two leaks which I fixed up... Now having a weird issue where it starts and at idle after a few seconds the fuel pressure suddenly goes to 0 and stalls.. if I blip the throttle, it will regain pressure and keep going though. I put a new fuel filter on and drained the old fuel and put new fuel in but maybe thats stirred up some crap in the fuel tank and blocked the intake or something??? Any other ideas? FPR stuffed?

EDIT: Also, when it stalls, the pump primes and pressure goes back up and it starts again as per normal and happens all over again.

Edited May 15, 2024 by Tim32

[joshuaho96]

1 hour ago, Tim32 said:

Hey guys, I did a smoke test this week chasing vacuum leaks and found two leaks which I fixed up... Now having a weird issue where it starts and at idle after a few seconds the fuel pressure suddenly goes to 0 and stalls.. if I blip the throttle, it will regain pressure and keep going though. I put a new fuel filter on and drained the old fuel and put new fuel in but maybe thats stirred up some crap in the fuel tank and blocked the intake or something??? Any other ideas? FPR stuffed?

EDIT: Also, when it stalls, the pump primes and pressure goes back up and it starts again as per normal and happens all over again.

Check your FPCM. What happens if you bypass it? The ECU will bypass it during prime + initial start, then the FPCM kicks in and slows the pump down.

[MBS206]

If you've been mucking with vacuum lines, start there, and check you have your vac line on properly, and on a vacuum line that isn't flowing air, it's just a vacuum source.

 

When you measure your fuel pressure, are you measuring absolute fuel pressure, or gauge pressure compared to the inlet manifold?

[Tim32]

Hey guys, thanks for the replies! (We are noobs so sorry if some of this seems obvious or is completely wrong)

@joshuaho96 I did go looking for the FPCM (after googling) but it's an old drift car so I think that has been bypassed already, the fuel pump is wired directly to the battery.

@MBS206 OK I'll double check vacuum lines. I'm just looking at the FPR gauge, shows around 40PSI while priming and idle, until instant drop to 0, not even a slow drop, like instant.

 

[MBS206]

1 hour ago, Tim32 said:

Hey guys, thanks for the replies! (We are noobs so sorry if some of this seems obvious or is completely wrong)

@joshuaho96 I did go looking for the FPCM (after googling) but it's an old drift car so I think that has been bypassed already, the fuel pump is wired directly to the battery.

@MBS206 OK I'll double check vacuum lines. I'm just looking at the FPR gauge, shows around 40PSI while priming and idle, until instant drop to 0, not even a slow drop, like instant.

 

How is the Fuel pressure gauge connected, show photos, what's it referencing etc.

 

Get photos of how the fuel pump is wired.

I'm assuming there's a relay triggering it, if so, measure what the signal wires are doing. From memory the ECU pulls to ground, so find the coil wire that goes to the ECU and measure resistance to ground.

 

If it's an old drift car, what ECU is in it?

[Tim32]

Here's some pics, ECU is stock. thanks again.

[TurboTapin]

9 minutes ago, Tim32 said:

Here's some pics, ECU is stock. thanks again.

That wiring looks like it was done by a 12 year old with a methamphetamine addiction. Start by redoing that disaster. What you're describing has nothing to do with your FPR vac source. 

[Tim32]

HAHA yeah its been quite the experience sorting out the wiring, I had an auto electrician come and do some cleanup under the dash. Its a work in progress.

[MBS206]

I gotta agree that FP wiring looks, interesting.

And questionable.

 

I'd be doing some probing to see what circuit is shutting off.

It actually sounds like FPCM wiring is partly in use.

Priming, starting are all at full power from memory, then a few seconds later drops to half strength, and it looks like that signal isn't getting to your FP, as a hypothesis that is.

 

Needs more probing on a multimeter being done.

[Tim32]

OK thanks for that, I'll do some probing on the weekend and clean up that wiring mess a bit, your hypothesis sounds logical, so I'll start there. Really appreciate the advice. 👍

[joshuaho96]

1 hour ago, MBS206 said:

I gotta agree that FP wiring looks, interesting.

And questionable.

 

I'd be doing some probing to see what circuit is shutting off.

It actually sounds like FPCM wiring is partly in use.

Priming, starting are all at full power from memory, then a few seconds later drops to half strength, and it looks like that signal isn't getting to your FP, as a hypothesis that is.

 

Needs more probing on a multimeter being done.

This is exactly what I was thinking, I just wasn't expecting whatever that hackjob was. Skyline owners never disappoint.