Strange Fuel Pressure Issues....ANYONE chime in please

[jspaeth]

Okay, a while back, I was having some fuel pressure issues, so I decided to spend the money on an Automotive FPR and a quality liquid-filled Marshall FP gauge.

With the vacuum hooked up I set it to 34 psi, which is what it should be (my vacuum at idle is roughly -500 mmHg, or -9.7 psi.....this gives about 43-44 psi without the vacuum line on.


Now, the problem is, frequently I pop the hood and check it, and it reads well below 34 psi (sometimes as low as 28-30 psi)......

Sometimes I just ignore it, and it goes back up to 34psi, other times, I bump it back up to 34 psi......but then sure enough, I check later and it is UP to 38 psi (since i THOUGHT I needed to raise it).


So my question is, why is it reading LOW? If there were a vacuum leak, it would read higher (i.e. closer to 43 psi, which is what it is with no vacuum line hooked up).

No fuel leaking anywhere, and this is truly sporadic, and worries me that it might go lean on me under heavy load, and the POOF!

Could this be a fuel pump related issue?......i DOUBT that the internals of the FPR are not quality enough to keep the fuel pressure the same without needing constant readjustment :-/

Anyone else ever experience this?>

Again, I am running a high quality FPR and gauge, and a Walbro 255 lph pump....since the gauge is under the hood, there is no way for me to monitor the fuel pressure while i am driving the car....

Seems scary that after fine-tuning a standalone ECU to make a ton of power and whatnot, that something as simple as keeping the fuel pressure's behavior the same as when it was on the dyno could be a point of failure.....

[S14DB]

Cap the line and only set it based on barometric pressure. Then hook is back up to vac.

[jspaeth]

I've done this before.....but I like your idea.

So I think what you are suggesting, is set it at 43 psi without the vacuum on (and vacuum plugged to assure the idle and everything is the same).

Plug vacuum back in, and it should be 34 psi.

THEEEEEEEN the next time the pressure drops or changes while the car is idling, pull the vacuum line off and check the fuel pressure....?

This should help me diagnose if it's a vacuum related issue or something else....?


Again tho, the fact that it is always low and never high is strange, bc if it was a vacuum leak, then the IDLE fuel pressure would be HIGH.


Also, my car is VERY consistent at holding ~500 mmHg of vacuum at idle, so if there was a vacuum leak, wouldn't I see a change in my vacuum reading? (this reading comes off of the manifold directly via my PFC and AVC-R both.)

[jspaeth]

Cap the line and only set it based on barometric pressure. Then hook is back up to vac.


UPDATE!!!!!!!:

Okay so I just went outside, car warmed up or whatever, fuel pressure was okay........

Stats: idle RPM....800
vacuum.....500 mmhg (about 9.5 psi)
fuel pressure.......34 psi
pulled off vacuum line, and it read 43 psi
Turbo timer reads........13.8 Volts

So, that means there was no vacuum leak and the fp was just fine.

Went outside to store for 30 minutes, came outside, started up car, and this is what I found:

Stats: idle RPM....800
vacuum.....500 mmhg (about 9.5 psi)
fuel pressure.......28 psi
pulled off vacuum line, and it read 37 psi
Turbo timer reads........13.8 Volts




So, again, the difference in fp when taking off the vacuum line corresponds EXACTLY to the manifold vacuum reading (WHICH is GOOD).

Also, the alternator was putting out the same voltage.


So, based on this new data, it looks like it's gotta either be something weird with the FPR or maybe the fuel pump is not pumping consistently?

I am leaning towards inconsistency in the fuel pump, bc this EXACT same problem happened before with my Nismo FPR, and I blamed the FPR that time.


:-(.....any thoughts people?....really don't wanna blow up my car

[jspaeth]

BUMP please

[devonkyle77]

i am subscribing to this, as i would like to see what you find.

[jspaeth]

i am subscribing to this, as i would like to see what you find.


Thanks broseph. haha

I am thinking either I need a NEW fuel pump period, or just need to rewire it.

Again, the fuel pump's only 3 years old, and the alternator is putting out consistent voltage.

I doubt that BOTH my old Nismo and my new Aeromotive FPRs are to blame.

Seems like a fuel supply issue related to the fuel pump.

Oh yeah, maybe I should also change my fuel filter, or try shaking that the next time the pressure gets low....it's new but could be clogged somehows.

[MongolPup]

Wow, very scary phantom situation. Fuel pumps are relatively cheap if it comes down to that.

[JohnC]

Hi,

Going along with the fuel supply as the cause of the problem, give the voltage getting to the fuel pump a check. if the voltage drops the lph output of the pump will change as it depends on the voltage its getting and Maybe (no expert) that can have a real effect on fuel pressure. If you see a drop in voltage at the pump when the fuel pressure drops that might just be the problem. If the voltage at the pump doesnt fluctuate it may be the pump it self.
Even if it is not the problem I suggest hard wiring the fuel pump if you have not done it yet.

[jspaeth]

Hi,

Going along with the fuel supply as the cause of the problem, give the voltage getting to the fuel pump a check. if the voltage drops the lph output of the pump will change as it depends on the voltage its getting and Maybe (no expert) that can have a real effect on fuel pressure. If you see a drop in voltage at the pump when the fuel pressure drops that might just be the problem. If the voltage at the pump doesnt fluctuate it may be the pump it self.
Even if it is not the problem I suggest hard wiring the fuel pump if you have not done it yet.


I like this idea, and this is probably next on my list....HOPEFULLY, it's not something stupid like a fuel filter being plugged.....shouldn't be the case though, bc the fuel fitler is only 6 months old and I don't let the tank get too empty before filling up.

[JeremyR]

any updates on this?