Fuel pressure high at idle, low in boost

[Haifisch M3]

I show 67 psi fuel pressure at idle and 62-63 psi at 14 psi of boost, AFRs are lean at 12.6+ right now


E36 M3 TT st 2 with aeromotive 340 fuel pump and stock FPR

before throwing a new FPR at this , just wondering if this could be caused by something else?

[Turbohugh]

I show 67 psi fuel pressure at idle and 62-63 psi at 14 psi of boost, AFRs are lean at 12.6+ right now


E36 M3 TT st 2 with aeromotive 340 fuel pump and stock FPR

before throwing a new FPR at this , just wondering if this could be caused by something else?

Hose going to the fuel pressure regulator ok ?


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[Haifisch M3]

Hose going to the fuel pressure regulator ok ?


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haven't really gotten under there yet

[Braymond141]

Vacuum line is an obvious first step, but that fuel pressure reading, if correct, is sky high. I've pulled 5 FPR from junk yard cars and tested them with wildly different results (ranging from 2.5bar to 4.35bar)... so it is possible your FPR is bad.

That said... I could never get the stock 3/2 valve setup to work properly with a high flow fuel pump. It over powers it. When you go to replace the FPR, make sure you use a new Pierburg only. Do not bother with a Bosch unit, they come out of spec (two brand new ones read 4.0bar on my bench tester). Don't be surprised if fuel pressure is still an issue, even with new parts.

[Haifisch M3]

Vacuum line is an obvious first step, but that fuel pressure reading, if correct, is sky high. I've pulled 5 FPR from junk yard cars and tested them with wildly different results (ranging from 2.5bar to 4.35bar)... so it is possible your FPR is bad.

That said... I could never get the stock 3/2 valve setup to work properly with a high flow fuel pump. It over powers it. When you go to replace the FPR, make sure you use a new Pierburg only. Do not bother with a Bosch unit, they come out of spec (two brand new ones read 4.0bar on my bench tester). Don't be surprised if fuel pressure is still an issue, even with new parts.

looks like I am over 4.5 bar with mine
Never heard of the Pierburg one, do you have any experience with aeromotive 13109 FPR?

[someguy2800]

Sounds to me like your pump is just overrunning the regulator at idle, which is when nearly all the fuel needs to go through the regulator. The pressure in boost is about right.

[cj.surr]

Sounds to me like your pump is just overrunning the regulator at idle, which is when nearly all the fuel needs to go through the regulator. The pressure in boost is about right.

This. I always encounter this problem with Walbro 450 and stock fpr.

[Haifisch M3]

Sounds to me like your pump is just overrunning the regulator at idle, which is when nearly all the fuel needs to go through the regulator. The pressure in boost is about right.

so this is nothing to be concerned about, does not affect fuel-supply in boost?

[someguy2800]

It’s going to make idle and part throttle difficult to tune. Should get an aftermarket regulator and then get a tune revision.

[Haifisch M3]

interesting thing is that it idles fine but AFRs are a bit lean in boost

[ba114]

A 340 wont overpower the stock regulator. Not from my experience anyway....450 is a different matter.

[bmw328m52]

A 340 wont overpower the stock regulator. Not from my experience anyway....450 is a different matter.

I'm running the 340lph from rally road. It definitely overpowers the stock 3/2 valve causing idle issues and rich conditions in boost. Went to the aeromotive 13109 and haven't had any problems since. I also had a lean condition at some point and it was the vacuum line going to the fpr. Make sure it is still in tact.

From previous builds I've seen, it looks the stock 3/2 valve can only keep up with the 255 pump. Anything higher than that should really have an adjustable fpr to be safe.

[Braymond141]

I wouldn't say it overpowers the FPR (I've never tested a standalone FPR before), but 340lph+ most definitely overpower the 3/2 valve setup. If I recall correctly, even a 255lph pump can cause issues.

[chikinhed]

You can disconnect the 3/2 valve from the fuel system when you install an aftermarket FPR, just leave the electrical connector connected to the 3/2 valve.