Thread: WOT fuel pressure

Anyone else have a fuel pressure gauge installed? Before I top swapped, at WOT I saw about 60PSI. Now, I see 70PSI at WOT. Just wondering if that's normal because idle is like 50PSI and if I gun it from right there it's shocking to see the pressure go up 20PSI that quickly.

Are you using the stock fuel pressure regulator?

-Riggs.

Yep. Still using the one that came in my L67 fuel rails. It holds pressure at key off just fine and idles at the right pressures, so I presume it's good.

I don't have a gauge, but I imagine the regulator in the L67 rails keeps a Higher pressure due to the higher flowing injectors and the hungrier Boost fed motor

i will have my gauge in within the next couple of weeks if you want to wait, hah.

Originally Posted by Bio248

i will have my gauge in within the next couple of weeks if you want to wait, hah.

I'm not going anywhere.

It's actually good to know I've got that much pressure up top to play with. The pump isn't anywhere near running out. And my 42.5# injectors are only getting up to about 15-16ms IPWs. Lots of fuel if I ever need it.

I just want to make sure that much pressure is normal and okay for the system.

i would say that youre okay. in the 60-70 range WOT doesnt seem like too much to me since idle is like 40-50.

I almost forgot about this thread.....

Anyways, the stock L67 fuel pressure regulator will show 50psi with the vacuum source disconnected.
So, under vacuum at idle it should be 30ish I think.
Either way, the stock fuel pressure is 50psi when not under vacuum.

-Riggs.

30PSI at idle? That's way too low. Every source I can find for the L67 says around 50 PSI at idle and up near 60PSI with the engine not running.

I just can't find info for what it should be at WOT. Seems no one expects someone to have a fuel pressure gauge installed in the car.

Interesting. Could be that my FP gauge isn't 100% accurate, but it's never given me reason to suspect it. I see right under 70PSI at WOT. Pressures at all other times are normal according to tech data.

Yeah, a stock L67 regulator as far as I know. I figured I'd try the one that came in the L67 rails I bought and for all other times it works just fine. No reason to change it so far.

I'm making only about 6-8PSI. I've got a 3.8" on there right now and I have 1.9 rockers.

And baby steps? I thought I was taking baby steps by tuning on a 3.8".

Sounds good. Makes me feel better that at some point someone else has seen the same thing.

30PSI at idle? That's way too low. Every source I can find for the L67 says around 50 PSI at idle and up near 60PSI with the engine not running.

You do realize that the FPR is adjusted according to manifold pressure, right?
That's the reason that the regulator is hooked up to a vacuum source. When you're at high vacuum (like at idle/low RPM's, etc) the fuel pressure will be low, and when you're at low vacuum (or boost on forced induction) the fuel pressure will be higher.
This is to adjust the fuel pressure according to fuel demand, as the injectors will flow less at a lower pressure and will flow more under higher pressure.

As someone mentioned before, it adjusts at a 1:1 rate so if you're seeing 50psi under atmospheric pressure (at idle with the vacuum source disconnected) then you would see ~30psi when you are at idle (which is about -20psig manifold pressure).

The factory service manual specifies that the fuel pressure should be between 53 and 59psi with the fuel pump on and the engine off. The 50psi at idle that everyone is referring to is 50psi at idle with the vacuum source disconnected. The FPR is set to 3.5bar when the vacuum source is at atmospheric pressure, which would be exactly 50.67psi IIRC.
-Riggs.

At idle, vacuum connected, mine is at 50 PSI. I see as much as 60 PSI during normal driving, and WOT is up near 70PSI. Key on but engine off is 60PSI.

Nowhere in the GM SI does it refer to taking the vacuum source off to measure the 50PSI idle pressure. And the GM SI talks dumb to you, too, labeling every step. It says to simply connect the FP gauge and start the car and you should have 50 PSI. Goes for the L36 and the L67.

I saw the same idle and cruising numbers on my L36 as my L67 and it had a brand new FPR (and they go with what my Haynes and GM SI say), so I doubt they're wrong. I've never seen it go below 50PSI with the engine running.

I'm just going by what both PRJ and ZZP told me, as I had inquired about setting the pressure on my adjustable fuel pressure regulator. They both said to unhook the vacuum line and to set the fuel pressure to around 50psi at idle to set the pressure.

The regulator will fluctuate a bit according to manifold pressure though, which would explain why you are seeing a much higher pressure at WOT than at idle.

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ClubGP Message Forum troubleshooting


Where exactly is your fuel pressure gauge hooked up to?

-Riggs.

Originally Posted by BiggRiggs

The regulator will fluctuate a bit according to manifold pressure though, which would explain why you are seeing a much higher pressure at WOT than at idle.

I understand that part, but my question was if 70PSI was where it's supposed to be. It makes sense, though. If the L36 uses 60PSI of pressure for WOT, and the L67 makes about 10lbs of boost stock, then the L67 fuel rail pressure would need to be 10PSI higher than the L36 to keep the same flow on the same injectors. The difference between manifold pressure and fuel rail pressure is still 60PSI in both cases.

I'm not disputing PRJ's method, but that's not the data GM gives for a stock FPR.

Originally Posted by BiggRiggs

Where exactly is your fuel pressure gauge hooked up to?

Schrader valve on the fuel rail.