fuel pressure while driving
Thread Starter
Drifting
Joined: Apr 2014
Posts: 1,699
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From: Hawthorne CA
fuel pressure while driving
I attached a fuel pressure gauge and I have ~37psi at idle and 42 psi at idle with vacuum hose off the FPR. I taped the gauge to the windshield and went for a drive. I noticed the fuel pressure can be as low as 32psi when decelerating but as high as 44psi on open throttle. Is this normal ranges?
Team Owner
Joined: Jul 1999
Posts: 65,492
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From: Orange Park Florida
24 lbs inj, LT1 red intake, 43.5 is the operating pressure.....idle I was running 30+ range and running RICH and engine black smoke, stalled at idle, rough as ****....idle adj all over the joint.....no go....FINALLY found out the pressure should have been 43+ and the only explanation I can figger out is that with the lo pressure, the comp says stay on longer and to instead of a decent spray the fuel dribbles out and so lots of it goes into the pipes and afterburnt, making the black smoke, and of course idles like crap.....the mystery was that ~2500+ rpm under load, everything smoothed out.....but I unsure about exhaust color/smell.....
Drifting
Joined: Jun 2018
Posts: 1,678
Likes: 229
From: Michigan
I attached a fuel pressure gauge and I have ~37psi at idle and 42 psi at idle with vacuum hose off the FPR. I taped the gauge to the windshield and went for a drive. I noticed the fuel pressure can be as low as 32psi when decelerating but as high as 44psi on open throttle. Is this normal ranges?
At Idle you are pulling vacuum (in ref to gage) or have low absolute pressure. This reduces spring pressure in your FPR opening the return side and lowering pressure
When you open the throttle up, vacuum decreases (Manifold absolute pressure rises) and this reduces the pull and allows the spring to close the opening. Raising Fuel pressure.
An adjustable fuel pressure regulator operates by allowing you to adjust the spring pre-load.
During deceleration, the vacuum rises above idle state and thus you will see a bit lower pressure. I am surprised you see a bit more on acceleration that with the vacuum line off without boost. It's pretty close though, could just be error in the gage or you eye *****.
Last edited by KyleF; Jul 29, 2020 at 09:47 AM.
Drifting
Joined: Jun 2018
Posts: 1,678
Likes: 229
From: Michigan
Yes, this is what the LT1/L98 injectors are rated at and typical WOT/VAC line removed pressure target.
This sounds more like a tuning issue. Both my L98 and LT4 have fuel pressures below 40PSI at idle. Could ask a lot of questions, but mainly what injectors? Sounds like it could have been the Voltage offsets that get ignored on this forum but are known to be a tuning issue on another L98 based platform forum.
I am running B3's in an Iron headed L98 (IROC). It ran OK and recently I moved over to an EBL system so I could start working on my tune. Once the injector offsets were adjusted, the car idles smoother, and even my wife commented on the lack of exhaust smell in the garage after I start it or bring it back in after a drive. My IROC had the cat removed by the PO so you can really smell the mixture changes. It did always run fine in the upper RPMS. I actually had very little corrections to do to the Low Speed VE table after the injector voltage offsets were changed. I have read a few ways guys get around doing it this way, but in the end it is about tuning no matter how you approach it.
.....idle I was running 30+ range and running RICH and engine black smoke, stalled at idle, rough as ****....idle adj all over the joint.....no go....FINALLY found out the pressure should have been 43+ and the only explanation I can figger out is that with the lo pressure, the comp says stay on longer and to instead of a decent spray the fuel dribbles out and so lots of it goes into the pipes and afterburnt, making the black smoke, and of course idles like crap.....the mystery was that ~2500+ rpm under load, everything smoothed out.....but I unsure about exhaust color/smell.....
I am running B3's in an Iron headed L98 (IROC). It ran OK and recently I moved over to an EBL system so I could start working on my tune. Once the injector offsets were adjusted, the car idles smoother, and even my wife commented on the lack of exhaust smell in the garage after I start it or bring it back in after a drive. My IROC had the cat removed by the PO so you can really smell the mixture changes. It did always run fine in the upper RPMS. I actually had very little corrections to do to the Low Speed VE table after the injector voltage offsets were changed. I have read a few ways guys get around doing it this way, but in the end it is about tuning no matter how you approach it.
Last edited by KyleF; Jul 29, 2020 at 09:56 AM.
