Fuel pressure...is it supposed to drop @ WOT?

More specifically, is it supposed to drop @ WOT in the higher RPMs? From what I've heard others talk about it, I was always under the impression that fuel pressure raised with the opening of the throttle...is that wrong? The reason I ask is, I have a 1993 Camaro with a 255 lph Walbro pump, Racetronix fuel pump voltage booster, 30# fuel injectors, and an Aeromotive AFPR. Anyways, I just recently installed a electric fuel pressure gauge, and it reads 43 psi @ idle. If I take it WOT, the fuel pressure slowly drops progressively with the RPMs rising. At WOT @ ~5900 RPM, the fuel pressure is as low as 34 psi. There's no noticeable performance loss, nor is there any irregular "behavior" exhibited by the car; the only noticeable change is on the fuel pressure gauge. Is it supposed to do that? If it's not, anyone have an explanation as to why this is happening??

You don't want your fuel pressure to drop at WOT, you're running the engine lean. Stock fuel pressure is 41 - 47 psi with the vacuum line removed from the FPR (simulates WOT), so with the vacuum line on at idle, your fuel pressure would read around 36 psi. My Walbro 255 is able to supply my 593 rwhp setup with stock fuel lines. Try installing a new fuel filter as an old one will restrict fuel flow.

The FPR is vacuum referenced. Typically, vacuum is lowest at WOT so FP should drop as well. How much is it dropping?

Rich

It drops progressively as the RPMs increase, as much as a 9 psi drop @ 5900 RPM...

It drops progressively as the RPMs increase, as much as a 9 psi drop @ 5900 RPM...


If you have a vacuum gauge, you can be sure. But it seems like it may be dropping a bit more than I would expect. If you have 16" of vacuum at idle and it drops to 2" at WOT that would equate to a 7psi drop. 9psi equates to 18".

Rich

The FPR is vacuum referenced. Typically, vacuum is lowest at WOT so FP should drop as well. How much is it dropping?

Rich

I agree with you Rich that fuel pressure is vacuum referenced, but it is inversely proportional, that's why you see a rise in fuel pressure when you remove the vacuum line from the FPR when the engine is idling, as the regulator is now seeing atmospheric pressure (no vacuum).

I agree with you Rich that fuel pressure is vacuum referenced, but it is inversely proportional, that's why you see a rise in fuel pressure when you remove the vacuum line from the FPR when the engine is idling, as the regulator is now seeing atmospheric pressure (no vacuum).

DOH!!! Damn dyslexia (really, I have it). Less negative = more positive!!!

Rich

So, you're saying there's too much vacuum? You've lost me... :confused:

At idle you have full vacuum, fuel pressure should be lower than the pressure the injector was rated at. At WOT, there is little intake manifold vacuum, so the pressure should be higher than it was at idle.

The way to check:

-let the engine idle and pull the vacuum compensation line off (block the line to prevent a vacuum leak). Measure the fuel pressure. If your "tune" is based on LT1's standard fuel pressure, you would want to see 43.5psi. GM accepts anything from 41-47psi as being within specs.

-reattach the vacuum line. Fuel pressure at idle should drop in proportion to intake manifold vacuum. A drop of 8 psi would not be unusual with a stock cam, as little as 3 or 4psi drop with a more radical cam.

-tape the pressure gauge to the windshield, take it out on the road to max load/max RPM. The fuel pressure should not drop more than 2psi below the number you got at idle without the vacuum line.

Given the description of your system, its either a weak fuel pump, or yet another screwed up Aeromotive fuel pressure regulator.

Doesn't an Aeromotive rep frequent this board? Anyone know his screen name so I can contact him...?

The guy they sent to give me a hard time no longer works for Aeromotive. Not aware of anyone from Aeromotive currently on the board.

Wow old thread LOL

i have the same thing happeneing, only difrence is it is only happening when i am at 6000 rpms or so wich happens to e wen my car starts breaking up.

Anyways i have all stock fuel system, my question is can a stock FPR restrict the flow at WOT, or is that just with at aftermarket ones?

Gawd Injuneer you have been errrm "commenting" on Aeromotive for a long time LOL :thumb:

The LT1 fuel pressure regulator is not in the fuel supply line, so it can't restrict fuel supply side flow. The FPR is in the return line. It controls fuel pressure by pinching down on the fuel return flow. If you are using more fuel than the pump can supply, the FPR will close down completely, and the fuel pressure will start to drop. If the FPR has a leaky diaphragm, it may not hold the required pressure, since it would be fully closed and the diaphragm would be leaking fuel directly into the intake manifold via the vacuum line.

im still a little confused, and just need some reasurance.
At idel i took off the vaccum line and got about 45 psi. and when i put the vaccum line back on i got around 40 Psi (a have cc306 cam so less vaccume then stock) So this means my FPR is working correctly right? Also their is no gas in the vaccume line.

At WOT with the vaccume line still off my fuel presure stayed right at 45 psi (didnt move on psi from idel) But as i climbed the RPM's up to about 6000 the PSI went down to about 35-40(i couldnt stare at it to read the exact number) As soon as the car shifted and the rpms went down the PSI went back up. I heard that this is normal and nothing to worry about and also heard it means i need a new fuel pump (going to try the filter first). This could not be caused from the FPR correct? Since it holds the PSI allway untill i hit 6k RPM's

OK one more thing i noticed, if i hit the gas hard from like a 20 mph roll and put a lot of load on the engine the presure will drop for a second to about 40 PSI- Is this normal?

NOW if those 2 things are not normal and it cant be caused by a faulty FPR, can it be caused by my injectors staying open to long (the car is running at about 110% duty cycle at 6000 rpm's) or is it the pump that is not abel to supply the PSI that quick?

I know this is long and i have another post asking some of the same questions, i just didnt have all the knowlege when those where posted, so i am going to delet them.

im still a little confused, and just need some reasurance.
At idel i took off the vaccum line and got about 45 psi. and when i put the vaccum line back on i got around 40 Psi (a have cc306 cam so less vaccume then stock) So this means my FPR is working correctly right? Also their is no gas in the vaccume line.

yes

At WOT with the vaccume line still off my fuel presure stayed right at 45 psi (didnt move on psi from idel) But as i climbed the RPM's up to about 6000 the PSI went down to about 35-40(i couldnt stare at it to read the exact number) As soon as the car shifted and the rpms went down the PSI went back up. I heard that this is normal and nothing to worry about and also heard it means i need a new fuel pump (going to try the filter first). This could not be caused from the FPR correct? Since it holds the PSI allway untill i hit 6k RPM's

you'd like it to stay up there or you might run lean. You might try a voltage booster or larger wires.

OK one more thing i noticed, if i hit the gas hard from like a 20 mph roll and put a lot of load on the engine the presure will drop for a second to about 40 PSI- Is this normal?

same thing - not enough fuel

NOW if those 2 things are not normal and it cant be caused by a faulty FPR, can it be caused by my injectors staying open to long (the car is running at about 110% duty cycle at 6000 rpm's) or is it the pump that is not abel to supply the PSI that quick?

not enough volume

I know this is long and i have another post asking some of the same questions, i just didnt have all the knowlege when those where posted, so i am going to delet them.

tnx for the reply that cleared it up for me, ill seach for a voltage boster and see how that works.

1: Have you tried changing the fuel filter?

2: You have to get larger fuel injectors as they are too small.

Re'

Ok i got the larger injectors installed (LS1 28#'s for now) and i have NOT changed the fuel filter yet. Ill do that as soon as it stops raining.

IS their a way i can boost the fuel pumps performance temporarely, some one mentioned larger wires, and some ones i taked to said run a wire straight from the battery to the pump harness on the rear.

Can you guys explain a little more on how to do this?

An LS1 28# injector is the same size as a stock LT1 injector, when you run the LS1 injector in the 43.5psi LT1 fuel system. You haven't gained anything, unless you boosted the fuel pressure to the 58psi level the LS1 uses. You could have done the same thing with the stock injectors and turned them into 28 #/HR injectors.

I will assume you are reading your injector DC from DataMaster. From my experience, the DC reported in DM is higher than actual. Appears they calculate it by dividing the injector BASE pulse width by the time available for 2 revs of the crank. While that would be correct if the BASE pulse width is the same as the actual pulse width, I'm not so sure they are correct. With a good tune, the stock injectors would be supporting about 425 flywheel HP at 100% DC (which you don't want to run them at).

If you are making appreciably more than 425 HP, you need larger injectors. You also need a larger (or healthier) fuel pump. If the pressure is dropping, it isn't because the injectors "are staying open too long". The injectors are staying open as long as they need to, to support the HP (assuming the tune is correct). The fuel pump is not able to supply adequate fuel flow to meet the requirements of the engine.

Poor voltage at the fuel pump would cause it to flow poorly. Have you checked the voltage at the pump? Have you ever checked your system voltage at 6,000rpm? Is it maintaining close to 13V under those conditions? If the voltage is falling off, the pump will lose capacity.

Tnx Injuneer, you caught something that i would have never, thanks for really paying attention and thinking about my problem!

You know i got these freaking injectors at a f-body performace shop, i told them i need to get bigger injectors and they sold me the LS1 ones :mad:
I have to find a LT1 tuner, Some of these guys really know their sh*t about LS* engines but are in the dark when it comes to the LT1.

As far as me reading the Duty Cycle im doing it from auto tap, it show the pulse width (witch was 22.5 and 22.3 at 6025 rpms) and from another post of your i calculated the 112% DC. I know it isnt going to be 100% correct but with that and my plugs being all white after a WOT run i am pretty sure i am running lean up top. NOW if understand correctly this could be do to the fuel pump not keeping the presure at those RPM's and the computer trying to correct that buy opening the injectors longer. I dont think i am really near the 400+ HP mark, but i will find out to day on the dyno. If im not i will just chalk the injectors up to general mantinance :rolleyes:

I will check my voltage to the pump (is at the conector on the rear ok?) and check the voltage at high rpms (At the battery?) If that is fine then i will buy a performance fuel pump and see where i am with that.

Gawd i hope i get this fixed soon!

The PCM can't try and compensate for the low fuel flow/pressure at WOT (PE mode). It is ignoring the feedback from the O2 sensors at that point. The pulse width is based on the standard fuel calculation, dividing the MAF air flow by the target A/F ratio to get mass fuel flow, then multiplying that result by the long term fuel correction for Cell 15 (assuming the PCM was adding fuel in that Cell).

Your AutoTap scan will tell you what the "system voltage is". To measure the voltage at the fuel pump, get as close to the pump as you possibly can. In most cases, that's is the harness connector over the rear axle.