Please help, Fuel pressure problem supercharged L98

[Tomas]

Hello everyone Need help, I have 50 psi fuel pressure at idle, but when i hit the gas pedal a little bit under load the pressure drops to 40 psi. Also under boost the pressure drops to 40psi . What can be wrong? A have Walbro 340 pump, new fuel filter, new fuel regulator and FMU( I have turned the screw full clockwise but the fuelproblem stays. I hope someone will answer me soon thanks regards Tomas

[texican2K]

Hi Tomas,
I just saw your post about eratic fuel pressure. You might go to this link and see if these guys can help. Do a search when you get there if the thread I linked to does not help. To me it just seems you may have a vacuum leak. Is this the regular FPR or is it the FMU?
http://www.thirdgen.org/techbb2/show...hreadid=115936

Good Luck & HLV
t2k

[Mo_Bandy]

could possibly be a ruptured diaprham inyour fuel pressure regulator... are you sure your gettting a good 13.8 volts to your fuel pump..?

coulpe of thoughts...

Mo

[AKS Racing]

I've seen you post several times on this topic. As no one has really taken an interest, I thought I might offer some advice.

First, this will be a static test. The car has to be somewhere so that the fumes of an overly rich engine will not kill anyone near the test (complete test takes 30-45 minutes).

You will need a source of continuous compressed air (air compressor), a handheld psi guage (0-30 psi or somewhere there about), an easily accessable fuel psi guage (may read off of the fuel rail), a fabricated manifold (details below).

Manifold:
quick disconnect fitting: male QD x 3/8" NPT
isolation valve: 3/8" valve (ball valve from Home Depot??)
swedge fitting: 3/8" x 1/4"
regulator valve: 1/4" gate valve or oriface valve (auto parts store)
hose barb: 1/4" NPT x 3/16" barb
hose: 3' of 3/16" hose
tee fitting: 3/16" x 3/16" x 3/16"

Connect QD that matches air hose supply to isolation valve. Reduce line size using swedge. Attach regulator valve. Convert to rubber hose using hose barb fitting. Connect approx 3" piece of hose to end of barb. Add tee to air line with 3" piece off one side, and 2.5' piece off of oppoosing side of tee. Connect psi guage (will be called boost guage in test) to 3" stub. This manifold will allow static testing of the fuel system.

Once all items have been gathered, and the manifold has been fabricated without leaks. Proceed with test.

Test:
Close adjustment screw on FMU completely (screw in). Place mark on body and screw of FMU, so that adjustments can be accurately recorded. Plug vacuum line leading to FMU (oriface must remain in line that will be used for test). FP regulator on engine will remain connected (operational during test). Close manifold isolation valve. Connect manifold to compressed air source. Connect manifold to inlet to FMU (with oriface inline). Adjust regulator valve to closed. Adjust FMU screw to 4 turns out. Start engine. Allow to warm up. Stick screwdriver behing throttle stop to allow engine to run at 1800-2000 RPM.

Please note, do not exceed 80 psi (typically safe from 4 turns out to 2 turns out), as the system will attempt to find ways to relieve the pressure. Raw fuel, hot motor, disaster!!!

Record FMU screw position, regulator guage, and FP (vacuum/no boost). Barely crack regulator valve (boost guage on manifold should still read 0 psi). Open isolation valve. Adjust regulator valve to 2 psi. Record FMU, boost guage, and FP.
Turn FMU screw to 3 turns out. Record FMU, boost guage, and FP.
Turn FMU screw to 2 turns out. Record FMU, boost guage, and FP.
Turn FMU screw to 4 turns out. Adjust regulator to 4 psi. Record FMU, boost guage, and FP.
Turn FMU screw to 3 turns out. Record FMU, boost guage, and FP.
Turn FMU screw to 2 turns out. Record FMU, boost guage, and FP.
Ect., Ect., Ect.
Keep going up in regulator psi, simulating boost.

Take note of how much FP is being added for each reference pound of boost. This test will take probably 30 minutes to run, and the engine at times will run very rich. Take precautions to deal with the fumes.

If the system never shows additional FP, the FMU diaphragm has probably been ruptured. This can be caused, if the system was initially started with the FMU screw full in. The ATI supplied booster pump in the system will push to 120-130 psi and the FMU will fail.

If the system psi increases, then proceeds to fall off, the fuel system is not adequate. Upgrade time! When mine was at the 500 RWHP level the factory system had been upgraded with 3/8" line and different filter. I never saw degredationin fuel pressure prior, but did not want the risk.

Hope this has helped. I put together a graph at 1/4 turn FMU increments, and 0-10psi boost levels that detailed the ATI supplied FMU.

If all else fails, F.A.S.T. is an option (though a rather expensive option.

Aaron

[Tomas]

Hello Aaron

Thanks for test program that you gave me. Before I start the test i have some few questions. When I installed the Walbro 340 fuelpump Jack at Racetronix told me that I should take the ATI booster pump away because the pump will only reduce the fuel volume. Is that true?? Or should i install it again??Can I open the FMU and see if the diaphragm has been ruptured??

I hope you will answer me soon

regards

Tomas

[AKS Racing]

Tomas,
First there is a difference between psi and volume. I do not know what a Walbro 340 is rated at deadheaded, but it may be that the Walbro is made to push a lot of volume, but not handle the psi. What are the specs of the Walbro 340? Your system, with the FMU is designed to max the psi while delivering adequate volume to deliver the required mass of fuel. True, the ATI may not deliver equivalent fuel, but it is designed to deliver adequate fuel at a much higher psi in conjunction with the factory pump. Did you remove both the factory pump and the ATI booster pump? The factory pump is still required in conjunction with the ATI booster pump. My new system utilizes an aftermarket pump to supply the volume and a booster as well to get the psi required with the FMU (also have a 2nd pump in the tank for the stand alone system for the N2O). I already have plans to replace the in tank and the booster upon change to the low pressure system in conjunction with FAST if there are any signs of degredation of fuel supply.

Aaron

[gcrouse]

Is that 50psi at idle with the vacuum line connected?

The more vacuum applied to the regulator, the lower the fuel pressure; fact is there is more vacuum with the throttle slightly to 1/4 open than there is at idle (because the rpm's are higher and the engine is sucking more air). FYI, mine is 10-11" at idle and 20" at 2000rpm; so the fuel pressure drops.

As for the lack of fuel pressure under boost, that usuallly means a flow restriction somewhere in the system: fuel pump, fuel line, fuel filter; or maybe a bad regulator or FMU. You have a good fuel pump there and the other stuff is new; you're stock lines should be fine unless you're over 500rwhp. It's weird because when that FMU slams shut under boost the pressure should spike pretty high; but it sounds like your injectors are flowing more fuel than you can supply- so the pressure is dropping. Is it possible you have a flattened fuel line underneath somewhere?

It could also be that ATI inline pump if you haven't removed it yet- it doesn't flow a lot of volume and is there to help the stock in-tank pump maintain higher fuel pressures- especially with an FMU. If you have the Walbro (which is quite capable of 80psi), you need to remove the inline since all it's doing is restricting the flow at this point.

Also- just for fun, repeat your testing with the vacuum line disconnected from the regulator (and plugged) just to rule out weird vacuum probs. And make sure the vacuum source you're using is straight from the plenum- not ported vacuum from the throttle body; or vacuum from behind a check valve!


[Modified by gcrouse, 1:41 AM 7/7/2002]

[krivera]

Well i would really like to know the problem if it gets resolved? i am having the exact problem? 45psi at idle and under boost...46psi? but i think it is the FMU or Fuel Filter? i just changed the diagram on the reg blew it testing the FMU :o