Fuel Pressure Drop - Baffled...

Problem:
From -18 to 0 psi, fuel pressure rises to about 46psi. At 1psi the fuel pressure drops down to where it normally is at idle (about 38psi). As boost comes on, fuel pressure rises at what appears to be .5psi for every 1psi of boost (not 1:1 ratio).

What I've done so far:
-Replaced vaccum hoses
-Replaced FPR
-Bypassed FPR Solenoid
-Replaced Hotwired Supra FP with Walbro FP (later reinstalled Supra)
-Re-did the Hotwire
-Swapped TruBoost controller for Halman PRO (MBC)
-Pressure tested (slight leak at throttle body butterfly - spring area)

I don't know what else to do or check, so I'm hoping for ideas on what might be causing this.

Fuel filter? Didn't see it on your list

I have been told about the fuel filter, and though I do plan on changing it out this weekend, I just don't see how that could possibly be the culprit considering the way fuel pressure rises but then drops (the drop being tied to going from a negative pressure to a positive pressure.)

But I appreciate your input and hope that it does cure this weird symptom.

FWIW: This weekend I'll also be doing spark plugs, motor oil, installing a new UEGO WB sensor and, if time permits, differential.

Wouldn't a clogged filter cause your fuel pressure to increase? Like putting your thumb over the opening of a hose.

Faulty gauge? Whats the wideband read from -18-0?

What kind of fuel pressure regulator?

Quote:

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Originally Posted by Frostedbutts

Wouldn't a clogged filter cause your fuel pressure to increase? Like putting your thumb over the opening of a hose.

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Not when you're measuring after the restriction...

Fuel filter replaced. Problem persists. Any other suggestions I can try now since I'm still under the hood at the moment wiring in a serial cable to the uego wb02 so I can log using evoscan?

Quote:

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Originally Posted by rawavr4

Faulty gauge? Whats the wideband read from -18-0?

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This was my thought. What are you measuring the fuel pressure with?

-Chris

Both the adjustable FPR has a fuel pressure gauge (in engine bay) and I have a Prosport Fuel Pressure gauge (electric and in cabin). Both read the same at idle and if I use the FPR to up the fuel pressure or bring it down, both gauges show the change.

I don't know how i can monitor both gauges while actually driving the car. But I figured since both are showing same change when the fpr is used to set fuel pressure, I was good.

As to the AFR during this episode, I'm still putting in the wiring as i mentioned earlier. So I don't know the AFR. NB02's show .95/.96 WOT at 1bar, fwiw.

As your fuel pressure drops and then fails to keep up with boost, do your IDC's rise higher than usual, the ignition timing stays low and the car is SLOW? I guess what I'm wondering... Did the fuel pressure ever rise 1:1 with boost? If so, when? What changed?

IDC's look the same as always. This past Tuesday, I replaced the 550's with 780's and used ECUFlash to modify injector size.

Timing isn't changed from what I'm used to seeing when I log. The car is ok as far as performance, except there is alot of breakup (hesitation, stumbling) when you just stomp on the gas (go WOT). If you ease into the peddle, that breakup (hesitation, stumbling) is far less pronounced.

Here is a log (after I installed the 780's), please let me know your thoughts. The pink area is low-RPM high load knock that I need to figure out - but I believe it's due in part to this fuel pressure issue I'm experiencing.

http://i139.photobucket.com/albums/q.../WTH_Knock.jpg

Fuel pressure, in the past, did rise 1:1. This is new to me and have yet to figure it out. Car has alot of what seems like backfires and seems quite a bit quieter when idling than I'm used to - even had several non-car friends say this to me out of the blue.

At about 3k rpm, the gas peddle seems to lose it's resolution as it feels like it's an on/off switch for gas and not a smooth transition in power as the peddle is depressed. But this only happens just over 3k rpm and this symptom I've become aware of with this fuel pressure issue.

What`s the voltage measured across the pump when the drop happens?. Is the FPR on a dedicated vac line?.

Only other thought is you have a problem with the fuel pump hanger/ hard line inside the tank?.

Voltage at pump while idle is just over 13v. I would need someone with me to see it during actual drop. FPR is on stock vacuum line (aka: is on a dedicated vacuum line).

On the fuel pump hanger:
The portion that meets up with the fuel pump that looks like a bell has been removed and a piece of high pressure fuel hose was used to go from the fuel pump to the remainder of the hardline. The hose is clamped down with two (2) clamps on each end (2 on the fuel pump side, 2 on the hardline side) and all this is new; replaced when switching out the fuel pump.

What could cause this weird fuel pressure problem?? I'm stumped :/

I'm on page 62 of a 115 page thread over at the other site...but still looking for help on this...

Im Still thinking. This is weird. It seems like it has to be vacuum related. Also you can make some long (5-6ft) alligator clips and, hook em to the pump while you drive it and check the voltage from the drivers seat.

Make sure the oring on the top of the fuel pump is not cut/pinched/compromised or otherwise not installed incorrectly.

Make sure the tank can breath. If you did a vac reduction and or charcoal can removal, capping all the lines can create a vacuum in the tank making it harder for the pump.

-Chris

Quote:

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Originally Posted by rawavr4

Im Still thinking. This is weird. It seems like it has to be vacuum related. Also you can make some long (5-6ft) alligator clips and, hook em to the pump while you drive it and check the voltage from the drivers seat.

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I will try this, time permitting, this weekend. There is an airshow that I really like to attend - they have lots of Warbirds :)

Quote:

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Originally Posted by Ninja Performance

Make sure the oring on the top of the fuel pump is not cut/pinched/compromised or otherwise not installed incorrectly.

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The "bell" portion of the fuel pump assembly/tray was cut off and a piece of high-pressure fuel hose was used to go from the remaining hardline to the fuel pump. This does away with the need for the rubber o-ring and the two plastick pieces that go around the o-ring. This host is clamped with two clamps at each end of it.

Quote:

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Originally Posted by Ninja Performance

Make sure the tank can breath. If you did a vac reduction and or charcoal can removal, capping all the lines can create a vacuum in the tank making it harder for the pump.

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No vacuum reduction done. However, I did have to relocate the charcoal canister, several years back, to install the GReddy FMIC kit. I will check this to make sure nothings wrong.

Yesterday, I had a friend pick up another FPR. This one is bran new and after installing it, the same thing happens. Going from -17 to 0psi, the fuel pressure would rise from 38psi to about 46psi. As soon as the boost gauge see's about 1psi of boost, the fuel pressure drops back down to about 37-38psi and if boost continues to rise, the fuel pressure will rise; but not in a 1:1 manner, more like .5:1 ratio.

Can the EGR cause what I'm seeing? I ask because it's the only thing on the plenum that could potentially cause a rise/drop in pressure within the plenum.

Checking in and a little update..

Over the past couple days, I've checked all the vacuum lines (had to reroute a couple as they were incorrectly connected) to make sure all was correct and in good shape. I noticed that one of the vacuum lines (at the EVAP) appeared ripped and so I replaced it. Some more boost leak testing was done and another FPR was tried.

Problem presists. I have noticed, though, that my fuel pressure is better (after the vacuum line check) and seems to rise closer to 1:1. I think I would leave this alone if it wasn't for the initial rise (coming out of vacuum), then drop, then rise again (in boost) as it causes the EUGO WB to swing wildly from rich to lean.

Any other suggestions? Can the EGR cause this?

Thanks


What I've done so far:
-Replaced vaccum hoses
-Replaced FPR
-Bypassed FPR Solenoid
-Replaced Hotwired Supra FP with Walbro FP (later reinstalled Supra)
-Re-did the Hotwire
-Swapped TruBoost controller for Halman PRO (MBC)
-Pressure tested (slight leak at throttle body butterfly - spring area)

I would do a pressure test too. I have been fighting this problem for a while, but now im the middle of a complete teardown so I haven't really had it in the top of my priority list.

My fuel pressure would drop like a rock once I hit boost and then slowly come back up. I replaced FPR, fuel filter, fuel pumps (im running twin walbros), checked all lines etc. After repalcing the filter it improved, yet still did not build like it should. Once conclusion I came to is a possible fuel leak somewhere and only does it under boost when the pressure tries to rise, i/e at the fuel rails, however I smelled no gas from under the engine but its possible something back at the tank could be causing a leak.. It was perfect for years with this setup. Im hoping after this teardown its gone, because I will have a chance to really go thru the system. Im running -8an all the way from the hanger to the filter, so Id doubt there is any restriciton. Another suggestion is the source of the vaccum, I changed the source to a direct line from the plenum, while in theory this should be a better vaccum source, its possible that port has a restriction, i used what used to go to the bov. So I plan on relocating the source to a vaccum dist block. I know there is some sediment in my fuel system due to the car sitting for 3 years with no stabilizer, so during this teardown im gonna do something to refurbish the tank and get all sediment out. Im sure all of the above will cure it, but I never really did pinpoint it.

when you did the hotwire, did you also bypass the changeover relay? Doesn't sound like the culprit, but it does have a history of causing issues if left in there.

What fuel pressure regulators are you using? Both your original and the test one. I had a problem with the AEM fpr where the orifice ball seat was worn dropping pressure because it wouldn't seal. Also do your fprs have changable orifice sizes? Maybe try a smaller one.

This probably won't help much since you already know but here is a profile of rpm/boost (subtract 14.28psi)/and injector duty cycle. Stock fpr and hotwired walbro.

https://dl.dropbox.com/u/29716828/3si/boostvsfuel.PNG