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Fluctuating Fuel Pressure

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Old 08-14-2018, 05:55 PM
  #21  
Pwnage1337
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The saga continues guys.

Car was off the road for about a month, blew a head gasket. That was my fault for beating on it in 100+ degree weather. I retarded my cam and ran a little thicker gasket to get my DCR into a more acceptable range since you can't buy 93 around here anymore, and you could when I built the engine. Also installed a 160* high flow T-Stat and a FlowKooler WP.

Initially my thought for the lean drivers side bank could have been the HG starting to let go. I was wrong. After putting the car back together, the issue persists. I've tried a couple things...

- Swapped injector harnesses, maybe a bad connection at an injector is causing one to not fire. - No Change
- One at a time, isolate vacuum to the FPR, climate control/cruise control, PCV system - No Change
-Pulled the plugs, and they all look pretty similar. Not like one is pure white while the others are nice and brown.

Thoughts of what I need to do/can do to diagnose:

-Move the fuel pump return in tank, what Bjankuski suggested.
-Check header primary temperature to see which cylinder could be running lean - ordered one of those thermal guns
-Swap injectors from bank to bank and see if the problem moves
-Swap the 02 sensors from bank to bank and see if the problem moves

Right now my order of business is:
Move the 02's - if the problem moves, I think i can be confident that i have a bad 02
Move the injectors - if the problem moves with the injectors, pretty safe bet that i have a plugged/faulty injector.
Burn the car to the ******* ground and start over

Other questions I have:
Is it possible for injectors to "partially fail" The car runs dead even at WOT, it's just under light throttle that the issue exists, and gets noticeably worse as the underhood heat gets up there.
Would an injector failing to fire cause any disruption in the fuel pressure and make it fluctuate? Doesn't seem likely but I'm not sure so that's why I'm asking. I could see maybe if the entire bank was failing to fire. If an entire bank was failing to fire i doubt the car would even run.

Edit: I haven't ruled out a Vacuum leak. I have tried finding one (for an hour straight) and can't find anything. Unless its pulling air from the lifter valley...

Thanks in advance

Jeremy

Last edited by Pwnage1337; 08-14-2018 at 07:46 PM.
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Old 08-17-2018, 08:14 AM
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bjankuski
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Does the WB sensor read lean or rich with the higher fuel pressure (45 PSI vs the 38 PSI) and when doe sthe car stumble, high or low pressure?
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Old 08-17-2018, 10:41 AM
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Did you try simply swapping out the fuel pressure regulator?
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Old 08-17-2018, 12:20 PM
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Originally Posted by bjankuski View Post
Does the WB sensor read lean or rich with the higher fuel pressure (45 PSI vs the 38 PSI) and when doe sthe car stumble, high or low pressure?
The WB reads "on target" (matches the drivers side bank) when the pressure is normal and reads lean when it swings down. The car stumbles under low pressure. The pressure swings very fast...like from 56-50 psi within a second, as fast as the computer screen can update its jumping around. I'll try and grab a video

Originally Posted by 383vett View Post
Did you try simply swapping out the fuel pressure regulator?
Its a HSR setup, it came with a regulator, which i presumed was bad, so i bought one of the Kirban billet regulators and the issue didn't change one bit.

I will add that I have bumped the FP up to 53 just to see if it was better - the car doesn't stumble and has better manners on tip in throttle, but the AFR on the drivers side bank is still wild and ends up running leaner than the passenger.

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Old 08-17-2018, 12:51 PM
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Please note that this was on a cold start up...the issue gets worse as the car is hot. I also wasn't driving (obviously)
Under driving conditions, I would expect to see the pressure fluctuate all the way from 50-58, rapidly, and the AFR would jump between 15.5-17 under light cruise. anywhere from 60-80 mph. Little touch of the gas to get above 10% throttle opening and the banks will balance out until the car returns to sub 10% throttle position then goes right back to 15.5-17 AFR



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Old 08-17-2018, 12:55 PM
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Had the same problem. Turned out to be the fuel pressure sender for the fuel pressure gauge.
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Old 08-17-2018, 01:00 PM
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I am curious about the fuel rail setup. From my understanding, this is how it is for the USM HSR's

The bottom of the regulator is the return, with the feed being at the front of the passenger side rail. There is a crossover pipe, underneath the orange "U Bend" that feeds the drivers side rail. Do you think with the regulator being so close to the feed source, and the drivers side rail being further away that it could cause fuel distribution issues? The vacuum line in the bottom left picture is the regulator vac source hook up. Removed it to see if issue persists without vac applied to regulator and it does. Port is capped at manifold

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Old 08-17-2018, 01:02 PM
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Originally Posted by sstonebreaker View Post
Had the same problem. Turned out to be the fuel pressure sender for the fuel pressure gauge.
Are you running an EZ EFI style system? I would think that the FP is a pretty big player in the overall scheme of the engine runnability. Just price checked and they are 80 bucks
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Old 08-17-2018, 01:44 PM
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Originally Posted by Pwnage1337 View Post
Are you running an EZ EFI style system? I would think that the FP is a pretty big player in the overall scheme of the engine runnability. Just price checked and they are 80 bucks
No, it was just an Autometer gauge and sender. We put a mechanical gauge on the schrader valve and fuel pressure was fine. Turned out to be a bad sender.

On your system, is the computer basing the injector pulse width on fuel pressure? I've never heard of a system doing that - usually they monitor AFR via the O2's.
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Old 08-17-2018, 01:53 PM
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I'm not sure if the FP is factored into the duty cycle of the injectors or not. I could try to find out. Either way, I need to get a gauge on it to see if it's actually moving like that. Probably have to find a 6an to Schrader valve fitting. My pressure tester is just a Schrader.
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Old 08-18-2018, 10:08 PM
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Originally Posted by Pwnage1337 View Post
I'm not sure if the FP is factored into the duty cycle of the injectors or not. I could try to find out. Either way, I need to get a gauge on it to see if it's actually moving like that. Probably have to find a 6an to Schrader valve fitting. My pressure tester is just a Schrader.
….. If the fuel pressure is actually changing then so is the fuel flow through the injectors ! …..
… ps … I would personally change the location of the fuel feed to the FPR … my feeling being that the bottom feed to the FPR and to the opposite fuel rail would allow any air that gets into either fuel rail to get trapped in the rail … there is no flushing action of fuel flowing through the rails … mine is plumbed with fuel into the passenger rail front ... crossover from rail to rail at the rear … and fuel out to the FPR from the front of drivers side rail … the only fuel going to the bottom of the rail is through the injectors … I'm NOT suggesting that this is causing your problem … it's just my Bobservation ! … Bob …..

Last edited by C409; 08-18-2018 at 10:26 PM.
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Old 08-19-2018, 11:20 AM
  #32  
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Thanks for your input!

i was looking at it and it is a confusing setup. Holley recommends exactly what you said. Fuel in one, crossover, then fuel out the other
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