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**UPDATED** Fuel pressure dropping on boost referenced setup.
I'm going nuts trying to figure out why my fuel pressure isnt 1:1 with boost and drops as RPMs go up. I've been researching for weeks on this and havent found the answer. Tried and tested everything I could think of. I'm ready to throw in the towel with the boost referenced setup and just set the FP at static 65 and be done.
Here's what I have verified so far:
Tested regulator with a gauge and compressed air.
10PSI and 15 psi. The regulator and in car F.P. gauge responded perfectly.
10 gauge power from alternator to relay. 10 gauge wire from relay to connector that goes to the bulkhead.
10 gauge ground from fuel pump to body of car.
Voltage at pump at idle 13.8V and 13.5V while driving
Fuel pump is a brand new Walbro 535 (no check valve)
Note: **Experienced the same issue with dual walbro 255s except pressure was overall higher than the Walbro 535**
With the dual 255's, fuel pressure would jump to 66psi and then taper down to 50psi at the end of the pull.
With the single 535, pressure jumps to 56psi and tapers down to 50psi at the end of the run.
So different fuel pumps, different fuel lines, different fuel wiring = same issue.
The fuel rails are the same I've always used. The regulator is the same. Regulator seems to respond to compressed air through.
Here is a sketch of my fuel setup.
Last edited by 30th t/a; May 31, 2023 at 08:57 PM.
Have you verified voltage at the pump AT WOT? The bigger the pump the more sensitive they are to voltage under load. You might also check that the line in tank post pump doesn't have a tear or weak clamp ot something in it.
Have you verified voltage at the pump AT WOT? The bigger the pump the more sensitive they are to voltage under load. You might also check that the line in tank post pump doesn't have a tear or weak clamp ot something in it.
Thank you for the reply.
I have not checked voltage while WOT. That I will do.
As for the hose, I'm very confident that it is good. It's a brand new section of hose.
Also, with the regulator working how it should when I put compressed air to it and the FP rising 1:1, that should eliminate the hose from the equation.
Last edited by 30th t/a; May 28, 2023 at 12:26 PM.
I just did a 3rd gear wide open throttle and voltage stayed at 13.7.
I don't know what else to try other than buying another fuel regulator even though mine tested out fine with compressed air and maybe different fuel rails.
I know there is people on the Corvette forum running the same rails making a lot more power than I am
Yes. I'm sure on that. The pump is good to 700rwhp on e85 from my research. My setup is right at 700 with 18 degrees timing...but I pulled 6 degrees timing out so I know I'm not anywhere near 700 now.
Do you think the 8an feed line going into the 3/8" feed side of the corvette fuel rail could cause this issue?
someone mentioned that I should try an aftermarket fuel rail with 8an fitting for the feed.
Yes. I'm sure on that. The pump is good to 700rwhp on e85 from my research. My setup is right at 700 with 18 degrees timing...but I pulled 6 degrees timing out so I know I'm not anywhere near 700 now.
Do you think the 8an feed line going into the 3/8" feed side of the corvette fuel rail could cause this issue?
someone mentioned that I should try an aftermarket fuel rail with 8an fitting for the feed.
People have made much higher on a stock rail, I would venture to say even a stock fuel system except pump, injectors, and wiring. Are you saying the regulator is PRE-rail? I'm not a big fan of that.
Do you somehow have the pump in a stock style fuel bucket that could be sucking dry at high fuel flow? That used to be a common issue many years ago when people dropped higher flowing pumps in otherwise unmodified fuel buckets.
People have made much higher on a stock rail, I would venture to say even a stock fuel system except pump, injectors, and wiring. Are you saying the regulator is PRE-rail? I'm not a big fan of that.
Do you somehow have the pump in a stock style fuel bucket that could be sucking dry at high fuel flow? That used to be a common issue many years ago when people dropped higher flowing pumps in otherwise unmodified fuel buckets.
I got holes cut into the bucket.
I had the same issue with dual 255s.
ah firewall was blocking the pic. It may be worth trying a different regulator given that you tried different pumps.
Thanks. I'm going to try new fuel rails.
I considered changing the pump and still might
but like I've said, I've had this issue with dual 255s.
Hopefully the rails fix the issue.
Any chance that your boost reference line to the regulator could be getting pinched when the engine torques on it's mounts?
no chance of that. As you can see here. I've had it on and off a bunch of times through out this trouble shooting process and never saw anything wrong with it.
Rails??
Move the psi sensor to before the rail position and another gauge into the side of the regulator.
Do they match?
And, lose that shitty scotchloc wire thing! All that does is cut wire strands.
Are you SURE that sensor is accurate? I've seen many that are not linear.
What's with that screw hose clamp on the reference line?
its not clear what fuel you are using. also what injectors?
are you sure there isnt a restriction where you adapt the -8an line to the rail feed?
does the pressure go down correctly with vacuum?
id try dropping base pressure and see if it still caps out at 56psi
I am running this same exact fuel rail and line set up. Are you running the BAP on the 525? also add a mechanical gauge to the end of the rail to help verify your pressure sensor, might try that ?
After talking to Andy at A&A Corvette, I was explaining my fuel pressure with him and asking him about the Corvette rails that I was running and if they could be the issue. He said probably not. I told him I had a KB BAP sitting around that I was planning on using but I wanted this issue fixed before I installed it. He told me to go ahead and install it now, and if it fixes my issue, then the problem is my brand new 535 pump, if the issue is still there...then its something else. So I installed the BAP and did a 4th gear WOT pull.
Cruising fuel pressure is at 48-50psi , then fuel pressure jumps to 65psi when the BAP kicks in, 61psi at the end of the pull where I am making 8.5psi @4844rpm.
Compare this to the 535 by itself. 53.75psi at the hit and 54.81psi with making 8.5psi of boost @ 4800, at 5400rpm boost at 10.7psi, fuel pressure is at 53.12psi
This tells me that my new pump is weak. This 535 pump is suppose to support 700rwhp on e85 by itself.
Last edited by 30th t/a; May 31, 2023 at 10:03 PM.
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**UPDATED** Fuel pressure dropping on boost referenced setup. 
