This is weeks of troubleshooting, sorry for the novel. This is an NA LS3 swap on a '99 C5, stock LS3 injectors and rail. TSP cam, headers, etc. Never had a problem with fuel with the LS1, but this is obviously a thirstier engine. After the first failed dyno pull, the tuner said I needed a new fuel pump, the duty cycles were too high and it was leaning out. Got a walbro 255lph. Went in for a second dyno pull, still a problem.

There's no reason a 255lph pump won't work for a 500HP NA engine. I replaced the regulator/filter, got a pressure gauge and measured at the rail. Seeing 65psi during run, drops to 55 at engine stop and stays there, doesn't seem to be a leak. Third dyno pull, still a problem.

Took out the pump and checked the lines, everything looks good. Checked the lines from the regulator to the pump, nice and round, blew through the lines, no blockage. Replaced the whole fuel line with steel braided, added an electric fuel pressure sending unit and installed the gauge in the dash, now seeing 72psi running (hmmm.. that's high). Went out for some pulls and noticed a slight drop on short WOT runs but never got below 65psi (I won't risk my license to test it out to redline 4th the way it's done on the dyno). Fourth dyno pull - guess what - still the same problem. Not as bad, but he won't run it past 5500 rpm because AF gets into the 13s and duty cycle is over 120%. Decided to track down this high PSI issue, must be part of the same problem.

Replaced the regulator again, still seeing about 70psi running. Installed a direct voltage wiring kit for the pump. Now seeing 75-80psi running (hmmm... that's really high). And as the pressure increases, the car is harder to start. It turns over forever and fires off randomly until it finally settles into idle. Took the pump out of the tank again and checked everything. Cleaned off the return line on the sending unit and tried blowing through it. Seems like there's an obstruction, sounds like a trumpet. I removed the sending unit from the pump canister and popped off the end of the return line. Under there's a small piece of red plastic on the end of the tube. I pried it off and replaced the end cap. Now there's no obstruction in the line. Why would that be there from the factory?. Left the new wiring kit installed, figure it can't hurt.

Refilled the tank, started up, it started hard again, and shows 75-80 psi running. What could be keeping the regulator(s) from working? It regulates by dumping gas down the return, and I've confirmed that there is no blockage there. The lines will only connect one way at the regulator, and I've confirmed they're correct (green/blue/black). If you guys have ever seen the pump, the return line is on a spring that allows the pump to move around on a hinge, the tube moves in and out of a grate with an end cap on the underside of the sending unit. What's the reason for all that? Could it still be blocked in some way by that mechanism? I won't try another dyno pull until I get the pressure regulated at least. - Thanks for any advice.

 

Lot going on here. Being LS3 injectors and rail, that means the FPR on the 99 rail with the intake manifold reference is not there right? Is your fuel system closer to the one on my 2002 with the filter/regulator combo near the tank and no return from the engine bay?

 

Hi oharal - my '99 system was return-less with just a pulse damper, no FPR at the rail. FPR/filter was always on the frame rail in the back. The LS3 stock rail connected up the same way my LS1 did. From what I think I know, the 99-2003 models have a non-adjustable 58psi FPR that keeps the downstream pressure regulated via a spring diaphragm, and any excess pressure is bled out the return from the FPR. That being the case, either the FPR is broken (both new OE units), or it can't handle the flow from the new pump and allows the pressure to build at the rail. My plan is to try replacing the line from the FPR to the tank with a standard 3/8 hose, and if that doesn't work, dumping into a bucket to see if the pressure is reduced.

 

Hey so thanks to everyone who viewed this, I know it's a lot of info. After re-tracing my steps and re-testing a few things, I'm now seeing there's a problem with the check valve on the main feed. Pressure only gets to 30 when primed, and drops off to 30 after key off. Ordering a whole new 6AN setup for all the lines from the sending unit to the FPR and siphon feed. I don't know enough about fluid dynamics to say how that could be the cause of the high pressure issue, but it certainly is causing hard starts/long cranks.

 

so it goes up to 30 then back to zero if i'm understanding this or u start the car goes to 70 then falls 30psi??

 

With key-on and primed (2 sec of pump) it rises to about 30-40, it used to come up to 50. If I get it started the pressure shows 75-80psi running, then when I shut off the engine it drops immediately to about 30, used to stay at 55. Hoping the check valve has been causing the pressure problem, but that doesn't explain the drop in flow on dyno, hopefully the racetronix harness will fix that. Lots of speculation but feels like I'm getting closer.

 

my eyes r freaking out reading it over and over. have a memerior issue. did u try passing the fpr all together?? is there an adjustment on fpr??
My set up for the pass 12yrs is it primes to 60 then when i shut it off goes to zero. litery before i even turn the key on i'm at 60 and never had an issue. i guess what im saying is losing psi should not be an issue. i forgot r u using the stock return line??

 

@helga203 - It should never go to zero with key off unless the gauge is electric, my mechanical gauge used to lock at 55psi for a while after the key was off, and I believe this is proper. The problem now seems to be that it can't build pressure on the 2-second prime because it's leaking back. I did replace the return line with a shorter test 3/8 hose and it made no difference in running PSI (remember there's two problems - prime PSI is too low and running PSI is too high).

 

i here you ,mine is out of the norm. should stay psi, but im not opening up a can of worms i have enough problems with this car.
but if u thing about it, thats what mine is doing i have no check valve if i don't start the car it immediately. it drop to zero in a few seconds. see what in getting at it should hold to start. and for some reason when my car gets really hot it takes about 3 primes to get to 60. literary between primes my psi drops if i dont go fast enough, but im going to bust my brains to think what's going on i mean wtf it only a stupid fuel line. sorry might have missed this but when did it start happening?

when it does prim the second time u shoudl check the volts.

 

Turns out the ultra-high pressure was a bad regulator. Right out of the box it let 80PSI to the rail and eventually stopped regulating at all, so at WOT it would drop into the 50s, and the problem I thought was the check valve - yea it was also the regulator. It was like the diaphragm was broken and it was always letting enough through to build pressure but not enough for flow, then it let everything back down the return when the pump turned off. I almost had to call my wife from the gas station because it was showing zero pressure key-on. I was able to build up pressure by flipping the key on several times until I could start it.

I put the first replacement back in, now I'm back to a solid 72psi no matter what, and it holds pressure all day. I put the mechanical gauge back on the rail and compared it to the electric, and the electric is 5psi higher across the board. So, that means I'm really at a constant 67psi which seems much better. I hope with the hotwire kit it's able to keep up the flow on the dyno, which was the reason for starting this whole damn process.