STRANGE fuel issues.... - Lateral-g Forums

Keith · #1 11-26-2004, 02:47 AM

I posted this on P-T as well because I'm just desperate for an answer. Do any of you guys have a clue as to what might be going on? I definitely appreciate it!
Myself, along with some very experienced people are having an issue with a car we cannot figure out. To start, it's an 88 camaro IROC-Z that was bought less motor/trans to be a fun car to test stuff on and to beat up a little in the process for the sake of numbers. Currently the motor is slightly worked 355 backed by a TH350 and run to some 4.10s. The issue we're having is with the fuel system. Because it's a retrofit carbed application, the pickup in the stock tank was modified and then checked twice for clearance in it's appropriate baffle and we capped all lines going into the tank (total of 4) except the vapor return line, which we attached a breather to, and of course the main feed line. Main feed drops approx. 14" down to a filter, then holley blue, then aluminumn line approx 6', another filter before the regulator set at 8psi, then to the Edelbrock 750. With the exception of the fuel tank line and the aluminumn line and it's connections, everything is stainless braided and uses -6an. The problem we're having is after the car has been running for about 1/2 hour, regardless of how we treat it, we start loosing fuel pressure slowly but surely. Doesn't matter if it's WOT or idling, we loose it at about a PSI every ten minutes, the only constant being the car running. If we shut the car down and then restart, it just picks up where it last left off. If the car cools down completely, it's starts right at 8PSI and stay's there until about a half hour again. The car has been through three fuel pumps, two regulators and a completely re-done fuel system including a different carb and the same problem no matter what. We checked voltage to the pump at the pump and it doesn't change and we upped the car to a 90Amp alt just to be certain. It just doesn't make sense because it's not temp related...we could cook it as soon as it fires up and the temp could be holding at 160 and then pump won't crap out until about the 1/2 hour mark. The only way we know is the A/F gauge starts going hot and sure enough we've dropped down to 5 or 4 PSI.
I'm sorry this is long but wanted to give you guys everything to scratch your heads over. Everything in the fuel system has been replaced which would indicate that it's not the fuel system but we're all miffed over here. Any suggestions....questions? Thanks guys...

**XcYZ** · #2 11-26-2004, 07:39 AM

Where are you reading pressure at? Are you running a return line back to the tank, or is the system dead-headed? Have you tried running a return line back from the regulator to the tank? Maybe the fuel is being cavitated, and when you shut the car off, it's just enough for it to settle. I don't know, just a thought. It sounds like you've tried everything else.

**camcojb** · #3 11-26-2004, 10:24 AM

What filter do you have before the pump? You generally can't run the real fine filters in front of a pump, only after. The ones in front are much coarser to keep large debris out of the pump. A restrictive one will cause issues.

Jody

Keith · #4 11-26-2004, 02:11 PM

I didn't think with the Holley blue I would need a return line @ 8PSI, although it was considered. The filter before the tank is an Earl's replaceable element style with -6an's on both sides. We're running the pressure gauge at one of the regulator outlets....is a return line really necessary? I've seen it on more serious applications without it but running a return would be no issue if you recommend it.
What is so strange about this issue is that the car runs perfect and the A/F is dead balls on until just about the half hour mark.....I promise to let everyone know what it turns out to be. Thanks Scott & Jody.

**XcYZ** · #5 11-26-2004, 02:34 PM

Keith, I just remembered something... A long time ago, I had a problem that was very similar. I would drive the car a good 20 or 30 miles, then it would start doing lean surges, and once in a while just quit. After messing with everything more times than I could count, I got pissed one day and drilled a hole (yes, I can hear you guys laughing now, lol) in the gas cap. It never happened again. It ended up that the venting gas cap went bad and wasn't venting, and after the motor would run for quite a while, it would start vapor locking.

That sounds like what could be happening to you, and could help explain why your A/F starts to change. I know you said you have a vent in the tank, but perhaps you could look into making a change where you're 100% sure the tank is venting.

BTW, that was with a Holley Red pump on a 350.

**Hydratech®** · #6 11-26-2004, 03:04 PM

When I switched my 442 over to EFI, I went through about a fuel filter a week, for about 6 weeks, until it finally cleaned itself up and stabilized. Why? In carburated form, even though there was a recirculatory function, the fuel only experienced filtration upon entering the "carbonator". Once switched over to EFI, the entire contents of the fuel tank gets recirculated through the system and filter at a substantial pace. I was cutting open the fuel filters everytime, finding just crazy amounts of muck in them! A very very fine particulate, cumulatively amounting to most likely 2 cups total before all was said and done! It would act just about exactly as you describe, responding perfectly if I'd shut it down and restart it, as it would backflush the plugged filter just enough to allow it to flow again (atleast until the backflushed muck got back into the filter again).

Could it be this simple?

Keith · #7 11-26-2004, 04:43 PM

Gosh I wish it was that simple.... the tank is new and so is the entire fuel system!!!!! I think vapor lock is the culprit and we just have to check our breathing....thanks.

Novacane · #8 11-26-2004, 09:28 PM

I like the pressure in the tank theory!

Keith · #9 11-27-2004, 03:06 AM

Bad news...popped the gas cap off and same problem. Has to be a air slowly coming into the line somewhere at the tank. We're going to run a gas can with a stainless to -6an to the pump and see what happens......I'll keep everyone posted....

**camcojb** · #10 11-27-2004, 11:07 AM

Keith,

You might also try removing the inlet filter if it's easy to do. Any restriction like that can cause cavitation at the pump and would not show up immediately, but after running and heating for a while. If that cures it replace the filter with one with a much coarser filter and use the fine filter for the pressure side. You do not want any restriction on the inlet side of the system.

Jody