View Full Version : The strangest demon ever....

11-26-2004, 12:28 AM
Okay, I need everyone's thoughts on this...I mean even the people who normally don't post as anyone's experience would be helpful. 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...

11-26-2004, 12:54 AM
And you are positive it has adequite venting in the fuel tank?

11-26-2004, 02:38 AM
We have a 1984 project car doing somehting real similar, it is also a carburated pee wee. It is running out of gas (fuel pressure) out of the blue. Then will pick up pressure for no reason.
We have not spent anytime checking out the problem yet, but I am fairly cetain is is cavating some how, or sucking air. I will be looking to the rubber lines that conect from the regualtor / pick up to the body line? Or the pick up line in the reuglator inside. I had a pick up line that developed a hair line crack on the inside, against the regulator retaining ring. It drove me frickin nuts trying to chase a "vapor lock".
I'll be watching to see what you find. We probably will not mess with this one for a couple weeks. I will let you know what we find.

11-26-2004, 03:32 AM
Does the line run close to the exhaust or any other hot parts? It could be boiling the fuel in the line maybe?

11-26-2004, 05:12 AM
Hmm...I wonder-try using only one filter. Run it just before the carb under the hood and see what that does. -Jabin

11-26-2004, 07:35 AM
I doubt it's vapor lock...that doesn't go away that quickly normally.

My bet is 1) the stock injection pickup sucks for a carburetor pump, or 2) the tank isn't vented good enough and you're pulling a vacuum in it. #2 being where my money's at.

Q ship
11-26-2004, 07:51 AM
Have you tried running it without the tank filler cap on? Cheap way to see if it's a venting problem(which is what it sounds like).

11-26-2004, 11:57 AM
We've run this combo before with only one filter and now both filters are the very nice Earl's replaceable element style using the -6an's. The only heat the line is exposed to is a very brief encounter with one of the tranny lines (for about 1" and it's not quite touching) Now, the pickup was originally for an in-tank pump w/ screen on a TPI setup. The diameter is 3/8 so we measured where the stock pickup ended in the tank (and where) and matched that length and location with aluminumn line. The hairline crack is a possibility and I didn't think to pop the gas cap off to check. I'll let you guys know...definitely. I've never encountered a problem like this before. Thanks again you guys.

11-26-2004, 01:35 PM
I agree with Brian D in the gas cap issue. The one thing I keep wondering about is: Did the car work fine with the original engine?
If it did, can this maybe have something to do with the fact that the return line was used then but not now?


11-26-2004, 02:24 PM
I agree with Brian D in the gas cap issue. The one thing I keep wondering about is: Did the car work fine with the original engine?
If it did, can this maybe have something to do with the fact that the return line was used then but not now?

Don't know about the original motor except it was a TPI 350...we got the car as a roller...a return line is needed in a high-pressure fuel system but never figured I'd need one in a mild street-car with a low-pressure carb system....

11-26-2004, 02:56 PM
Dont think you actually need one, but maybee it helps preventing vacuum in the tank if the vent dont work well.
It may also be an advantage with a return line if you have problems with boiling fuel. The sirculation prevents this


11-26-2004, 03:34 PM
I agree that the tank is not venting properly. Make sure you do not have a clogged vent. Since the motor was out of the car when you got it, someone may not have capped the lines and something has blocked the vent line. Trying taking the vent line off and back blowing it to see if it is clogged.


11-26-2004, 06:26 PM
I agree it SOUNDS like venting. That system if originally FI should vent through the charcoal canister. check to see is some lines where left plugged...easy way to test system would be to see how it reacts with the gas cap off.

11-27-2004, 01:05 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....

11-27-2004, 05:42 PM
I'm stickin with my prediction, it's sucking air and losing prime. Let us know then I will know what I need to do, LOL!

11-28-2004, 08:00 AM
I'm gonna just throw this out there. I had a corvette (i know im sorry i was young ) that had a inner liner in the gas tank. It was a factory tank it was seperating from the steel outer liner and finally it was so bad it would pull up against the pickup and close it off. After it sat it would pull away from the pick up and it would run again for about a half hour. I tried all kinds of stuff to fix it finally i filled the tank and discovered it would only hold 15 gallons and it was a 20+ gallon tank. I talked to another guy who said he had the same problem. I replaced the tank to fix it.

I dont even know if gm still uses that type of tank. But it might lead you to check the pickup, sock, etc..

Good luck John

12-05-2004, 07:06 AM
So what was the prognosis?

12-06-2004, 11:53 AM
I've been so busy with some other cars, we haven't got back to his yet. We're going to run a 10 gal fuel cell we have lying around and run the car around the block and see if it does the same thing or if we can isolate the problem to the plumbing in the gas tank. I'll let everyone know as soon as we find out :)

David Pozzi
12-10-2004, 09:46 PM
If those fuel filters are the bronze type element, try a paper filter. The bronze type can plug with very fine microscopic dust. The dust is so fine it will go through a paper filter. Try blowing thru them to be sure they flow without restriction.

I don't like having a filter before the pump, adds a restriction.
I wonder if the hose between tank and pump is sucking shut after it heats up?

12-30-2004, 07:00 AM
Fixed ours yesterday, Dropped the tank (Lord, when my time comes, please allow me into heaven, even after the things I said while removing the tank), what a pain in the ass!
Long story short, as expected, it was cavitation. The connection between the pick up tube and the original (not used anymore) fuel pump was ruptured. Eliminated the fuel pump, got a piece of 3/8" tubing, a couple flare nuts and a union, and extended the pick up tube, problem solved.

01-01-2005, 04:42 AM
Good to hear that the problem is solved :)
Yeah! gas tank removal....uhh!


01-01-2005, 10:34 AM
Glad you got it fixed...the camaro still just sitting waiting for us to have time to put into it........the gas tanks in 3rd gens are just as bad...took three of us almost a half hour and some "massaging" just to get the stupid jerk in. :D

01-02-2005, 01:05 PM
Ours is a 84, hated the job.

01-02-2005, 03:38 PM
Ours is a 84, hated the job.

OH MY GOD!!! Same car! Can I pay-pal you $$ for a beer? What a PITA