Persistent fuel pressure problem

[peter]

I have been trying to solve this for 2 weeks. When cold 7 psi- runs great. Within 15 minutes pressure drops to 3 psi and within 30 minutes it drops to 0, and the car dies. If I let it sit for 20-30 minutes pressure come back up and it runs fine and then it cycles back to 0.
So far I have replaced the electric fuel pump and main fuel filter, and removed an extra filter at the carb. I do not hear a rush of air when I open the tank fill, so I dont think its vaccuum in the tank. I am wondering if the tank intake has a screen and its getting blocked in the tank? Other ideas to look into? thanks peter

[Naumoff]

It does sound like a venting issue. There is no screen. I would make sure the vent is clear. And if that does not work I would pull the gas tank and clean it out.

[Brent]

Sounds like a vacuume issue in the tank.
Pop the gas cap loose and see what happens.
If your fuel pump is above the fuel level it takes VERY little vacuume to cause a pressure issue.

[peter]

I checked the vent with compressed air and can hear the air coming up through the tank -- it's clear. I also opened the gas cap and there is no vaccuum pressure so I dont think its a vent issue.

The pump inlet is about 6" above the tank outlet and I wondering if the fuel line from the tank up to the pump is the problem. I figure it might be the hose under vacuum is collapsing. It appears to be a braided covered rubber hose and it has worked for the past 4 years.At this point it is predictable, I get about 1/2 hour of run time until the car stops running -- I let it sit 1/2 and get another 1/2 hour... The only other possibility is to drain the tank and check in there for something blocking. Given the predictable timing I am thinking its the hose. Sorry for rambling.... looking for other ideas. thanks peter

[Naumoff]

Rambling is good and asking for advice can lead you to thinking about different things that can cause the problem. Replace the hose and see if it does the trick. Let us know if it helps.

[Brent]

Draining the tank is easy enough to do . Strain the fuel as you drain it so you will know if you found anything. You would have to have some serious issues to collapse a braided hose unless it is that "fake" braided crap that just slips over a regular rubber hose.

[peter]

When I installed the new electric fuel pump, which an exact replacement for the one I have been using (Holley Red) the directions that came with it said it should be installed so the pump intake is not higher than the output from the tank.

My current set up the pump intake is about 6" above the tank output. Maybe this is the problem and I have been lucky for the past 4 years?

[Brent]

The red and blue pumps are push pumps and to operate to their potential they need to push fuel not pull it, however, they will work as a puller. I have seen pumps mounted on the inner fender 12' in front of the tank. Not ideal but it will work.

You can change it but I think something else is the culprit.

4 years of running great is a clue something has happened.

Good luck!

[peter]

must be blocking in the tank. is it a big deal to remove the tank and clean it out?

[weaver]

Peter, I still have your floormats and rear end plate, call me and let me know how to send them.

Alan

[Brent]

I would take the fitting off of the outlet and drain it through a strainer. Put a jack under the right rear so the car leans forward and to the right where the outlet is. Put 5 gal more fuel in it, slosh the car around, and do it again. You might put a short piece of line on it so the fuel does not get inside the fiberglass belly pan the fuel tank sits in. You will have a horrible smell for a looooooong time.

To remove the carpet and the fuel tank is a waste iof time in my opinion.

[ralphscott]

I used a siphon pump first to get most of the gasoline out. Then I disconnected the fuel line. I also did as Brent has suggested to get the last of the fuel out. I ended up fabricing an insert to inset into the fuel outlet that keeps stuff away from the fuel outlet. I have a piece of racquet ball in my tank and it won't come out. It is stuck in one of the baffles against the tank wall. I did not want to cut an other access hole in the tank in hopes of finding it. I have been considering replacing the tank with a fuel cell.

Ralph

[Justin Upchurch]

Could this be vapor lock?

Justin

[peter]

It does not seem to be weather related-- but instead time dependent. I am going to take the fuel out tomorrow and see if there is something in the tank

[JeepSnake]

Peter, I know the Holley pumps are pretty loud, so this may not work so well. But...

Can you tell a difference in the way the pump sounds when your pressure goes to zero as compared to when everything is running ok? If you're creating vacuum on the suction side of the pump - either from faulty tank vents or a collapsing suction hose - you're going to be cavitating the pump. If you can pick it out from the normal pump noise, this will sound not unlike sand, pebbles, or gravel rattling in the pump housing.

Even though your pump is 6" above the bottom of the tank, 75% of your running is probably with more than 6" of fuel in the tank. That still means you've got positive suction head on the pump. If you consistently run with the tank below half full, I'd look into moving that pump down. Sure, Holley probably covered their a$$ with a safety factor on suction head that will cover the 6", but you can't be sure. I mounted my Carter pump down on the side of the frame rail.

Anyhow, just something you might be able to hear.

[peter]

When testing the pump with the engine is off, it is loud and consistent but I have full pressure -- about 7 lbs for about 10 minutes and then it progressively drops down to 3 and then the pressure will float back and forth between 2-3 lbs every 20 seconds or so. The pump tone does change as the pressure changes. When i remove the pump from the mounting bracket and move the pump up and down the pressure/ tone does not change. Its like its trying to suck and something is blocking. If it is blockage its either the tank outlet or inside a hose. It makes sense to me that some stuff is floating around the tank- the car runs fine until the "stuff" gets sucked to the tank outlet-- then no pressure. When the car rests it disloges and is ok again, until it gets sucked in.................

[will butterworth]

to that pump for a different sound and you might eliminate something, and a rubber fuel line will collapse from the inside and you can,t see that, when it heats up, look rubber lines close to engine where it heats up and could collapse on you .My bet is a collapsing fuel line-4yrs.old getting hot , next would be fuel pump itself, last thing I would do is break out a fuel tank, but if it comes to it you have to do what you have to.Like the man said when he was characterizing a cobra , the other woman, and you,d better look out.Brent always has helpful ideas and so does all this bunch---heck--wish we were all there to help figure this out,everybody likes a mystery, cause with a cobra all of us gonna experience some funny stuff.Will-alabama

[peter]

Ill take credit for that :) -- I think it was me that characterized the cobra as the other woman-- and it would not make my wife happy to hear it.

[scott h]

i have a boat that would run out of gas, after much searching i discovered a piece of electrical tape was caught in the pick up tube and would flapper against a screen at the top of the tube. it took me two years to discover it, having been talked out of disassembling the gas tank fittings by more than one "expert". My point? Don't rule out anything! without checking. You seem to be on the right track.

[peter]

I removed the fuel tank tonight and double checked all the connection to the tank, pump and filter.Everything checked out ok. The tank was clean and all the fuel line and connectors look fine. Ill drive it around tomorrow and see. Ill check for the hoses near the engine for damage. I might also lower the fuel pump.
Thanks everyone for your input.

[peter]

Still having the same problem. I have been completely through the fuel system. The fuel pressure problem was related to the fuel pressure guage. Its filled with silicone indicated low pressure. I popped the plastic stopper on the side of guage and now it reads constant 5-7 lbs. So I think now I am chasing an ignition problem. I replaced the coil and will be replacing the MSD box tonight.

[rdorman]

I am eager to here what the problem is once you figure it out. From the begining it sounded like the pump was overheating but with the simptom being the same before and after pump replacement, that does not seem likely. I would indeed lower the pump so that it does not have to 'suck' the fuel out of the tank. Like some one already pointed out, they are generally not great at that. Plus, if I recall correctly, a constant flow of fuel through them is often how they are cooled during operation.

[Naumoff]

Still having the same problem. I have been completely through the fuel system. The fuel pressure problem was related to the fuel pressure guage. Its filled with silicone indicated low pressure. I popped the plastic stopper on the side of guage and now it reads constant 5-7 lbs. So I think now I am chasing an ignition problem. I replaced the coil and will be replacing the MSD box tonight.

It is not the MSD box. Try replacing the magnetic pick up coil in the distributor.

[bittentoo]

The magnetic pick up should ohm out at around 700 ohms. Even if it tests good I think that I would replace it to eliminate Gremlins and it only costs about 30 bucks.

But this sure does sound like a coil problem.

Could it be the ignition switch?

[peter]

It was the magnetic pick-up in the distributor. When the temp rises it shorts out.

[peter]

and, thanks everyone for all the help

[Naumoff]

Great. Now back to having fun.:shift:

[will butterworth]

what we going to do now?