Low fuel pressure or restriction?

Okay heres the problem.... when I first start the car up fuel pressure is about 8psi then after about 15 minutes of idle or a short drive, the fuel pressure slowly starts dropping to about 1.5-2psi then will go up to about 4psi then slowly fluctuates back & forth between 1-4psi till I shut it off for about an 1 hour (with little to no fuel in clear filter after pump) then I'll start it back up and it'll repeat above pretty closely. I am running an Airtex electric fuel pump rated at 5.5-9psi. I already changed the fuel pump out once so know I have a good working pump and already tried taking the gas cap off when it started acting up in case of a vent restriction. I have dropped the fuel tank to check for trash in the tank and it looked like the tank has been cleaned recently , there was only a little sediment in bottom of tank and I cleaned that out. I am using the original hard metal fuel line from tank to engine. I'm thinking there has to be a restriction in the line but you'd think I would have low fuel pressure all the time. Also do you think I should move the pump as close to the tank as possible, I have it mounted on fender well in the engine compartment...where all my friends have mounted theirs and they have no issues. Please help I do not want to drop the tank again. P.S. unless there is a big piece of trash I missed that when I 1st start it up the pump slowly pulls it against the pick up tube in the tank blocking it off and when I turn the engine off it settles back down.... and if you ever seen the in side of one of the tanks the pick up tube opening is very small so it doesn't take much. :arg:

If there are any of the original braided, flexible fuel hoses still in place they are subject to internal collapse under the suction of the pump. It happened to my '58 F-100 with the stock pump. The truck would run out of fuel and stop dead. A little air pressure to the fuel fill or a couple hours parked would correct it temporarily. Once I replaced the original hoses it was fine and has been ever since. Sometimes having an electric pump in the engine compartment can cause vapor lock (similar symptoms) but I would start by checking the hoses. There should be at least two. One from the tank to the metal fuel line and another from the chassis to the engine. If either of these are on the suction side of the pump, they are suspect.

Lou Manglass

Have you tried blowing the line out from the engine back with the tank cap off and then checking pressure? I have an 59 jeep that is my daily driver with a gunky tank and every so often I blow it out and change the filter, It's getting a new tank this spring. Otherwise a bad flex line as mentioned might be the trouble, I've had problems with flexable brake lines acting like a one way valve by blocking flow back. One other thing, what would the flow do if you turned on the pump and ran it into a gas can for awhile would the flow drop off?
By the way that's a great looking engine and duel carb setup you have, I have a 55 2 door with a 223 I'm modifing right now, hope it turns out as nice as yours.

Brad

Thanks for the compliments on the engine....it was a lot of work. I dropped the gas tank over the summer and the tank was pretty clean...but went ahead and really gave it a scrub and blew the line out from the tank to the engine and didnt really notice any restriction but am gonna run the engine straight out of a gas can this weekend and see what happens?

A couple of thoughts.

First, most elec fuel pumps are really meant to be pushers instead of pullers. They will work as a "puller" pump, but they tend to burn out faster since they're not meant to suck fuel. There is also the vapor lock issue, as has been mentioned. The rough rule of thumb I've always heard (and use) is no more than 12" higher than the tank and no more than 36" from the tank, but really as close as practical. Could be it's getting hot and starting to stall out due to vapor lock/deadheading against the system, hence the changes?

Second, what kind/brand of regulator are you running? those little Mr Gasket/Purolator/Autozone specials (all the same reg., just in different packages) suck. Really. The little rubber diaphragm gives up the ghost in short order and no matter how much you move the dial, it doesn't change anything. Or it changes without you wanting it to. Sounds like it could also be responsible for your flucuations as well. I ran a couple of those before switching to the Holley style ones. Haven't had a problem with pressure since. I could watch the changes on my fuel pressure gauge in real time, so I have a bit of empirical data on my part. It may not happen to everyone, but it was definitely an issue on my cars. There are certainly more expensive regulators out there, but for the street, it's hard to beat the Holley ones for price/performance.

Lastly, How and where are you measuring the pressure? At the carb or further downstream? Is there anything between the carb and gauge that could be influencing the readings from the other end? I.e. building up a pressure head that the reg/pump has a hard time overcoming?

Yeah, I'm gonna put the pump closer to the tank, that was the plan in the begining but got in a hurry to start her up and then got lazy. As far as fuel pressure regulators I had one of those Mr. Spectra Gasket fuel pressure regulators and it was the biggest POS waste of money and threw across the street in the neighbors yard and spent more money on a Holley Regulator. And now I'm not even runnin' a regulator because I have a 2bbl Barry Grant Demon 98 and they run at the same fuel pressure as a 4bbl Holley. I tested the dead head fuel pressure with my Mac Tools guage straight out of the pump. There is a K&N fuel filter between the Airtex fuel pump and the fuel block, the liquid filled Jegs fixed fuel gauge is on the fuel block and then Y's off to the carb(s). There is no way it could be vapor locking in that I made sure to run the fuel line out of harms way of heat. Oh and there are no braided hoses. Again thanks for all y'alls thoughts if u have any others holler at me...for now when the weather clears up a little gonna replace complete fuel line tank to carb and move fuel pump right out of tank in trunk for easy access.
This is a picture of the engine with the holley 94's but you can still see the fuel lines are way out of the way from heat.


These are the carbs I'm runnin' now got tired of messing around with leaking Holley 94's.