Fuel starvation

[jak8822]

1971 f100 360. I have a problem with fuel not reaching the fuel pump consistanly. Truck is completely stock with all emissions tubing in place. I have taken all the lines loose at least three times and checked for any restiction, replaced the fuel pump and all the rubber fuel lines. Each time it occurs I remove the line at the pump and there is no fuel present. If I blow into the tank at the cap fuel flows freely, I re-attached the line and it starts right up. When I remove the fuel cap there are no signs of vacuum or pressure so at this point I am hoping someone has encountered this problem. Thanks

[ToughOldFord]

You might have a pin hole in the pick up tube and it's sucking air. Have you noticed if it's any better with a full tank of fuel?

[papabug71]

I was going to say it but ToughOldFord beat me to it. Sounds to me like its getting air into the line & the pump is loosing prime.

[jak8822]

I havent noticed if it was related to how much fuel is in the tank but its worth a look. I thought about the pick up line in the fuel tank but it appears to be soldered in to the tank and not easily removed. I did use suction to draw fuel into a clear line and didnt see and drain back to the tank in the 10 minutes or so it held pressure. Its strange that it doesnt occur at the same duration after start up. Sometimes it will run for a couple of days and the other morning it only ran for about 5 minutes before it quit. Just long enough to get about 300ft down the road.

[ForingaMex]

Maybe your tank isn't getting any vacum. Have you tried taking the filler cap off the tank so the tank gets vacum? Does it make a difference?
Jake

[FORDification]

Maybe your tank isn't getting any vacum. Have you tried taking the filler cap off the tank so the tank gets vacum? Does it make a difference?
Jake

You don't have problems with a tank not getting vacuum...it's not supposed to get vacuum. It's when vacuum forms in the tank from using a non-vented gas cap that you have problems. Vacuum in the tank is a bad thing...removing the gas cap to a non-vented tank will allow it to revert back to normal atmospheric pressure.

So...since your truck should have an evap tank, it should also have a non-vented cap. If the line going to the charcoal canister is plugged off, the tank is no longer vented and a vacuum will form as fuel gets used.

[70_F100]

Once upon a time, many years ago, I was working on a GMC 6500 mobile home hauler with a 427 gas engine.

This truck would run BEAUTIFULLY when it ran, but it would totally shut off randomly, and would not start.

Each time it cut off, I found that there was no gas getting to the carb.

I replaced both of the fuel filters, checked the lines, replaced the fuel pump, gave the engine a complete tune up, and inspected everything from the front to the rear of the truck. Of course, being a mobile home hauler, there wasn't a lot of distance between the front and the rear.

I worked on this truck for two days. Twice, on test drives, it shut off completely and had to be towed back to the shop. Other times, it would shut off, and if I let it sit for a few minutes, it would fire back up.

Unfortunately for me, my hair was much longer back then, and I pulled it quite a bit trying to pull it out. I only succeeded in making my head sore!!

Eventually, as a last resort, I drained the fuel tank and pulled it off the truck, thinking that the pickup tube might be cracked.

What I found when I pulled the tank was a leaf that had gotten into the tank somehow, and it would get sucked over the pickup tube at random times, blocking the flow of fuel.

Cleaned the tank out, got rid of the leaf, put the gas back in, and it ran fine. Saw the truck come into the dealership a few other times, but never for a problem with performance.

I had something similar happen a few years later on a Scout that my wife and I raced off-road. Seems that the seal came out of the lid of a gas can I used for refueling, and it got into the tank. Caused us to DNF at a couple of races.

The moral of this story is, check and make absolutely sure you don't have any trash floating around in your fuel tank before you spend a bunch of time and money chasing problems that may not exist. The truck is 38 years old, and there's no telling what you might find.

Your troubles sound just like what I've described.

[ForingaMex]

You don't have problems with a tank not getting vacuum...it's not supposed to get vacuum...

You are absolutely correct and that is what I meant. I used the wrong terminology, sorry.
Jake

[BobbyFord]

I had the same problem on my '72 when I first got it. Turned out the canister was somehow plugged. I replaced the tank and filler neck and cap with a non-emissions tank. (Then a year later swapped in the Mustang tank).

[DIXONBOY]

There is alot to read so hopefully no one has posted this yet. I had the same problem on my 69. In the tank on in the pick up tube I had a peice of trash that would float around the bottom and get sucked up to the tube, stop the gas flow and make it die. Once the truck died the trash would float down to the bottom and run fine. Kicked my butt!!!!

[70_F100]

There is alot to read so hopefully no one has posted this yet. I had the same problem on my 69. In the tank on in the pick up tube I had a peice of trash that would float around the bottom and get sucked up to the tube, stop the gas flow and make it die. Once the truck died the trash would float down to the bottom and run fine. Kicked my butt!!!!

Just what I said, about three posts up...

Thanks for the reinforcement, though!!!

[DIXONBOY]

Sorry. Im at work and did't have time to read all the post. But like you said i guess its good to have the reinforcement!!

[BobbyFord]

There is alot to read so hopefully no one has posted this yet. I had the same problem on my 69. In the tank on in the pick up tube I had a peice of trash that would float around the bottom and get sucked up to the tube, stop the gas flow and make it die. Once the truck died the trash would float down to the bottom and run fine. Kicked my butt!!!!

Just what I said, about three posts up...

Thanks for the reinforcement, though!!!

That's an old trick you do to someone that ain't on yer Christmas card list; stuff a glob of celophane down their filler neck. (Cause sugar sure as heck won't do crap).

[jak8822]

Thanks for the info. The previous owner did stated that he pulled the tank for cleaning, and as I look inside it appears brand new. The odd thing is that when it dies the pump will not prime by cranking. I need to blow into the tank or apply suction to the line then the fuel flows out in a heavy stream that appears unobstructed. I dont even drive the truck lately as I am tired of this issue leaving me priming the pump on the side of the road.

[Dragon]

Wow my truck has the floater problem. To clear my line I pull it off the fuel pump, I wipe the gas dribbles off and blow back at the tank. Then I can put the line back on and drive away. It happens at 1/8th a tank.

My electric fuel pump is mounted under the tank on the frame so it is easy to get to.