Troubleshoot fuel interuption

What are the possible causes of fuel interption from the fuel tank to the fuel pump. 1979 f-150 351M, automatic xmission with a/c

1979 F-150 351cuin, hmmm,

Depending on where the tank is located, (behind seat in cab or under the rear of the truck between the rails) that will give an indication of the tanks condition.

Dirt/rust or water in tank being picked up by the pump;
Loose or cross-threaded line fittings;
Damaged (crimped, kinked, cracked or abraded) fuel lines/rubber hoses; (check both feed and return fuel lines)
Paper filter or a porous bronze filter at the carb may be dirty;
Fuel pump worn or leaking (possibly from pump gasket);
Possible worn camshaft causing a disruption of fuel pump operation.

I think that’s about it.

I have a Chev/GMC 1967 thru 1982 workshop manual for when I used to have a '78 Jimmy 4x4, so if you need carb info and such I may be able to help.

Not having a photographic memory (Or even one with a retention mechanism) I don’t know if Ford used the same models of carbs as GM did.

Thanks for the reply, i’ll give a little history. The engine has 70.000 miles on it. I put a new cam in it when I rebuilt it. That should not be worn that bad i hope.
I emptied the fuel tank… I only got 1/2 teaspoon of water and very little sediment. no rust to inhibit flow. I only run high test fuel in it for years.
I replaced the pickup tube and sending unit in the tank. It came with a new screen. I saw nothing wrong with the old one.I checked the fuel lines to make sure they are clear. They are. I replaced all of the rubber hose connections in the line, replaced the tubing from the pump to the carb. I also put a new carb on it BEFORE I started having the following symptom. I have never had this problem till the new carb was installed. Everything else was checked and replaced as possible factors after the carb replacement. My fuel pump went out(due to runnining dry) so I replaced it.

The problem occurs after I idle standing for approx five minutes then try to accelerate to drive on, it acts as though it is out of fuel.(even when tank is full) When I accelerate the vehicle stalls. As if it is out of fuel. I noticed that the fuel filter that WAS (took it out) installed between the tank and the pump was without fuel. Fuel stops getting to the pump. I then disconnect the fuel line from the pump, syphon fuel from the tank, get a flow then recconect the rubber hose to the pump, prime the empty carb, then start it and get going again. Some times if I am trying to accelerate(only after sitiing at a red light) on the road it jerks from fuel interuption. the rpm and momentum i guess re-establish fuel flow to the pump then it finally stops hesitating and drives fine.
I have no problem while operating, only when accelerating.
I guess it could be the carb. Causing some sort of vaccuum on the line. I am stumped bad. I seem to be missing a concept on flow, vaccuum and gravity and the association with the pump. The carb is 200 dolloars. The reason I replaced it was because a venturi body malfunction occured. Thanks in advance for your support.

I need some more tips please

I am no expert but after reading your post…problem happens after new carb and only on acceleration and you guess it is the carb…did you get a new one or a rebuilt carb? why not get a new one and try that…

I am not an expert either but a new carb is probably 400 plus dollars. i can get a whole other vehicle for that cost. lol It was a remanufactured carb of course. I wanna fix it without any more cost or labor. Here I

It sounds like possible vapor lock. This happens when the fuel heats, and begins to vaporize in the lines. The pump doesn’t pump vapor, and stops pumping. Check the fuel line routing, and make sure it doesn’t run near high heat sources, like exhaust. There are also thermal wraps you can install to help keep the fuel lines cooler. Use it around the fuel line under the hood until it runs down past the firewall, and towards the tank.

It is definitely a vapor lock. It has to be occouring in the undercarriage of the vehicle between the tank that is located in the rear of the vehicle btween the rails. What is making it occur after thirty years of problem free operation. It is a lock of fuel without a doubt. The thermal wrapsounds ok. I will check that out. A second thought could be a pinhole in the supply tube to the pump. Its not big enough to see a fuel leak but could cause an evaporation air pocket and lock. We are close… I guess I could do a pressure check on the tubing in question. that would answer the question of a micro pin hole that is not easily located. What the…(&%^( else could it be. Let me know

I’ve read this post over and over, and I’m thinking that the stamped metal eccentric that mounts behind the crankshaft timing gear for the fuel pump is worn. But the only way to find that out is to remove the timing cover.


We are still onto something. I am sure that this eccentric part is mounted on the front side of the cam. I see your point. It does have 140 thousand miles on the part. I rebuilt the engine 7500 ago and my inspection of it indicates to me that it is not that part. Although you could be correct I do not suspect this. ??? I know you have to read my history post over and over because I am so distraught that I have not solved it my self I am excited over fixin it. lol I seem to be a little confusing in the post. Keep them coming though. Maby this eccentric came loose but I got no indication of any noise. I am tempted to say it is the remanufactured fuel pump I installed. I am not that easy to convince to replace parts though. The more I consider it the more confident i am that the eccentric operator on the cam was and is still in good condition. I have confidence in my latest assessment but I gotta give it to ya for even mentioning it. I aint removing the timing chain cover to check until the tranny has to come out. Then, I’ll put a new rear main seal in and then new front and rear oil pan seal along with oil pan gasket. Uh Uh. not yet. 70.000 not yet


Because you said you DID NOT have this trouble BEFORE you changed the carburator makes me think that’s where the fault is. The remanned carb.

Does this carb also have that 70k miles on it and is there any warranty still on it?

To quote one of your statements: " Everything else was checked and replaced as possible factors after the carb replacement". Unquote.

Nothing replaced corrected the fault, so we’re back at the replaced carb again.

I’m thinking there may be a float problem or perhaps an accelerator pump fault in the carb.

Do you know or can you find someone with carb rebuilding experience to dismantle and check out this carb? Unless you feel you can do the job yourself?

Thinking of the accelerator pump reminds me of the time I had the same ‘hesitation’ (no gas) in a '65 Pontiac.

I had some help from a mechanic friend and a blow-up diagram of the carb I had in hand to do the job. All seemed to go well back together, double checking as I went.

Ok, mounted the carb back on the motor, hooked it up and start the engine after a few pumps of the gas. It fired up and when I tried to move the car, the hesitation was still there. What the…?

The mechanic said to remove the carb again. ARRRGH!

I removed it and stripped it down and lo’n’behold! there were 2 springs in the accelerator pump! Another wrapped tight inside the one I seen.

Removed the second spring, put it all back together, fired it up and took off down the road like a brand new car.

I’m not suggesting this is the case here BUT…

I can certainly rebiuld the carb. There really is not much to it. I am finding that the problem does not occur when ambient temp is down. I understand and I am aware and knoweldgeable of the hesitation of a faulty accelerator pump. I have not fininshed my test but I think I have a micro fracture or breach in the fuel tubing btween the firewall and the end of the tube in the engine compartment. I think heat from the engine compartment is causing a vapor lock right before the fuel pump. I am gonna check that before I pull the carb. I have a carb kit on standby. Reman is not always good but I am confident the carb may not be the WHOLE or should I say HOLE problem. I do not like to replace parts to find they did not work or fix the problem. I dislike this feature in any mechanics shop. Any. Unfortunately, time is money. My time is…labor without the expense of others time costing me. I thank every one for their input and I will update when I discover waht has happen.