i have a 1980 f100 with a 4.9l 300
ive been having this problem for a few weeks now. its started off i was just driving down the road about 40mph and the truck just drops down to ideling rpm (like 600rpm when in gear) and if i tried to give it gas it would act like it was gonna die so i just coasted with it ideling until i got down to about 20mph and i gave it gas and it was fine. it kept doing this every now and then. then about a week ago it did the same thing but this time it drop down to idle rpm for about 3 secs and it died so i put it in neutral and tried starting it but it wouldnt start till about 30secs and it finally started but was running rough for about 30secs but then it was fine. then today it did the same thing but would not start back up for nothing not even after about 20min so i had my dad come and he pulled me about 3miles back home. when we got back it still wouldnt start for nothing. we got to looking and the fuel filter was bone dry so i figured the fuel pump went bad. we took it off and cleaned it up and it seemed to be working fine we even hooked the inlet fuel line back up and put the fuel pump back on and turned the engine over a couple times and fuel was pumping out just fine so we hooked the other line back up and wouldnt you know it it started right up. so im wondering if its getting vapor locked or if the pump is just getting weak. the truck isnt over heating or anything either but it has been in the 90s the past month or so. so should i get a new pump or what? thanks for any help

 

Sounds like it's starving for fuel. I use a hand held vacuum pump with the catch jar and hook it to the line from the tank. Pump it and note the return. If it has a lot of bubbles your rubber line by the tank might be bad. It can suck air and not leak gas. If you have good return, hook the lind back up to the pump and take it off at the carb. Disable the ignition by removing the hot lead to the coil and crank it over while catching the fuel in a recepticle. Note how the flow is and how much you catch. Just a couple seconds should be several ounces. If you have good supply to the pump and poor at the carb, it's the filter or the pump.

I had a lot of vapor lock problems on my 72, mostly I believe because the metal line goes over the top of the engine. It would vapor lock on a hotsoak and not restart until I cracked the line on the output side of the fuel pump and released the pressure. I finally cured it with a fuel pressure regulator and it would not starve for fuel with it as low as 1 1/2 psi. Never had another issue after I installed it.

Just another thought, do you still have the metal line from the tank to the pump? If it has been changed out to rubber that can cause a problem when it gets hot because the line will just flex with the fuel pump pulses instead of pumping fuel.

 

There's a couple things to check. First off, did you inspect BOTH fuel filters? There's usually one up by the carburetor, and then another one on the fuel line just a few feet from the gas tank. On my Bronco, it's on the frame on the driver's side.

Next, the filler neck has a small, ribbed hose inside of it that it uses to vent air out of the gas tank. Sometimes this can get a kink in it and your tank won't vent anymore. I fought with that one for a while. You'll have to remove the filler neck from the gas tank to inspect it. It can be done without dropping the tank, but it's a hassle.

Your fuel pump could also very well be a culprit. They have an internal diaphragm that goes out over time, making them weak. They're fully sealed, so it's not something you can inspect. Fortunately, on these older engines, the fuel pumps are dirt cheap. Around $15 - $30 or so.


If it were me, I'd inspect the in-line fuel filter, and then replace the fuel pump. If those don't do it, inspect the vent hose.

 

Are you certain this is a standard feature on the F-series pickup trucks? I have a 1980 F-150, and have never heard of another filter beyond the one that screws into the carburetor. If there's another filter in the system I need to get in there ASAP, because mine's bound to be full of rust and gunk.

 

It could be too that there is debris in the tank plugging the outlet.

 

I can't tell you with 100% certainly, but I think so.

Mine has a metal line coming out of the tank which ends at the frame. Then, about 4 - 6 inches away, another metal line continues to the engine bay. In the gap between the two metal fuel lines is a rubber fuel line with a filter spliced in between. As far as I know, it's factory. I can't see why it'd only be on Broncos and not the trucks, but who knows.

I can check my Chiltons when I get home.


It'd be easy enough to tell. Just follow the fuel line out of the gas tank.

 

Yep...I just got out from under my truck; no fuel filter between the tank and fuel pump. Steel line all the way.

Of course, this doesn't really mean anything, given how many combinations of fuel tank layouts there are on these truck. I have a longbed with just the single midship tank, which is certainly different than your Bronco, and may well be different than the OPs truck as well.

I'm just glad there's not a filter I've been neglecting.

 

A rear filter on a carbed engine would be something new to me, but I learn new things all the time. I think an 80 just has the one where the fuel line goes into the carb. Those filters migrated all over the place on efi trucks.

 

i looked for a second fuel filter but the only one is the one in-between the pump and carb. also from the sending unit its a rubber hose for about 12 inches then its all steel until about 12 inches before the pump then its rubber again then after the pump its steel to the fuel filter which i believe my grandpa rigged the filter on the steel line with a 3 inch long rubber line. and also i did drop my gas tank about a month ago cause the float fell off the sending unit and i had to take the filler hose off the filler neck instead of taking it off the tank because i couldn't get to the clamp on the tank and when i pulled it off the plastic vent hose did come out and I'm not sure if i got it back in right so that could be my problem.

 

Be sure and check all the rubber hose between the fuel tank and fuel pump. Hoses can become delaminated with age, causing them to pinch off internally and starve the engine. If your hoses are pretty old, it would be a good time to change them, even if it turns out they aren't the problem now.

 

You will find a pickup sock strainer on the end of the pickup tube inside the tank.
Some people call these filters, but they are at least 100 mesh to only protect the pump from large debri.

You can blow back thru the fuel line with the gas cap off to test.
Or you can try to syphon to test for flow from the metal line going into the fuel pump.

Also make sure the system is vented properly.
You can test by trying with the gas cap off or loose.

 

A friend who is a Ford Technician once shared with me that when the "sock" is suspected of being plugged, they sometimes would, as you previously mentioned, remove the gas cap and then blow backwards through the fuel line with compressed air, which would literally blow the "sock" off or tear it, thereby allowing good flow of gasoline. They would then install an inline filter in the rubber fuel line between the hard fuel line and the fuel pump. This was quick and did not require removing the tank.

 

Maybe that's where I got my rear filter?

However, when I removed my gas tank, the sock was still present. Granted, the tank looked like it'd been replaced (it's an aftermarket) so maybe they put a new one on at that time. A lot can happen in 25 years.

If anyone ever needs them, JeffsBroncoGraveyard sells the sock filters for in the tank. Not very expensive.

 

This filter between the pump and carb, exactly where is it located?
I've had issues with an add-on filter between the pump and carb that was located right above the intake manifold, in hot weather. Right below the intake, is the exhaust, and that sucker gets rather warm. Warm enough to boil fuel inside a filter, I've seen it happen. On my 80 F150 300.
I relocated the big filter to just ahead of the pump, and put the small factory filter back on the carb, and haven't had any further issues in the last 14 years. Including last year with 110F+ temps a few days.

That space between the end of the steel line and carb *looks* like a great place for adding a filter, but it's not, particularly in hotter climates.