76 F250- Non Vented Fuel Cap Causing Issues

MrSony

Geezer
Nov 15, 2014
6,185
113
Des Moines, Iowa
I may have narrowed it down to the fuel cap I have.

Long story short, after some time on the highway the truck will begin to aggressively chug, pop, spit, and backfire out of the carb. however, if i put it in neutral and rev it, its perfectly fine. revs up normal. pop it back into drive, immediately back to stumbling. at first i thought it was the fuel pump going out, so i replaced it (it had some age to it, and i did not replace it when i put a timing chain on the truck) and previously went through the carb. both of those did not fix it, but i noticed when i take the fuel feed hose off of the pump (the line from the tank), it is gushing out as hard as the average faucet and would seemingly empty the contents of a full tank if left to drain. my regal doesnt have this problem, its usually no more than a half cup maybe. so I got to thinking, did i buy the wrong gas cap? there are vented caps for the non evap system trucks and nonvented for the trucks with that system. i have a non vented cap in a vented system.
I think the fuel system is under a lot more pressure than i should be and its either overloading the needle and seat on the qjet and just drowning the carb (entire top of carb ends up being wet with fuel), or its pseudo vapor locking itself and free revving it kind of clears it up a bit.

I'm gonna pick up a vented cap tomorrow and see what that does. the truck had a vented cap on it when i got it, but the tweakers before me ****ed up the lock and i almost broke a key trying to take it off once. i needed a cap, and the non vented is all the store had at the time.

it's either come down to the cap, or maybe an intermittent spark problem. it has a new duraspark box and coil, wiring, plugs, wires, etc from when i did a tune up last year.

i still dont understand why it will rev up fine in neutral but in drive it goes back to sputtering. only does it with sustained highway rpm, or if i mat it from a stop light it will just cut out and almsot die if i dont back out of it in time.
even though its a ford, i know a few of you have owned these trucks or at least know someone who has. any input is appreciated.
 

melloelky

Comic Book Super Hero
Oct 22, 2017
3,195
113
mass
I may have narrowed it down to the fuel cap I have.

Long story short, after some time on the highway the truck will begin to aggressively chug, pop, spit, and backfire out of the carb. however, if i put it in neutral and rev it, its perfectly fine. revs up normal. pop it back into drive, immediately back to stumbling. at first i thought it was the fuel pump going out, so i replaced it (it had some age to it, and i did not replace it when i put a timing chain on the truck) and previously went through the carb. both of those did not fix it, but i noticed when i take the fuel feed hose off of the pump (the line from the tank), it is gushing out as hard as the average faucet and would seemingly empty the contents of a full tank if left to drain. my regal doesnt have this problem, its usually no more than a half cup maybe. so I got to thinking, did i buy the wrong gas cap? there are vented caps for the non evap system trucks and nonvented for the trucks with that system. i have a non vented cap in a vented system.
I think the fuel system is under a lot more pressure than i should be and its either overloading the needle and seat on the qjet and just drowning the carb (entire top of carb ends up being wet with fuel), or its pseudo vapor locking itself and free revving it kind of clears it up a bit.

I'm gonna pick up a vented cap tomorrow and see what that does. the truck had a vented cap on it when i got it, but the tweakers before me ****ed up the lock and i almost broke a key trying to take it off once. i needed a cap, and the non vented is all the store had at the time.

it's either come down to the cap, or maybe an intermittent spark problem. it has a new duraspark box and coil, wiring, plugs, wires, etc from when i did a tune up last year.

i still dont understand why it will rev up fine in neutral but in drive it goes back to sputtering. only does it with sustained highway rpm, or if i mat it from a stop light it will just cut out and almsot die if i dont back out of it in time.
even though its a ford, i know a few of you have owned these trucks or at least know someone who has. any input is appreciated.
I'm not much help but if you suspect the cap,try driving without a cap see what happens?it's a cheap try anyways.
 
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MrSony

Geezer
Thread starter
Nov 15, 2014
6,185
113
Des Moines, Iowa
I'm not much help but if you suspect the cap,try driving without a cap see what happens?it's a cheap try anyways.
I wanted to but I don't think spraying cars with gas sloshing from the open neck would be cheered.
I take a right too hard that's $20
 
Oct 14, 2008
7,540
113
Melville,Saskatchewan
What motor and carb? Most Motorcraft carbs are junk FYI. It would not surprise me if that is the culprit.
 

ck80

Comic Book Super Hero
Feb 18, 2014
2,904
113
I wanted to but I don't think spraying cars with gas sloshing from the open neck would be cheered.
I take a right too hard that's $20
You can do a normal drive without whipping around like crazy. And it doesn't need to be 100 miles either. Just test drive to the point you usually get problems and see if they don't pop up.
 
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MrSony

Geezer
Thread starter
Nov 15, 2014
6,185
113
Des Moines, Iowa

MrSony

Geezer
Thread starter
Nov 15, 2014
6,185
113
Des Moines, Iowa
You can do a normal drive without whipping around like crazy. And it doesn't need to be 100 miles either. Just test drive to the point you usually get problems and see if they don't pop up.
It only does it on the highway at sustained 70-80mph mildly worried wind would suck fuel out.
 
Oct 14, 2008
7,540
113
Melville,Saskatchewan
So you got rid of the Motorcraft 4350. You could try the farmer proven fix, stuff a rag in the filler neck😋. Order a proper vented cap and go from there.
 

MrSony

Geezer
Thread starter
Nov 15, 2014
6,185
113
Des Moines, Iowa
So you got rid of the Motorcraft 4350. You could try the farmer proven fix, stuff a rag in the filler neck😋. Order a proper vented cap and go from there.
it had quite possibly the worst edelbrock in the world on it when i got it.
 
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79 USA 1

G-Body Guru
Sep 2, 2011
743
93
Cheektowaga, New York
:popcorn:
 
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