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1979 Ford F250 start sputtering at 45MPH


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junker30
New User

Apr 30, 2020, 10:21 AM

Post #1 of 7 (338 views)
1979 Ford F250 start sputtering at 45MPH Sign In

my 15 yr old son decided to rescue at 79 F250 from the Junk Yard. He swapped the bad motor for a 400M. It has a brand new fuel pump but doesn't seem to have fuel pressure. When you get to about 45mph it starts sputtering. He has drained the fuel tank to make sure there wasn't junk in the tank, ran new lines. Any ideas?


Tom Greenleaf
Ultimate Carjunky / Moderator
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May 1, 2020, 2:01 AM

Post #2 of 7 (300 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

? '79 anything. Making me guess and know a 351/400M well. Lines have to be right where they should or get hot or bent if new now. Tanks don't just drain the junk at the bottom you take those out or new if that lousy are not costly.
Other is the alcohol used in fuel wasn't thought of for '79 will really make rusty inside tanks, lines, pump(s) carbs a nightmare no surprises it's not happy if it was new again.


No machine or this likes just being abandoned for unknown time if it was junked IMO if you could get the whole thing should have voided VIN on it and be just parts for another if allowed to sell or give away this vintage at all?
Good truck just lots to do keep right on going. BTW - test the fuel pressure and VOLUME it can put out by that speed if automatic is high gear for a few seconds want volume and pressure to stay equal you know it doesn't but that alone doesn't end the problems coming,


T



Sidom
Veteran / Moderator
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May 1, 2020, 8:34 AM

Post #3 of 7 (292 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

A coil breaking down under load can "feel" a lot like a fuel problem sometime.
Swap the coil off the old engine, take it out for a drive and see if that makes any difference.


Tom Greenleaf
Ultimate Carjunky / Moderator
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May 1, 2020, 3:36 PM

Post #4 of 7 (281 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

Good one Sidom. OP - if coil has a hairline crack/carbon trace you wont always see it. If suspect or not I use plain blue window cleaner to mist parts with high voltage will cause the problem sometimes in front of you - darker even see it. Good stuff wouldn't react at all so an easy test,


T



Ramntm
User

May 4, 2020, 4:12 PM

Post #5 of 7 (228 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

Did he remove the fuel tank and take the sending unit out? the sending unit has a filter on it, which can can clog up when the tank has old or contaminated fuel in it. Did he change the fuel filter at the carburetor or inline? Which fuel lines did he replace? all the rubber hoses AND the steel lines all the way from the tank to the fuel pump?


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 4, 2020, 5:49 PM

Post #6 of 7 (224 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

One problem I have run into more than once on the old carbed engine is a rotted metal fuel line.

It usually won't drip on the ground because an electric fuel pump pushes from the back but a manual fuel pump for a carb pulls from the front and if there is even a pinhole in the rear line the pump will suck air instead of fuel but likely won't leak on the ground.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Double J
Veteran / Moderator
Double J profile image

May 4, 2020, 7:45 PM

Post #7 of 7 (214 views)
Re: 1979 Ford F250 start sputtering at 45MPH Sign In

HT
Yep, Ran into that more than once.
One was one of my own cars back on the day
Had a 75 Olds 98
Cold start fine,idle fine on fast idle, Take it for a ride, under acceleration, fall flat on its face.
When it warmed up, problem went away,

Traced that line from front to back,could not find anything wrong with it and it wasn't even rusted away, it was a newer car at the time
Never leaked
Replaced entire metal fuel line , problem was gone


(This post was edited by Double J on May 4, 2020, 7:47 PM)






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