Help!
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Help!
Hello all! I have a 1978 F-250 xlt with 400 engine. I'm here in New York. Last night I started my truck with intention for it to idle awhile and warm up in this 30° weather. I got busy with something and left truck idling for about 25 minutes. When I shut it off the engine dieseled for a split second. Ever since the truck barely idles, it stalls constantly, and runs extremely rough. What did I do wrong!?!? Help!
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New User
Joined: Mar 2018
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That's what I was thinking it was but haven't had a minute to pull any of them to look. I can't believe this truck has gone from running like a top to its current state from idling! Wtf!?
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Well you might not have done anything "wrong" but it is a 40 year old truck. When was the last time the carb was gone through, etc. Could be a lot of things, hard to tell from here, maybe some junk got caught in the needle & seat and it flooded out etc. Just WAG, but you'll have to check a few things. Ignition is a definite possibility too. Pull a plug and look for fouling, and then check it for good hot spark.
Cargo Master
Joined: Jul 2005
Posts: 2,309
Likes: 364
From: Kansas
when it dieseled, the compression blowing back through the carb could have wedged the choke shut, it could have blown of a vacuum line, your timing chain could have slipped....
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Cargo Master
Joined: Jun 2014
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From: Central Florida
Yet another possibility: when the engine dieseled, it probably shook a bit more than usual. This could have resulted in a vacuum leak by disconnecting a line, cracking old rubber line, etc. Vacuum leaks make for rough idle. Look for something like this and correct if found.
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Senior User
Joined: Nov 2015
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From: FL & NY
Sounds like a vacuum leak - check the PCV hose at the base plate of the carb., check the maniflod vacuum "T" for a disconnected hose, check for a missing vacuum cap. For the enine to barely run at idle it's probably one of the larger vacuum hoses that's disconnected.
Lead Driver
Joined: Jul 2017
Posts: 9,545
Likes: 1,156
From: San Jose, CA
That too!
Do you know what carburetor is on there? Stock or aftermarket?
If it's an old Holley it could easily have a bad power valve. But if it only dieseled lightly, but did not actually cough/spit up through the carburetor, a blown power valve is less and less likely except for old age.
But they do spit up at least a little, so the possibility still exists.
When, if ever, have you added some anti-moisture additives to the tank(s)? Even in dry climates moisture can build up and do all sorts of mischief. Freezing in lines, breaking up deposits, weird stuff I tell ya!
Some rubbing alcohol will do it, but I use that stuff for other things so tend to just buy a can of additive from the store. Yes, it falls under the heading "snake oil" like so many other things, but I doubt they're lying and ineffective. I've never had an issue since I started using them in the '70's. Before that... Several rusted through steel tanks told me that even here in sunny CA we do have moisture in the air.
And in the tanks of most gas stations back then too!
Good luck. As the others have said, lots of things it could be with an old vehicle.
Paul
Do you know what carburetor is on there? Stock or aftermarket?
If it's an old Holley it could easily have a bad power valve. But if it only dieseled lightly, but did not actually cough/spit up through the carburetor, a blown power valve is less and less likely except for old age.
But they do spit up at least a little, so the possibility still exists.
When, if ever, have you added some anti-moisture additives to the tank(s)? Even in dry climates moisture can build up and do all sorts of mischief. Freezing in lines, breaking up deposits, weird stuff I tell ya!
Some rubbing alcohol will do it, but I use that stuff for other things so tend to just buy a can of additive from the store. Yes, it falls under the heading "snake oil" like so many other things, but I doubt they're lying and ineffective. I've never had an issue since I started using them in the '70's. Before that... Several rusted through steel tanks told me that even here in sunny CA we do have moisture in the air.
And in the tanks of most gas stations back then too!
Good luck. As the others have said, lots of things it could be with an old vehicle.
Paul
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
Virtually all gasoline is laced with at least 10% alcohol these days, so things like HEET in the yellow bottle stacked like cordwood by the cash register in the wintertime to prevent gas lines from freezing is pretty much a distant memory by now, it's not needed.
Lead Driver
Joined: Jul 2017
Posts: 9,545
Likes: 1,156
From: San Jose, CA
That's a great point Tedster. Never even thought about it (although now that I think about it, I believe I heard someone else say that before) being a factor.
We don't get as much as most areas I don't think. But if there's even 5 or 10 percent in there, then water staying separated in the tank should be a think of the past.
Thanks!
Paul
We don't get as much as most areas I don't think. But if there's even 5 or 10 percent in there, then water staying separated in the tank should be a think of the past.
Thanks!
Paul
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