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!

 

Sounds like the choke may be stuck or sticking.

 

out of the blue like that? What would cause that?

 

 

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!?

 

 

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....

 

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.

 

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.

 

Could have blown the power valve in the carb.

 

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

 

 

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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