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90 Chevy 454 4V intermittant engine rough running issue

Greetings! I have a 90 Cheyenne 4wd 1 ton with a 454 with 55k miles that I have had since 1994. I rarely use it, and it sits up alot and is kept garaged. I has been very well maintained, though I can't recall the fuel filter has been changed since I had. Often to sometimes it runs roughly and misfires somewhere through the warmup period, by not always. It does ok when cold till it is partway warmed up. Seems like if behaves better the more I use and drive it for a period. It acts like something is sticking, when it does this it backfires through the carb and is even hard to crank if shutdown. Recently, it does it after it's warmed up, left to sit a few minutes and restarted, like vapor lock, but it continues for up to 15 -20 minutes or so, even after warmed up, but then goes away and runs perfect. Ideas Please?!

Reply to
Frank Knight
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Is it really a carburetor on a 1990 454? I thought for sure they had switched to at least TBI by then.

If it really is a carb, when you don't use it for a long time the fuel in the float bowls evaporates and leaves a very sticky/gunky layer on the jets and floats (distilled gasoline additives, basically). If it accumulates on the jets, it effectively makes them "undersized" and causes the carb to run too lean, and backfiring is certainly a symptom of lean running. A jet can even plug up solid with that goo as it breaks loose. Running the car regularly will re-dissolve some of that, but it is pretty hard to dissolve. But it sounds like its gotten to the point that the carb needs to come apart and by thorougly cleaned and a new gasket kit put in.

Now if its EFI (TBI or MPI) I'd suspect a sensor- either temp sensor not correctly indicating the engine temp to the computer or something like the MAP (or MAF) sensor.

Reply to
Steve

Seemingly Solved, thanks. Turned out to be a pile of rust particals inside distributor, it must have gotten wet somewhere along the history of this truck as I never had it off and have only put 5K on it since acquiring in

Reply to
Frank Knight

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