rough idle while in gear

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Mar 24, 2005 | 12:58 PM

tronbot

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Joined: Oct 2004

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From: omha

rough idle while in gear

i have a 78 F250 w/351M 72,000 original miles. it sarted to idle very rough while stopped in gear. if i pop it into neutral it clears right up. it has not died since this started it's just bothering me. other than that the truck starts, runs and drives like new. any ideas? do i need to speed the idle up a bit?

Mar 24, 2005 | 01:09 PM

SwOkcOffRoader

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Joined: Jul 2003

Posts: 2,544

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From: okc

i would give it a tune up before making any adjustments(plugs,wires, distributer cap, rotor bug, etc..) if this happened fairly quickly then its probably a bad plug or something of that nature....a bad plug will cause the engine to idle roughly then when you accelerate the engine speed will cover up the miss.......start with a tuneup and air/fuel filter change and go from there, a good tune up never hurts even if its not the problem, at least you can eliminate a few things doing it......

Mar 24, 2005 | 02:24 PM

olfordsnstone

Lead Driver

Joined: Sep 2003

Posts: 5,241

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From: Other side of the sunrise

Also look for a vacuum leak on the manifold, carburator and vacuum lines. Also check the brake booster and vacuum line fitting for a leak.

Mar 25, 2005 | 10:16 AM

kwanderi

Senior User

Joined: Jan 2004

Posts: 157

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From: Buckeye, AZ

Another often overlooked component is the modulator valve on the tranny, I'm assuming you have an auto. I replaced the on on my C6 in the 65 I had, what a world of difference in idle quality. It must have developed a vacume leak that caused the truck not to want to idle under 1000 RPMs. After it would idle at 500 just as smooth as could be.

Keith