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Old 02-07-2013, 02:27 PM   #1
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Default High Idle Problem

I have my beloved 1988 GMC 4WD ext cab K1500 5.7 Throttle body. The truck starts good, and idles smooth. Problem: The longer it runs, the faster it idles. At times when I stop and put it back in gear when it is running so fast, it dies just as soon as I put it in gear. Re-starts, but high idle.

Also, my heater doesn't switch from heat to defrost. I am assuming it is a vacuum problem, and the two might be related, but I cannot find a bad hose or leak anywhere. Any suggestions on where to start or what to check?

Thanks,
Gene
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Old 02-07-2013, 03:38 PM   #2
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the engine problem sounds related, vacuum leak will do that. spray around areas with carb cleaner and listen to the engine. any older tube, intake manifold, around tb base etc
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Old 02-07-2013, 04:54 PM   #3
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I agree with Justin on the vacuum leak, but based on the problem you have with the heater I suspect a vacuum leak in the heater control module. Don't know where it is on your truck, but on Bonnevilles it is located behind the glove box. Take a look at this link and see if it helps:

https://www.gmforum.com/mechanical-1...2000-a-279092/
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Old 02-07-2013, 04:55 PM   #4
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Of course, a large vacuum leak in the engine bay could affect idle and the heater control, so you need to check there like Justin suggested.
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