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

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

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/

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.