Ok, so my brother buys this truck for my dad. I decided since it has 280k on motor to tear down the top end to make it last another 200K. Went through replaced all sensors, cleaned the **** out of the throttle body, painted it everything up made it look new... Everything seemed fine was working properly. I loaded my bike to go riding was halfway up the mountain and check engine light comes on. Curious I pulled in to get some gas and it's idling at like 1600. When driving around town it's on and off. When your sitting at a light it feels like it shifts a gear, the idle will kick up, and the cel comes on. This comes and goes it's not consistant except for when the light comes on. The guy that sold the truck said he had replaced the tranny, but now I'm not sure. When it's running properly the idle is at 600. When it first startes in morning 1600 holds a high idle for way to long then comes down to around 1200 to 1400 Any help would be appreciated.. Im wondering if it's in the ecm, the tranny, or possibly the abs computer( I think it has abs even though the brakes suck).........thanx in advance..............1991 4x4 chevy silerado, automatic 5.7L, 280k miles

 

This sometime shappens when cleaning is done to the throttle body. Some of the electrical parts will go bad if a chemical has been thrown onto it. Or maybe something didnt get plugged in right. Either the IAC(idle air control) valve or the TPS(throttle position sensor).

 

yeah did u bump the iac or anything? they take a while to adjust themselves so i have heard. i try not to even mess with them. i would try another iac if you have one.