Scotty, so my 93’ F-150 5.8 236k. has had issues with high idling, even after replacing the IAC with a OEM part and resetting the computer. I think it’s what is causing the transmission to shift a little rough. What else should I look for? It’s not tripping any check engine light so my shop is hesitant to diagnose. When I unplug the IAC, it idles back to normal but when I plug it in, it revs back up. I’ve also cleaned the throttle body with the spay cleaner. Thanks!
My first thought would be to look for a vacuum leak. It takes extra air for the engine to idle higher. If the main throttle body blades are closed, the idle air controller is supposed to be the only way for that extra air to enter the manifold. It's likely you could have gotten a faulty IAC, I've heard from other websites that the IAC can check good, and still be defective. If you get a shop to diagnose your truck and any codes pop up post them here, you may have an EGR problem or vacuum leak if the problem not directly related to the TB or IAC.
