94 f-150 with 5 liter. Cel on. Reading was tps below minimum voltage. Replaced tps with help from this forum and my book. Have checked voltage readings 2 times. Throttle closed .96- throttle w/o 4.62. Now the problem. As long as I drive slow as in traffic in town normal. When the throttle is advanced to highway speed (above about 45 to 50) or start up a hill something holds the throttle open to a high idle speed. By slowing the truck to a stop the idle will return to normal in about 2 or 3 seconds. ( Hard on brakes). Cleaned throttle body. Disconnected cruise control. What is connected to my throttle valve that I cannot see? Air bypass valve is mounted on the side. Could this be the problem? I keep thinking it is an adjustment on the tps, but it appears ok. Any comments appreciated. No cel on after tps replaced. Lon.

 

On a hunch, try to pull the codes anyway, even though the CEL didn't blink at you. Maybe we'll get lucky.

Ford uses the IAC not only for idle, but for acceleration as well. They move it to a more open state when you push the throttle down, to give you more of a responsive feel. It's possible that it's not broken, but just starting to stick.

 

I'll second Frederick's advice on this one. If your TPS is in range and you are having the slow return to idle it's likely a sticking/clogged up IAC. Try removing it from the throttle body and clean it up really well - be careful not to damage it. Reinstall and see if that doesn't resolve your problem.

Post back and let us know.

Cheers!

Mike

 

Removed the IAC and cleaned it. No change. Replaced it with a new one. Do not have the sticking problem when the throttle is advanced, but now have high idle at all times. When slowing down the idle appears to step down some, but does not go to normal idle. Cannot find a vac leak. Have not checked the codes yet. With the old IAC installed when the truck was stopped the idle returned to normal. Will try to check the codes when rain stops. Please comment if you have an idea. Thanks, Lon.

 

Ok, do me a favour here would you. Get a can of WD40 or some such and start spraying liberally around the base of your intake plenum with the truck idling at whatever speed it happens to be idling at. Listen for any chnages in idle speed. You might also want to try sprtizing a bit around your existing vacum connections just to be sure that there are no leaks. Have you looked at the coffee can thingy on the passenger inner wheel well? It is a vacum canister and they can sometimes develop large vac. leaks. Also please run the codes if there are any stored.

Thanks Lon!

Cheers Mike

 

While looking for intake leak decided to pull wire off my new IAC. Idle went to normal. Runs OK and no CEL. What will this do? Thanks, Lon.

 

just replace that IAC, sounds like a bad one out of the box.