OK guys, I've got one that has me stumped.

About a month ago I replace those little plastic inserts for that I M are C on the back of my 98 F150 4.2l

The truck ran perfect for about 2 1/2 to 3 weeks. I was just getting ready to start bragging on how well it was running when all of a sudden one morning it decided to idle high.

Now what I have is a truck that's very sporadically may idol very high or medium or may not I don't hide all the run fine. I have checked and checked and checked for vacuum leaks and cannot find anything. Now sometimes whenever I turn the truck off while it's idling high I have difficulty starting it back up. The truck doesn't want to start after I shut it down from idling high.

Forgot to add that it is also sporadically idling up-and-down from time to time meaning that it's almost like it's going to try to start hunting.

 

Could an IAC cause these symptoms?

 

First, try inspecting and cleaning the IAC.

 

If your having this problem and your check engine light isnt coming on the IAC is bad-probably.

 

Cleaned the idle air control her. It was dirty. Took truck for a drive it's doing the same thing. I can be sitting at a stop light and it will idol fine and then suddenly going to high idle. It feels like or seems to me at least, it's a sensor or something mechanical that is controlled by a sensor or the computer or something else. It's very inconsistent.

 

by the way, the hissing/whistle sound is still audible at high idle. I cannot get it to duplicate the sound while the truck is in park. It's only under load that I hear the sound while I'm driving.

 

take away the sound I'm hearing, could a TPS cause the idle issues.

 

Replace the IAC

 

Could a bad IAC increase idle even with no pressure on pedal? Sometimes it increases and I'm in park.

While I'm not completely green, this is an area I'm not familiar.

 

Replace the IAC. The symptoms your describing are IAC. If you had another problem like a vacuum leak you would have a CEL. You can order the motor craft part at rockauto.com I've used the autozone part without issues but others hate them.

 

The only code I have is for the engine coolant level. Plenty of coolant and just replaced the sensor a few weeks ago. Prior to that the old sensor wasn't working and had no issues with truck.

I understand this might cause some idle issues but I've never experience this high of an idle with a coolant issue before. But again, first time to have a ford.

 

Thanks everyone.

Thursdaymorning I checked the hose from the pcv to intake. It has a about. 110 degree bend. When I wiggled it I hard a loud hiss. Took it off, inspected and slid back on.

Then I checked the engine coolant level sensor. This is code that came on at the same time as the idle issue started. It's a brand new sensor.

I discovered that while the plug was on the sensor it was not "clicked" on. Pushed it back on.

Started truck. No high idle. Drove around town and idle only went up one time for a few minutes.

Have driven 2 days and this morning still no high idle.

If it was truly the engine coolant level sensor, I've never seen one effect idle like that before. The truck would pull itself 40-50mph.