My 94 F150 4X4, 5.8 w/auto idles VERY high. I've replaced IAC with new Ford part. I found the voltage at the IAC connector to be 12 volts. The repair book that I have says it should be 10.5 and I need to replace the Powertrain Control Module (computer?). I could sure use some help. Thanks.

jimbob

 

The only way to check the IAC valve solenoid is to measure the resistance at the pins with an ohmmeter; it should be between 7-13 ohms.
Why do you need to replace the PCM?

 

High idle is also a sign of vacuum leaks. The gasket between the intake and plenum is a common place to have a leak.

 

Both IACs ohms were between 9-9.5. The only reason I'm asking about the computer is that the repair manual says the voltage that goes to the IAC should not be higher than 10.5 volts. If so, the IAC is permitting too much air to bypass thus causing the high idle. I haven't changed the computer yet. I'm hoping someone has the correct voltage reading at that connector and if it is out of limits, what would be the corrective action. As far as a vacuum leak, I've checked and rechecked...but I could have missed something. I'll check again.

Thanks for your input...keep them coming!
Jimbob

 

Hey, something a Ford repair guy told me, check the Throttle position sensor (TPS), on the Throttle body. Mine is underneath on my 91, so I don't know where yours is. However, I found a bare wire that might have lead to a short out. It's a cheap part, around 35.00. The position also lends itself to shorts by water, and crud from the engine. It's kind of a pain to reach, I gotta pull the throttle body to get it off. Anyhow, he said before you spend the big bucks, it's a part often overlooked and it's made of plastic, subject to temp changes, etc...

Good luck.

 

Thanks, I'll look at it this weekend and let you know.

Jimbob

 

Hey, just a quick note in case you haven't already fixed it. I just got my high idle problem fixed, and it turned out to be the intake gasket. The repair guy said it just blew at one point. A quick way to check, if you can plug the throttle body with your hand and it don't kill the engine, then there is a serious leak, and needs a gasket probably at the intake air plenum, between it and the engine. It cost me about 350.00 but I also had an analyzer hooked up and some other minor work done. It sure runs better now.

 

Placing hand or something was what I used to do to until I had to replace an intake manifold gasket at my cost and time for doing it, I did the same thing and the truck engine started to idle higher after I did the close off the throttle body air trick, It just so happened that intake vacuum is very strong and it pulled the gasket into the intake manifold, try using Berkibile 2+2 carburetor cleaner, it will make the engine die if you find a leak somewhere, also the IAC numbers are to the Red wire charging system voltage with vehicle running and 9.5-10.5 volts dc engine idling on the white wire, (white wire is also on dc averaging voltage not straight dc, it will fluctuate all over the place in straight dc volts). Numbers higher than 10.5 = intake system leaking, IAC not working hard enough, IAC numbers lower than 9.0 IAC working to hard. If you need more help on this let me know, Electric1

 

[updated:LAST EDITED ON 04-Feb-02 AT 08:01 PM (EST)]Please let us know how this turns out...I am very interested because I have the exact same problem with my 89 F-150 5.0 EFI. I just purchase a code analyzer and the TPSensor was one ( of many) codes it gave me. I replaced the TPS and and IAC and still the idle is very high. I have several other codes listed...but I am mostly concerned about the idle right now. Also my throttle body was incrediably dirty, even worst was the plennum. LOTS of black ****...this can't be any good!!! I just paid $2.5K to have the engine replace..and it looks like they did NO clean up work. I believe I will pay them a visit this week.

 

I had a 5.0 in my 86. I found the egr port into the plenum blocked by carbon , the tps sensor bad, the O2 sensor had the end wore off also. These were the main contributers of my high idle and rich running. I ended up replacing the motor at 90k, the mains were noisy. That seams to be a problem on the older 5.0. When I tore it down, the timing chain was able to almost touch in the middle. Got a rebuilt motor from advance auto, boy is it tight!

 

I just got it fixed!!! I just couldn't find the time to work on it until now. I really couldn't find the smoking gun but I changed the gaskets between the throttle body and intake and the gasket between the upper and lower intakes. The truck runs fine now. I couldn't really see anything that was wrong with either gasket but I would change the throttle body first.

Thanks for everyone that kept pointing me to a vacume leak. You guys saved me from buying a computer.

Jimbob