I have run the truck with the ac ever sense I got it a few weeks back. today it was nice out so i ran with no ac and the windows down. the at stops the truck would idle down to about 400rpm. it never was rough or anything just low. taking off was no problem and it didnt stutter or anything. with the ac on it stays perfect at 650rpm.

i know the computer "learns" and was wondering if maybe it just is not use to having the compressor not running. i only drove it for about 5 min with no ac. didn't like the low idle so i turned back on the AC. I have checked for vacuum leaks and have not found any. i need to clean the MAP and IAC valve as well as the EGR valve. could those be causing the low idle?

and does anyone have a good diagram for the vacuum lines? I have searched here and with Google and found the 5.4l ones but not for the 4.6.

 

I'd start with the IAC and MAF.

 

The IAC has the only idle control as signaled by the PCM.
The fact that with the A/C on it moved the idle up tells you the IAC is beng controlled by the PCM but not how well.
Remove the IAC and spray clean it inside until the cleaner come out clean looking.
Move the pintel inside with a small screw driver to be sure it's free to move.
This part is what controls air by pass around the throttle plate.
Leave the Mass Air alone, it's not the problem. It only measures the airflow and reports for fuel and ignition adjustments.

 

My 97 4.6 will do this everytime time the battery gets disconnected. Computer has to relearn idle. Mine can take up to 3 weeks to relearn depending on how many miles I drive a day.

 

Thanks! i am going to clean the IAC tomorrow and see if that helps. I only drive maybe 10 miles a day if that. maybe I should just take it out on the hwy for a bit and see if that helps.

 

IAC is a good start. I have heard of vehicles having issues with the A/C idle control, causing strange idle symptoms. The truck should idle higher than it is now all the time. My truck runs about 900 w/ A/C on, 800 with it off. Good luck keep us updated.

 

I would try running it on a trip for a while with a/c on tried all the above with mine. Still did same. Computer has to learn idle again.

 

Best stuff to clean out the IAC with is WD-40.

 

well i didnt get a chance today to get it cleaned out. I will try using wd-40 on it when I get it off. what about the gasket is it going to need to be replaced or is it ok to reuse the old one? the Hayes manual says the factory idle is set at 650 (which it is on with ac on)

 

Right, I figured mine was a little high, but I don't know how to adjust it and it isn't causing issues, so I don't worry

 

The idle is not adjustable except with a programmer.