my truck revs fine in park and netrual at about 600-700 rpms but in drive with the ac on it goes to 500-550 rpms and sometimes ducks down to 300 rpms only happens when the truck has warmed up and when the ac is off and in drive it revs at 400-500 but when you first come to a stop alot of the time it goes down to 200-300 rpms and after 5-10 seconds down there it goes back too 400-500. any ideals on what it might be? sometimes in park or drive and ideling it feels "shaky"

 

anyone have any ideals?

 

Could be a missfire. Check all of your plugs.

Clean the air filter, and check your MAF.

 

As long as you are checking and cleaning things take a look at your Idle Air Control. It is the cylinder that bypasses air around the throttle plate that the computer uses to control idle speed. They will sometimes load up with carbon and will not move freely or not be able to pass enough air to keep the idle speed up where it belongs. You can clean it with carb or brake cleaner and I would then lube it with light oil ( WD40) and blow the excess out before reinstall.

 

on another post they said yopu can replace it, does anyone know how much is costs?

 

I have a 99 4.6L with 101000 miles on it and started having the problem that you are describing since about 8,000 miles. Dealer never could solve the problem while in warranty so about every 8-10 thousand miles, I disconnect my battery cable to reset the computer and it will run fine for about another 8,000 miles.

 

Replace your PCV valve ..It's sticking

 

well this last weekend i replaced the IAC valve for 44.95 out at the drag strip and after acouple runs the computer reset itself and everything went back to normal, the IAC valve have been cleaned out by another shop and it was still malfunctioning, so i replaced it and it is normal again

 

I have replaced the PCV Valve - no effect. This problem has been going on since 1999 and nothing effects it except resetting the computer.

 

Mine used to do the same thing. I had the plugs and wires changed and it's helped tremendously. It usually happened when I was stopped at a light/stop sign.

 

i replaced the IAC valve (44.95 at o'reilly) or (120 for motorcraft) and i made a couple dor drag passes and everything went back to normal! so that is what it was, by the way cleaning it didn't help tried that first.