I have a 2002 F-350 with a brand new 5.4 motor we just put in. After we first started it we had a problem with the idle going up and down. I sprayed starter fluid all around the intake and vacuum lines, no leaks, no spike in rpms. I replaced the IAC valve and PCV valve, seems to run smoother at an idle until I put it in gear, then the idle goes up and down again. I don't know what else to look for, any ideas? Thanks....

 

I'll try checking and cleaning the MAF sensor and see if that works, I'll keep you posted....

 

Make sure to check your charging system. I had a couple cars with bad alternators do that in gear only because of excessive AC voltage being put out by the alternator.

 

Thanks, I'll check that next, this problem is driving me nuts....

 

Well I tried everything, new IAC sensor, PCV, cleaned the Ram air sensor, new battery, nothing worked. I took it into the ford dealer and after $500.00 later they found out that I had a bad 02sensor and a bad injector, Truck runs great now, only thing is I have no money left to go anywhere. The 02sensor corrected the idle problem and the injector corrected the miss i was getting every so often. Just want to let you know in case anyone else ever posts an idle problem on here.... thanks....