Just rebuilt a new 360 for my 98, but having high/ fluctuating idle issues. I replaced the IAC with new oem mopar and the TPS with new Delphi. Added a 53mm throttle body and headers, otherwise all stock. What else can I look for? Mainly seems to do it on the way home from work when it’s hotter outside. Come to red light and rpm’s stay up, but will eventually settle down to 650 after a while, but then sometimes goes back up. Not sure about a possible vacuum leak causing this. I would think it would never idle correct if i had a vacuum leak somewhere. I did look at vacuum hoses, but all seem good

also, when I first start the truck before or after work, I do hear a sound, sounds like air, like from an air compressor blow gun attachment. It does that as I rev it but only for the first minute or so then goes away.

 

Did you clean the throttle body when you swapped the IAC? They like to gunk up, and the IAC won't work the way it is supposed to.

Are you getting any codes??

 

Throttle body was CNC’d to 53mm so it was cleaned. No codes. After reading my own post a few minutes after I typed it out, I decided to throw the smoke machine on it. TB has a new gasket, so not sure why the huge leak there. Guess I will pull it off tomorrow and see

 

Since you're installing a ported throttle body, are you using special bolts with a recessed head (I believe the threads are 5/16x18)? Could be that the throttle body or intake manifold base isn't perfectly flat. Would two gaskets take up the slop?

 

Fun fact: if you install the throttle body gasket backwards, it will cause a vacuum leak. Luckily, had an extra gasket and installed it correctly this time. Lesson learned.

 

LOL. Hey, at least it was an easy fix.