When I first start up in the morning my 99' 5.9l truck is exhibiting some bad behavior. The idle is high... around 2000 rpm and it won't settle down. It alsomisses and runs rough. Once the truck warms up it runs fine.

I recently replaced the throttle position sensor and that solved a lot of driveability problems, but this is still an issue. I haven't thrown any codes, so I am a bit frustrated as I have no way of diagnosing the problem. Can anyone help shed some light on what this might be?

 

on the back side of the throttle body is the idle air controller (iac). it has a plunger that is supposed to move in/out of a well, under pcm control. it is responsible for the idle speed. when all works properly - as it opens up the idle speeds up, and as it closes down the idle slows. this controls everything because if your foot is off the gas pedal, then the trottle plate is closed tight and no air passes it. if you haven't already done so, remove the throttle body, all 3 sensors from it, and clean it thoroughly with carb cleaner and toothbrush. some clean the iac,but i say just replace it for about $40-50. especially clean the well where the iac plunger moves in/out.
a bad or dirty iac generally causes idle surging up down 500-2000 rpm, sometimes it settles down temporarily, then comes back.

 

So does that mean if I get the nice new high-flow throttle body that I've been drooling over that this problem will be solved?