My truck 99 5.3 EC 166k miles, has problems at idle after it gets warm. It happens intermittently. I'll be idling in park is the only time it does this and it will speed up and slow down from about 400 to 700 RPM's or so, it doesn't idle evenly. I know my cats are going bad so I'm not sure if this has anything to do with it or not. Any help is appreciated, it's nothing major just kinda wondered why.

 

Pull out your IAC motor and see if it's carboned up. If that checks out ok, i'd probably start looking for vacuum leaks, maybe even a bad pcv valve acting as a vacuum leak.

 

The above is a good start......also look at your fuel pressure regulator.

There seems to be alot of the FPR problems going around....

 

IAC motor? sorry for the dumb question. I've looked around for anything that might not be connected and just sucking air, why would it only do this when it's hot and only intermittenly in park?

 

Wouldn't know where to look.

BTW Thanks for the replies.

 

The IAC motor is in/on the upper of your throttle body. It's held by 2 torx screws. That's what controls your idle airflow. It allows and restricts air at idle depending on air temp, engine temp, etc. When it's carboned up it'll stick or clog the airway causing your engine to surge and "search" for the proper air flow count.

 

Would I have this on a 99 since it's not electronic controlled TB?

 

It is not really a "motor" more like a valve. It does get gummed up with carbon and will cause the problem that he complained about.

 

Thanks for the help I will check this out tommorow.

 

It will be the one on the top right side of the TB(looking at it from the front of the truck). The one below it is the TPS.