2002 LS V8 3.9 running rough only during idle

[JohnPags]

I have a 2002 LS V8 with 190,000 miles that is idling rough. I replaced the valve cover gaskets around 100,000 miles and since then have not had any issues with failing COPs. Additionally, I changed out all 8 coils and spark plugs around 150,000 miles. Prior to replacing the valve cover gaskets, I would burn out a coil roughly once a year so I am familiar with the symptoms of a bad coil. When this used to happen, the car would periodically sputter at idle but also when accelerating around 50 mph. Currently, I am not experiencing any sputtering while the car is moving, only at idle, and the problem lessens when I put the car in neutral. No check engine codes have been triggered. Does anyone know the possible causes? As of now, I think the problem could be with the following components:

Spark plugs
COPs
Fuel filter
Fuel pump
EGR valve/system

Like I mentioned, I don't think its the spark plugs or coils since I replaced them within the last 50,000 miles and have replaced the valve cover gaskets. Could the sputtering at idle be caused by another component that I have not considered in my list?

While this is my first post, I have been able to diagnose and fix numerous problems with my LS based on the helpful discussions that have been posted here in the past. Many thanks to those who participate on this forum.

 

[joegr]

What speed is the engine idling at?
Have you searched for intake leaks (unlit propane torch is a good way)?

 

[JohnPags]

The engine idles around 625-750 rpm when warm. I have yet to check for intake leaks. Are you suggesting that I open the valve on a propane torch and without lighting it, pass it around the motor so that if there is a leak, propane will be drawn in and I will be able to hear changes in the way the motor runs?

I forgot to mention in my last post that I replaced the fuel injectors as well around 140,000 miles. I have also now added the idle air control valve to the list of possible causes. I still need to check for intake leaks though.

 

[joegr]

...Are you suggesting that I open the valve on a propane torch and without lighting it, pass it around the motor so that if there is a leak, propane will be drawn in and I will be able to hear changes in the way the motor runs?...

Yes, I am. Checking the IAC is also a good idea.

 

[GoFar]

Easy way to check to see if the IAC is working is unplugging it, starting the car, and there should be a massive decrease in idling RPM (~500). If the RPM's idle the same way before, it has malfunctioned. If it is working and you do see a drop in idling RPM, plug it back in, and it should spring to life again.

 

[joegr]

Never mind, wrong thread...