Stumble/surge issue at Operating tempuratre at idle. Help.

This is on the wife's 96 Caprice with a L99. When the car gets up to operating temperature, she will get a hesitation off the line and sometimes at a traffic light or at a complete stop it will surge up and also seem like it is going to stall. Any ideas? I can tell you what has been done so far. New EGR, and new TPS both with no changes. It is very pronounced if she runs the A/C.

How many miles on the car

I think I would check the fuel pump pressure.

When is the last time it has had a tuneup?

Tom C.

There are 102K on it. I will check the fuel pressure this weekend. It runs like a bat out of hell until it gets up to ops temp and then it starts to act up but not all of the time.

OK. I finally got a chance to check the fuel pressure. At prime it is a solid 40 PSI. at startup/run it is 38 PSI with the fuel pressure regulator vacuum applied. With the vacuum blocked off at the regulator the pressure is 46 PSI and the idle is a little smoother and slightly higher in RPM.

Check
-idle air control valve
-o2 sensors
-ground wires at the coil
-ground wire from back of block to firewall
-burned or loose spark plug wires

Any check engine lights?
Any funny sounds?

The only check engine light was a P0300 and that only tripped one time. The wife now says that sometimes it will buck a little on the highway. I have a new IAC that I can install. No funny sounds present. I will recheck the grounds and wires. Nothing looked out of the ordinary the last time I looked at them. I will do a little more digging and report back.
Thank you

Sounds like an ignition problem.
Inspect wires and plugs
Inspect coil and the grounds
Might need new opti

Those would be the easy things.
Po300 could also be from burnt valve or headgasket on the way out.... But those are on the extreme end.

Run the car after dark and look for arcing.

Ok, I just checked all grounds near the coil/ICM and the rear of the motor. Everything looks good. I replaced the ICM and the coil and have the same issues. I am leaning towards the opti starting to shit the bed.

When we bought our L99 new, from day 1 it would "hunt" a little at an idle, and about 1 out of 5 or 10 times I'd go to put it in reverse, it would stall. I took it to the dealer countless times and they never figured it out but we learned to live with it. One day at about 60k miles, I decided to take the TB apart and clean it. I did not have fixing this problem in mind, just figured it was due. After that, I noticed right away that it idled smooth, and it never stalled again! I didn't see a damn thing wrong with the TB, no "smoking gun" or anything, but that fixed it in my case.

^^that sounds like you fixed a vacuum leak^^