My 1999 Park Ave (L36 naturally aspired) SES light came on today. I took it to Advanced and they said it was Evap/ Service engine soon.
Checked the gas cap and it was on nice and tight.
Opened the hood and noticed an odd sqealing sound , sounded like a fan belt but when I checked with a stethoscope it became clear the noise was not coming from the front of the engine. In order to get a better look at the situation I took off the plastic cover that sits on top of the UIM. As soon as I removed the oil fill extension the sqealing noise stopped and the engine idled very rough, it tried to stall . Put my hand over the oil fill on the valve cover and I could feel a noticable vacuum. When I seal the opening with my hand the sqeal comes back and it idles normally.
Any ideas on what might be going on?

Check the PCV valve, sounds like it'* stuck open.

The pcv valve seemed fine. Shook it back and forth and the piece inside seems to rattle freely. Put in a new PVC valve anyway. No change in the problem. Put an engine vacuum gauge on it and started it back up. I am getting 19 inches of vacuum when idling. When I remove the oil fill cap from the valve cover the vacuum drops to 15 inches. If the PVC valve is functioning properly what else could cause such a condition?

Wonder if you have a bad gasket. Kind of seems like there'* a internal vacuum leak.

I put a new UIM about 14 months ago ( 20,000m). I used the aluminum GM gasket. Can you think of a way to test it to see how the oil gallery and the intake could be tied together?

Nothing that is easy. The only way I can think of off hand is a leak between the intake manifold in to the lifter valley. If it was me I would pull the top end and check it out.

I took the PCV valve out of the UIM and started the engine with the cap removed. It ran very rough but it would run. The vacuum on the oil fill was gone. I held the palm of my hand over the top of the PCV opening and the vacuum on the oil fill returned. The engine vacuum clearly comes thru the little white tube that enters the PCV valve area. When I put my finger over the little white tube there was no vacuum on the oil fill. It seems like the problem is in the operation of the PCV valve. The suction from the little white tube seems to draw air from the lifter gallery and creat a vacuum in the crankcase. It has too be something simple, but what?

Make sure you have the o ring seal on the PCV valve and the big o ring below the plastic part that secures down over the PCV.

O-ring on the PCV valve???? I have an O-ring on the black part that seals over the PCV valve but the PCV valve does not have one. I will go look at the schematic in the factory manual. I may have missed something. That makes sense. If there is supposed to be a seal and it is missing that is how the engine vacuum is getting past the PCV valve.

I did a lot of research on your problem last night, and that'* what I was able to fine. O ring and spring comes up. One person said he didn't have a O ring or spring in the new kit. He found a kit that had both, installed them and the valve cover vacuum went away.