I've got a 97 Bonneville w/105k miles.

When the car is running, and I unscrew the oil cap, a strong vacuum pulls air into the valve cover.

This seems odd, since most higher mileage cars I've owned have displayed a fair amount of outward pressure (blow by is what it'* called, IIRC)

Any idea what'* up?

Otherwise, the car runs great and has for the last 5k miles or so that it has been doing this (I've owned it for 5k miles)

I occasionally get a check engine light pointing to the EGR.

Also, I noticed that there is NO PCV valve. When I install one, the engine runs fine (a little less power), but there is a squeeling noise from the engine.

also...was any damage done drving the car around with no PCV valve?

Change the PCV valve, its an integral part of your engine and performance, and will DIRECTLY effect that ammount of vacuum you are experiencing when removing the oil cap when the engine is running..infact, it can make it bog down.
The pcv "positive crankcase ventalation" allows gasses and moisture that would pressure up in the crank case, vent out, keeping your oil a bit cleaner, and engine slightly more deposit free, get a new pcv.

Matt is absolutely right. If you install a PCV properly, it should remove the vacuum effect you're feeling at the oil fill neck. This is actually one of the quickest and easiest ways we recommend for people to check their PCV functionality.

When you get the PCV, you might want to see if you can buy a kit with the PCV valve, o-rings, and spring. Many 3800s end up without a PCV spring for one reason or another (usually it'* because some mechanic forgot to put on in), but I firmly believe having a spring as part of the assembly will insure proper sealing for optimal PCV functionality. Opinions seem to vary on this a little bit, and you may not even be able to find a PCV spring, so ultimately it'* up to you. Use the Search function to look up threads that discuss the PCV valve.

At the very least, make sure you get the valve and new o-rings. When you install it, make extra sure that all the old rings are out of the upper intake manifold. There is usually a black ring that likes to hide down in the neck of the PCV housing, which will keep the new rings and valve from seating properly if not removed.

Ben is on target with his recommendations from what we've seen. In most cases, the vacuum was caused by one of the items above. Missing spring, missing o'ring, or old o'ring left in the PVC housing.

And by missing the O ring/Spring.. you will get a squealing type sound. Also if it'* not installedseated properly.

just curious, shouldnt the car be squeeling if I don't have a PCV valve installed??

No, the squealing noise is likely caused by crankcase fumes being pulled into the intake manifold around the outside of the PCV valve due to a missing lower o-ring or a bad seal around the base of the PCV valve as the others have suggested. Just like a whistle, high velocity, low volume through a narrow slit.

Did you check for a lower o-ring on your PCV valve?

As long as it has not been for very long, you probably have not caused any serious damage, but don't keep driving it like this. The vacuum you feel at the oil fill pulls the lip seals on the ends of your crankshaft in and allows dirt to accumulate where the lip seal should run. It will cause the seals to wear in an abnormal way and can cause them to fail prematurely. The only way to change the rear seal is to remove the transaxle or the engine. Better idea is to fix the vacuum leak into the crankcase. The longer you wait, the more harm you do to your seals.

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This is why you should get a PCV in there soon.

Thank you, Bill. Always quick with the background info, and always appreciated

I thought I'd chime in on this one.

I ran my car by accident once without a PCV valve just after installing a new UIM and the sound the car made...dear heavens, screaming is not the word...I think "blood curdling death cries" would be more applicable.

Im not sure if that applies in every situation, but mine made some of the worst sounds ive ever heard a car make when I had no PCV valve.