2000 Lesabre - Massive vacuum at oil cap

Here it goes, never posted anything before. I just acquired a 2000 LeSabre/3800 Series II - only 85k miles. Pretty car, but I knew it would need engine work.
Most obvious symptom is a massive vacuum when the oil cap is removed. The car usually dies when you remove it. I am still trying to acquire maintenance records, but I believe a gasket set was done on the Intake Manifold.
I did a PCV kit (PCV valve, 2 o-rings, spring, cap and MAP sensor. No change to my symptom. The car sounds ok when it runs, but dies frequently, especially if you drive it.
I see some gasket leakage at the Intake Manifold and around the Throttle Body. I don't seem to find any coolant in the oil.
My current plan is to replace the Intake Plenum, gaskets and Lwr intake manifold gaskets. I come up with this from all the discussion on these failed items, but have a difficult time drawing the coorelation to the vacuum at the oil cap.
Any input would be helpful.

My first answer would be the o-rings at the PCV valve and MAP housing, but you have already replaced those....wouldn't hurt to double check though....

If not that, the only thing I can think of would be lower intake gaskets leaking.....

One thing to try: Remove the oil fill cap...get a can of carb cleaner....attach red tube to nozzle....stick tube in the valve cover at the fill cap......use your hand to seal the tube in the valve cover....have someone start the car.... then with the vehicle running, give a couple shots of carb cleaner into the valve cover....if the idle changes, then you know the carb cleaner is being sucked into the intake, which it shouldn't.....could be a bad lower intake gasket....could be the gaskets were replaced on the vehicle before and were incorrectly installed(moved during installation, not properly torqued, etc.)....could even be a crack in the lower intake, though I have never seen one....

Has vehicle been scanned for codes?

Has vehicle been scanned for codes?

Thanks for the input and great tip TechII.
It had codes when I parked it. All were for the MAP sensor. 2 for high voltage and 2 for low voltage (at the MAP). The sensor was physically broken, so no surprise there.
I replaced the MAP and other items mentioned and cleared the codes. I have not driven it since then, just started and let it run. No codes yet.

Yesterday, I finished taking the upper intake manifold off. It sure is an oily mess underneath.
I found a loose bolt laying on top of the lower intake manifold and appears to be out of the hole over in the top left corner by the #2 intake port.
My assumption would be that this is allowing a major vacuum issue (and oil up from the crankcase), though I do not fully know where that whole leads to.
Anyone know where that hole goes?

A loose bolt laying on top of the lower intake?......like I said before, sounds like someone was in there before and screwed up......looks like you should replace those lower intake gaskets now.....check the EGR hole in the upper plenum for degradation.....with the lower intake off, check for cracks and pitting of the metal around the port openings......you will need lower intake gasket setup, upper plenum gasket, throttle body gasket, change thermostat while accessible....not a bad idea for a coolant flush, too, if not recently done....and an oil change...

In case any of you were wondering how this project stands. Lower intake gaskets - replaced. New Upper intake manifold. New thermostat, PCV and o-rings. New L-shaped heater pipe and fuel injector o-rings.
All is back together and no leaks.
Massive vacuum at oil cap is gone. Runs great.
Dies or stumbles on-occasion and I think this is a seperate problem. Maybe crank position sensor.
So far I am pretty happy.

I suppose I should finalize this thread.
The project of replacing the upper intake manifold and the LIM gaskets went just fine. All that seems to be working out great. I am sure the cause of the massive vacuum at the oil cap was the bolt that was out; it was one of two that is on the inside and holds down the LIM. It was also what let oil residue up to the UIM and kinda created a mess. The inside of the throttle body was coated with oily buildup.
My seperate issue was the car dieing while sitting at idle after warm up. Or stumbled during acceleration. I replaced the crank pos sensor for that, but no change. It ended out being the MAF sensor. (Yes I used MAF sensor cleaner first.) Replaced the MAF sens and the car is running great.

Great, good work.