I recently helped a buddy replace the head gasket in his '86 with a '84 engine. It runs like a champ...except oil starts spitting out of the oil separator(?) when we remove the oil filler cap the pressure goes away. Is the this a vacuum problem? how can we get the pressure to regulate?
We want to get this car to Thunderhill in a month for the NASA event. Any advice/ideas are appreciated.

Is there a vacuum line diagram online? I looked at Bills 944 world...but nothing. They have them for turbos, but not N/A.

I have never seen one spit oil up from the separator . could that be an issue with blow by (ie rings are bad) ?

I hate to think negative, but I assume you did a leakdown compression test after the hg install?
it could be a bad piston or ring, warped head or faulty head gasket causing blow-by in the crankcase.
You could test the PCV and such, but since you already had headgasket problems I'd eliminate the obvious first.

It could be bad rings pressurizing the crank case. It could also be a problem in vacuum lines.

There is a silver sticker on the pass side inner fender that show the the vacuum routing.

If it is not there you will need to look it up. I don't have a copy handy.

is that the same diagnosis when the dipstick pops out due to crankcase pressure? and shouldn't the excess pressure be routed back to the airbox(intake)?