This is a classic blown head gasket (inner).  You could take it to a
mechanic with an exhaust gas sniffer to confirm.  I had the same problem
recently, precipitated a top end rebuild.  I found that the rubber seals on
the cylinders are o-rings about 1mm thick, one of mine had disintegrated
(it's always a relief to actually find the failed component!)  Of course
there is also a metal ring on the top, but this alone won't keep the
coolant out of the cylinder.  Additionally I had the classic external
coolant leak, patched for about 40K miles with Alumaseal, badly pitted
heads.  I was a little leary of taking the engine apart, but it sure was
satisfying getting it back together (with minimal leftover parts) and
having it run well (even more so passing CA smog).

I'd go ahead and get it fixed

Dan Fuller
'90 Carat