New pressure caps can be bad. I was suspect of my old one so ordered a new
one from GoWesty and installed. Thought I would test the old one so I built
a gizmo to test. It tested fine, just for kicks I tested the new one.....it
didn't start to bleed off untill nearly 30 #'s  ! Returned it and put the
old one back on. Also went through two "new" front heater valves that
leaked after only a month or two. Went to VW for the third one, two years
and its still good. Moral.....be suspect of new parts...

Roger B.

From:    Tom Carchrae <tom@CARCHRAE.NET>
Subject: Re: coolant overflow fears and thoughts

@Bill - I've blown through that pipe, it is not blocked.  In fact, I used
it to drain the extra coolant out of the bottle.  I wonder about it being
air tight though, although I've seen no leaks.

@Dennis - Thanks.  I've been tempted to build a cap tester just to confirm
my theory on that cap holding too much pressure.  I ended up replacing
bleeder valve assembly, radiator, and at least one hose after that had
happened.  The only other thing I can think of that may have caused the
pressure problems was a badly routed front heater bypass job (the heater
hose may have been pinched behind spare tire).

I have done the combustion gas sniffer test, and it came out ok.  I guess I
should have a compression and/or leakdown test done again and see if that
shows any signs.  As you alluded, I suspect it may only be failing when
driving/under load.

Any other ideas to confirm the issue before tearing the engine apart or
replacing... :/