one way is do a compression test watching for which cylinder produces the
bubbles.
Would be nice if which side it is shows up that way.

and/or ..
Do a leak down test basically ..
which is to position one cylinder at a time on TDC on compression..
put compressed air into the spark plug hole,
usually via  a threaded fitting..

you won't be generating that much pressure that way though ..
your air compressor might put out say 120 psi ..
and combustion pressure is much higher than that.
Though I did find # 2 was 'the cylinder' on a 1.9 wbxr engine that way
once...
with a leak down test.
it was quite clear it was on the right side of the engine, the head for  # 1
and 2.

I doubt air is trapped in the system.
perhaps residual air/exhaust from the real problem though , for sure.

Scott
www.turbovans.com

----- Original Message -----
From: "Travis Newhouse" <travisnewhouse@GMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, September 21, 2011 8:22 PM
Subject: Re: 85-H20 loss of coolant mystery

> Thank you for all of the suggestions.
>
> I tried the at-home pressure test.  Engine was cold.   I removed the
> expansion cap to make sure I wasn't starting with pressure in the
> system.  Replaced the expansion cap and started the engine.  I let it
> run for about 1 minute.  I revved it a little from the engine bay,
> trying to see some of the bubbles in the expansion tank that I've seen
> before.  I shut it down and let it sit for 30 seconds.  When I removed
> the expansion cap, I heard a small rush of air and I noticed the
> coolant level in the expansion tank rise up a little bit.
>
> So, it could be the pressure was caused by heated air trapped in the
> system, or it could be exhaust gas being forced into the system.  I'll
> call around to see if any shops nearby can sniff inside the expansion
> tank.
>
> If the sniff test is positive, what is the best way to isolate which
> cylinder head gasket may be leaking?
>
> Thanks,
> Travis