I got a Block Tester at Auto Zone today - a little tube that holds
blue fluid that will turn yellow when exposed to exhaust gas.

I opened the expansion cap, started the motor, placed the block
tester's rubber cone sealed against the opening of the expansion tank,
and began pulling air through the little tube with the bulb.  I tried
for several minutes.  Fluid stayed blue.

I checked on the temperature gauge, engine still coming up to temp.  I
tried again to pull air from the expansion tank through the blue fluid
in the little tube.  Fluid stayed blue.

Engine is starting to warm up now.  I noticed that the level in the
expansion tank has risen a little.  I try again with the block tester.
 Fluid stays blue.  I notice the level rising in the expansion tank,
and go to shut off the engine.  Too late.  Thermostat must have
opened.  The expansion tank overflowed and steam billowed out of the
back.

Doubting the block tester, I replaced the expansion cap and restarted
the engine.  I held the block tester near the tailpipe and pull air
through it.  Fluid turns yellow.

I'm feeling pretty good that exhaust gas may not be in my cooling
system, and the head gaskets are probably still doing their job.

Tomorrow I'm going to go over the engine and cooling hoses once again
and look for signs of leaks that could be allowing air into the
cooling system.  Unfortunately, I couldn't find a pressure tester that
fits the threads on the expansion tank to help me out.  Does anyone
know if any chain auto parts stores or car dealerships sell an adaptor
that fits?  Do any other makes share the same threads?

Thanks to all who have offered their help.
Travis Newhouse
Encintas, CA