Here is some background information which may be important:

The vehicle was originally taken into the dealer due to a heater
problem.  It took a long time to heat the interior because there was
very limited heat output into the interior.  The fan had to be on low
to get any heat at all.

The dealer replaced the heater core.  After this I am not 100% sure of
what happened when.  However, either because the car was overheating
or becuase the heat was no longer working, the dealer replaced the
thermostat.

After leaving the shop with the thermostat, the air conditioner would
not work.  A few miles down the road it started working.  I think the
overheating problem started after this but I am no longer sure.

At first the problem only seemed to occur after a very long drive with
stops after the drive.  It may have had the problem if I made shorter
trips but I normally drive about seventy files each way before I stop.
The problem continued to get worse, so I switched vehicles with my
wife.  The problem showed up on her twenty mile compute much more than
on my long distance commute.  She has to stop at lots of red lights.

I took the car back to the dealer and they said they detected a trace
of exhaust gas in the coolant.  That is when I decided to use the
block seal.  While flushing the system, I noted that the heater was
putting out well.  While installing the seal, I noted that the heater
was not putting out.  The thermostat was removed at the time.

After installing the block seal, I flushed the system and installed
new coolant.  The next day, I drove about 10 miles watching the engine
temperature the entire time.  The temperature was perfect when I
stopped the engine for about 30 minutes.  I then started and drove
about five miles down the interstate.  I exited and the temperature
was still perfect until I stopped at the fourth red light.  At that
point, the temperature shot way up.

I immediately started the drive back home while attempting to avoid
stops.  I made the first red light but had to stop at a stop sign. The
stop was very brief and the temperature was slightly high but did not
seem to increase (probably had already lost enough coolant to explain
the slightly elevated temperature).  I drove 15 miles and the
temperature remained the same.  I stopped at another red light about
five miles from home and the temperature started to increase after a
few seconds.  I drove the remainging five miles at highway speed with
the temperature as before until I stopped in the driveway.  The
coolant was bubbling out when I stopped the car and the temperature
increased.

Later that day, I replaced the coolant, added barsleak and idled the
car about 15 minutes.  The temperature increased to just below high
and then decreased slightly.

After leaving the car off for a couple of hours, I made a trip about
two miles up the road with three stops.  By the time I returned to my
drive way and stopped, the temperature was very high and the coolant
was bubbling strongly.

Two days later, I drove the car about five miles and stopped for an
hour.  No problem at all.  Got back in and drove two miles and there
was a stream of coolant on the concrete when I left the parking lot.
The temperature was fine for the five miles it took to return home but
the resevoir was low on coolant.

Here are the responses to your questions:

I did not find any coolant in the oil or any oil in the coolant.  Both
appear clean.  There was a trace of exhaust gas in the coolant which
is why I applied the block sealer.

Is there anyway to check coolant flow without pulling the water pump?

I flushed it completely, after I started having the problem after the
dealer changed the thermostat and couldn't find a problem.

I flushed it again to add the sealer.

It does not seem any worse and has not changed from before I added it.

I will check the voltage on my next trip out.

How do I check this?

I already replaced the cap which made no difference.  I have since put
the original one back on because the replacement seemed buckled.

I reinstalled the thermostat with the same orientation as the dealer
which appears to be correct.  The pointy end is toward the radiator.
The jiggle valve is up.  Unfortunately, I did not test the thermostat
while I had it out.

.. Thanks,
.. Commercial Pilot and CFIG
.. What do you fly?

I have about 183 K miles on car.  I have a noise from the "pumps" area
but I think it is the power steering pump. I will be getting rid of
the car soon but I would like to see it make it through the summer so
I can take my time deciding on a replacement.  If noremote t, I have a
couple of deals in the works.  Its a shame as it has four good tires,
excellent brakes, great A/C, and no problems other than overheating
(big problem).

The biggest remainging suspects are the water pump, cooling fan
circuit, and radiator.

I have seem a lot of reports of vanes corroded away in Taurus water
pumps so this seems a definite possibility.

There could be an intermittent problem with the coooling fans. I will
check their voltage when I drive the vehicle again.

It could be the radiator but I don't think so.  I got normal discharge
flow when I flushed and refilled the system while attempting to
resolve this issue.

I don't think the temperature sensor is the problem since temperatures
read as expected before, during and after the overheating occurs.

The problem could still be a leaking head gasket or crack in the
block.  The sealant should have fixed this problem and there was no
indication of such an issue when I checked the spark plugs, oil and
coolant.  No oil in the coolant.  No coolant in the oil.  No sign of a
problem on the four plugs I checked.

The spark plugs were supposedly changed earlier this year.  I checked
the front plugs and the easy to get to rear plug.  The front plugs
were in like new condition.  The rear plug had a normal appearance
instead of a like new appearance.  I wonder if the dealer "forgot" to
change the back ones. In any case, I will pull the other two plugs if
I don't resolve the overheating issue or trade the car in first.

I will let you know how I resolve the problem if I am able to do so.

Thanks.

I assume you mean the temperature system in the grey housing near the
thermostat.  Both radiator fans operate whenever the air conditioner
is on so I don't think it should make a difference since the PCM would
not need to turn the fans on since they are already working.  I have
never not seen the fans working with the A/C on but will make sure
they kick in when the engine reaches temperature.

I will also check for 2000 ohms hot and 40000 ohms cold across the
sensor.

Thanks.

You stated that someone had put in a new heater core and you think the
problem may have started around that time.  Have you rechecked all the
work that was done at that time - maybe they reversed the heater hoses
or installed the heater control valve backward or pinched/kinked one of
those hoses.

The upper hose, being the outlet hose, is not usually a problem.  A
spongy lower hose can collapse from the suction of the water pump - but
that is usually something that occurs at high rpm and your problem is
the opposite - low rpm.

I would check that all hoses are going to the correct place, maybe check
that thermostat again (even try running the car for a few days without
it).

I think you have a bad head gasket.  Block sealer was the wrong thing
to put in, it never works.  You should immediately check for water in the
exhaust, for water leaks from the block, and for water in the oil.

Assuming you still have a 50/50 mix of water and antifreeze, smell and
observe the exhaust when the car is hot.  Then let it cool down and
get cold then start it again, let it run a 30 seconds or so, then shut it
off.
If you smell antifreeze at any time from the exhaust, or any water dripping
from the exhaust feels slick between the fingers or smells of antifreeze,
your gasket has failed.

If the car is losing water and there are no leaks from the block the gasket
has failed.

If there's water in the oil the gasket has failed.

The reason your cooling has shot to hell is because the exhaust and
combustion chamber are at a higher pressure than the coolant, and
exhaust gas is being forced into the coolant, causing air bubbles.

I think your putting your head in the sand and hoping it isn't a head
gasket.

Ted