> Measuring the temps of the hoses into and out of the radiator is
> also useless without knowing flow information.  Imagine - say
> you have a 20 degree difference between the inlet and outlet of
> the rad, and good flow.  Your pyrometer/whatever would say 20 degrees
> difference.  But say you have a clogged rad so you are getting very
> low flow, but still getting a 20 degree temp change. 

Generally a radiator clogged enough to cause overheating will have
flow restricted such that what water DOES go through will come out
much cooler than normal. A clogged radiator doesn't really lose any of
its cooling capacity, it just loses its ability to pass enough cooled
water back to the engine.

> Also - you can't just take out the thermostat and run wihtout one to
> see what happens, cuz (1) the housing will leak like a sieve and (2)
> the thermostat does two things:  it cuts off hot coolant recirculation
> as well as allowing cooler coolant in from the rad.

The thermostat's just an open-close affair at the end of the coolant
return pipe from the radiator. The bypass pipe remains open either
way. Usually the way you "take out" a thermostat to prevent leaks is
to use an old one with the "works" snipped out of the center,
basically just leaving an empty ring.