You're right about making sure the car is indeed overheating. Any garage
worth it's salt today will have a digital pyrometer. In less than five
minutes they can aim the sensor at the thermo housing and tell you at
what temperature your coolant is running. The temp at the housing should
correspond closely to the thermostat rating.
An alternate, albeit more crude (read cheap, about $2) method, involves
going to Radio Shack and buying a thermistor. They come bubble packed
with an ohm/temp conversion scale printed on the back of the package.
Tape the thermistor to the end of a pencil and hook the leads to your
digital VOM (everyone with a vintage British car has one, right?). It'll
take about 1 - 2 minutes of holding the thermistor against the thermo
housing to stabilize the reading. Convert the ohms to degrees and you'll
know if your sensor/gauge combo is accurate or not.
If it is overheating, note at what temp the electric fan kicks in. When I
had one on my '80 Spit, the fan came on slightly above 200 and went off
around 170. As they say, your results may vary.
If your car overheats at highway speed, you could be looking at a clogged
radiator. Does the temp drop a little if you open the heater control
valve? That adds extra cooling coils to the loop. At that point, take the
radiator to a proper shop and have it flow tested.
> IM not sure of the correct terminology, or even if IM on the right
> but maybe somebody might have an idea what IM talking about.
> I recently purchased a 1979 Spitfire. It overheats, or I think it
> drive the car for 10 minutes or so and the needle on the temp gauge begins
> to rise to the hot mark. I pull over, pop the hood and the hoses are very
> hot, but no steam or overflow.
> What we have done is replace the thermostat, new head gasket, totally
> flushed the hole system. The water pump is working fine. We have come to a
> conclusion that the car might not be overheating, rather the temp sensing
> unit (which has never been replaced) is at fault.
> My question to the group is this, if I use an instrument to test the
> of the coolant, what temp should it be? 180 degrees? 200 degrees? And is
> there any thing else I can do to stop my car from overheating.. IM almost
> out of ideas!