Engine Overheat Problem

Before dumping any more money into it, I would measure the temperature going into and coming out of the radiator. This will tell you if it is doing its job or if it might be a sensor problem. Some engines will get hotter when shut off because of residual heat that is not taken away by circulating water.

How stock is the engine, fan, and radiator? Is it missing a fan shroud? Are the pulleys the correct diameter? Could be timing or carburetor adjustment. The obvious is the thermostat. Has it always done this or is it a new problem? Was coolant used when parts were new? water only would cause corrosion.

If you measure the surface temperature of the radiator, what do you get?

Is the fan actually turning properly? If high engine RPMs makes the light go out, it sure seems like you don't have proper airflow.

--scott

I may be wrong but I think he is saying that high speed driving is what causes the light to come on. He said "If I drive it below 75 mph the temp light stays out...". Of course he said other things that seem to contradict this also so we have to guess what he is really meaning.

But, if high speed is truly the only thing that leads to the light coming on, I would guess the gauge is accurate (at least relatively speaking). It clearly is getting hotter when driving 75 mph than it normally does which it shouldn't. The sending unit might be a little bit extra sensitive but there isn't anything about high speed driving that would cause the gauge to behave differently other than temperature. There are a couple things that immediately come to mind that might cause this. Lower radiator hose collapsing (partially blocking flow) and bad distributor advance. If the mechanical or vacuum advance in the distributor are not working the engine produces a lot of extra heat at high speeds.

-jim