Purchased an 86 classic 240 DL, had been sitting awhile. going through the car, it has overheating problems. Replaced expansion tank cap and pulled thermostat. When I pulled the thermostat I discovered that it had rusted away and probably plugged the radiator and maybe water pump. Pulled and replaced radiator, water pump, fan clutch. Water pump as expected had blockage on the intake tube side of the pump. Flushed it, and put it back together, still overheating. Heck its over heating faster now. Before it would go around the block. I have a feeling the expansion tank is not holding pressure, although it has a new cap. For example, the engine is reading red hot on the temp gauge, but you can unscrew the exp. tank cap and a) no water comes blowing out, and b) only hear a slight hiss. I know that these things will not cool if they don't hold pressure, but before I go overhauling anything else, I would appreciate some good advice.

 

I'm pretty sure the expansion tank cap, when it properly maintains the system pressure - only reduces the boiling point. Do a search here for "temperature compensation board". As already stated, the use of a IR thermometer is highly recommend.

 

Appreciate the advice. I will check it out.

 

The attached picture shows you how to bypass the temp comp board. By doing so you will improve the accuracy of the stock temperature gauge.

 

Well the bypass worked, thanks gang! The blockage in the radiator and water pump from the rusted out thermostat still played a major role in the problem, but its all working now, thanks so much. And I am buying an IR gun!