Overheating after turning off

[1983Pinin]

Hello,
My 1983 spider has overheated only twice in the 1.5 years that I've owned it. In both cases, I had been driving for a while-- at least a half hour, and it happens when I turn the car off for about 5 minutes ( to run into a shop, in both cases ). When I turn it on again the temp gauge pins, and the fan is not on.
The car has a good water pump,fan works whenever over 190 degrees, I replaced the temp sensor in the radiator when I bought it, no visible coolant leaks, inside heat works fine, no oil in the coolant, always checked before long rides and filled to the top using the correct technique ( usually doesn't need more than a half cup to top it up ), regular weekend driving, etc.
In both cases cabin heat was off, and in both cases after waiting for temp to come back down ( electric fan not on at this point ), I can go off on my merry way and the problem is gone.
Any ideas why I have had these rare overheating issues? I know the cooling system in the spider is a little wonky, but I was wondering if anyone had a suggestion, other than consider myself luck that it doesn't happen more often!
Thanks
--Peter

[courtenay]

Sounds like the temp sensor that pegs the needle might be faulty. It's the one closest to the radiator on the top of the engine block. Disconnect it next time this happens and see if the temp gauge changes.

[bradartigue]

If the fan is not on and the fan works otherwise...then the car probably isn't really overheating. The fan will work whether the car is on or not, so you've got a bad sender or air in the head.

[So Cal Mark]

all engines experience an increase in temperature when turned off, the water in the block isn't circulating and heat transfer will cause the temp to rise. With the fan switch in the radiator rather than in the block/head, you can't expect the fan to cycle on. Once the engine is started, the temperature will drop pretty quickly as the water circulates

[bradartigue]

Perhaps I should be more eloquent:

In a properly functioning system the fan will continue to function if the temperature in the radiator is high. If the car is truly overheating the radiator temperature will be high enough to cause the most visible sign of overheating, fluid running back rapidly into the overflow bottle. The fan will run, regardless of key position. My point being this does not sound like an overheating car, but one where the true engine temperature, at rest, is being exaggerated by either air in the cylinder head or a faulty sensor.

[1983Pinin]

Thanks everyone. Yes, no signs of overheating other than the temp guage, so maybe first thing to do is replace the sending unit in thehead.
--Peter