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1981 datsun heater cools down at highway speed

this is my first winter with a 1981 datsun 310. in the morning i start my car and let it warm up. when the car is sitting still, the heater will warm up after about 10 minutes and the interior will warm up nicely. but when i get on the highway and get cold air blowing across the radiator it cools down pretty quickly. after 10 minutes at 55mph the heater will be blowing cold air.



if i am in rush hour the heat will be hot enough to have to turn it down, but as soon as speeds increase the car gets cold again.



is it the thermostat, radiator or what?



i know it’s old and works differently than modern cars, but it’s my only way to get around. thanks for any help!!



this isn’t my only issue so if you know older datsuns please look for my other question.

Make sure the radiator is full of coolant. The thermostat may be stuck open. Simply changing the thermostat could be the simplest test of the thermostat.
If the electric radiator fan runs, and the engine isn’t hot, it could be a bad cooling fan switch. Here is what it looks like: http://www.autozone.com/N,15200115/shopping/partTypeResultSet.htm You will have to look at the old one to see if it is a one terminal switch, or a two terminal switch.

Missing or stuck thermostat.

Yes, I’d probably replace the thermostat but if the problem persists, you can put cardboard in front of the radiator to slow the cooling during the winter.

I’d vote for a malfunctioning thermostat also . . . change it now, you might think you have problems with it stuck OPEN, but CLOSED is another thing altogether . . . could cause overheating when you are in a traffic situation. T-stats are cheap and (usually) easy to replace. Happy Holidays! Rocketman