I did some repairs on my 1985 Volvo 240 DL today and had to drain the coolant from my engine. After replacing the coolant I was driving and the engine temperature gauge went to full hot and overheated within about 5 minutes. I then let it cool, unscrewed the radiator cap, started up the vehicle, and let it run to get rid of any air pockets. This seemed to work as the engine temperature did not heat up excessively. However, it did not heat up at all, even after driving for another 10 minutes. The temperature gauge stayed at cold and the heater was blowing excessively cold air. Obviously the thermostat is stuck in the open position now, but why? Is that a normal response when the vehicle had overheated before due to air pockets in the cooling system? Is there a way to reset the thermostat or do I need to replace it? Thanks.
When the thermostat is subjected to extreme heat, it can get stuck and would not close again.
There is no way of resetting it except to replace it.
After coolant has been drained, system has to be bled of air when refilling. You need to run engine for a few minutes with heater turned to maximum heat and radiator cap off. Top up as necessary and when coolant level stabilises. Close radiator cap and run for another few minutes.
Allow to cool and if coolant level is not visible due to reservoir not being clear enough, allow to cool before atempting to remove radiator cap to check level. Top up if necessary.