The heat in my '87 Celica has been pretty wonky. It warms up quickly but every once in a while it will go ice cold. I fuck around with the fan control and heat control and sometime the heat comes back, but I dont know if its because I touched anything.
I have coolant topped up. When the car is warm, there is some in the overflow tank. Could it be that I have an air bubble? The car doesn’t overheat though…
Is your rad cap working? Check pressure, if failing pick up a Stant cap (Good quality).
If you had a low level in your rad then i’ll bet air pocket, run it with out cap on (front end jacked up if you can) and rev it. Little air bubbles will come out and this process may take 30 mins to 1hr depending on your system.
Finally for mechanical, How is your thermostat? If it sticks open you’ll get a nice cold running motor. Replace and get the air pocket out again.
If it still drops off, well you know what its NOT.