Hello all...
Truck in question is a 1984 GMC Sierra with the 6.2L Diesel underhood. Runs like new and drives well, but it doesn't get out a lot as the body is bad and I only have a makeshift door on the driver's side. It gets started and moved around to keep the batteries up and people from complaining. I'm remodeling a house and sometimes use it to move heavy trash cans. Would like to restore it at some point, but I'm not there yet.
Today it did something strange. All of its life, the temperature gauge has reported 200 degrees F when fully warmed up. I started to back it up with the heat on. It was about halfway warmed up. When I parked it a few minutes later, there was cold air coming from the heater/defroster and the temp gauge was going up rapidly. It got to 215 degrees before I shut it off.
Am I right in thinking that this might be a stuck thermostat, or is it something more sinister? The truck shows no sign of having suffered a major cooling system failure (no coolant smells, milky oil or green puddles, plenty of coolant in the radiator and some in the expansion tank) and it cooled off pretty quickly once it was shut off.
William