I just bought an '81 240 2dr with some issues ( a little rust, pulls to the left, rattling muffler) but the first problem I decided to adress was the lack of heat. Driving it home from the seller's house, the heat was lukewarm at best and the temp guage barely moved. Found this forum, did some searching, and decided to replace the thermostat. Sure enough, old one was stuck open. Installed new thermostat and gasket, but didn't change coolant even though it looked a little brown. (No garage to work in, and it's cold out there, so I did it quick and dirty.) Now when I drive the car the temp guage rises to 9 o'clock in a few minutes and stays there. When I turn on the heat, I get wonderful hot air, but it only lasts about 30 seconds and then turns cold. If I shut off the fan and wait a few minutes and then turn it back on, I get hot air again but only for another thirty seconds. What do you think? Air in the system? Clogged heater core? Heater valve? Thanks in advance for your replies.