Short version, friend of mine has a 85 300TD that is her only car (original owner). She has no heat, car doesn't warm up. I said thermostat. She takes it too her mechanic of 30 years and he replaces something and the car overheats 20 minutes into her way home. She claims the temp gauge pegged . Excellent. Mechanic won't answer his phone.

I show up and pick her up, but first I am going figure out what went wrong. My competency with car repair is very high so don't be afraid to use some detail. For whatever reason I haven't found any good info about what this is and what it does.