I know my 88 GXL has a bad coolant seal inside the motor, but I don't think that to be the reason behind this. When I'm climbing steep hills at highway speeds, the car starts to run considerably hotter. On the way down the hill it goes cooler than normal operating temp. is this simply caused by a malfuntioning thermostate not handeling pressure changes correctly or is it something more serious like the cats?
Thanks,
Chris

 

How about..

Your engine is working hard, for the speed it's at, and the blown coolant seal is just dumping combustion gases right into the coolant. When you back down, the engine is barely working at all to keep the car moving, not as much work=not much heat, so it's not dumping that into the coolant as much.

Nice try on the thermostat, but once you reach a certain temp, it either opens, or it doesn't. If it doesn't, you'll know it.

PaulC

 

Check your oil level, and make sure your oil cooler is not blocked with debris.