Hey guys got a 88 trans am with a l98. It has the dual fans of course that I have wired up for around town to keep it cool. In the early spring when it hit 90 here in kansas, it had a bad time overheating, so I added a 160 t stat to the mix to keep it cool....that worked for a couple months but now its hitting 100 routinely again and its no longer working like it did. Today I pulled off of the interstate and drove for about 3 minutes at 35 mph, no stoping until I hit the gas station and I shut it off for maybe 5 minutes. I came back out to the car and it was sitting at 240....I got it back up on the interstate and tried to cool it back down but the most it would go down to was 215, and no a/c running. If I stop the car it will shoot up to 220 or 230 and not come back down, even if I'm going 60. My airdam is still there and I have checked to make sure there is no debris underneath there. It is full of 50/50 water/coolant of course too. It is a bone stock car with 285k miles. It burns about 2 quarts of coolant every week, which is about 400 miles, almost all highway, so whats the problem...cracked head(s)? Old radiator, water pump?

 

Using that much coolant every 400mi you have a leak somewhere in the system.It could be a head gasket,water pump(how long has it been since it was replaced or has it ever been replaced?)or the radiator.Have you noticed an anti freeze smell inside the car?That would be a sign of a leaking heater core(but then again with the temp control on cool there shouldn't be any water going to the heatercore).Have you pressured tested the system?That would give you a clue where to look.
Just a side note that maybe part of your problem.My '95 Jimmy(the one in my sig)would start to run warm on the highway with the A/C on.Turn the A/C off and it would come down,turn on the heater(lot's of fun at 92*F - heat index of 102*F)but the temp would come down.What I found,thankfully,was a stretched belt.I have a Goodyear gatorback belt in there now instead of the gates I've been using.No problem since,but it's only been a month.

 

I think the water pump was replaced before I bought it, I have never pressure checked it though, I will try that....is there any way I can tell where its leaking?