I had a new cat installed because my 88 iroc 5.0 failed emmisions. I have np doubt the cat wasnt bad but since installed, the car is running at like 230 degrees at highway speeds. Normally it was at 220 or a little below. Now it is only at 220 when i am going 30-40. i know it doesnt make much sense, i opened the radiator cap and let it warm up the tstat seems to be open and circulating fine. anyone ever seen this before? Is the exhaust having trouble getting out now?

 

my car ran alot hotter with the stock sigle small pipe set up but passed. Now cooler with the freer flowing duel cats and larger pipe setup. I doen't boil over that often any more.

 

Sounds like you have some other problem which is why the car failed emissions in the first place. Check over your spark plugs, wires, and distributor cap. Then check between your radiator and a/c condensor for plastic bags and other debris. Finally, make sure the black air dam under the car is secure.