How is the coolant mix? When did you last change it?
Feel the hoses and the rad that lets you know where the heat is circulating. Hot engine and only warm upper rad/hose means the thermostat is not opening fully. If this is on the highway I think it's most likely the thermostat. If you change it take note of the orientation, or you'll have to pull it apart again to fix it.
Fan clutch is possible but as long as you're moving you don't really need it. On my old car I removed it, and I'd just flick on the electric fan when I was sitting in traffic too long.
Sometimes the coolant sensor goes wonky, not as common but it happens.
Performance one step at a time.
Last edited by InfiniteDice; 11-18-2012 at 12:01 PM.