Does the cooling fan work ok? Some possibilities, thermostat--water pump-- coolant flow, a blocked passage somewhere-- A tranny issue? You can flush the cooling system, see if it helps.
You may need to use an analog gage in place of the sending unit, just to double check the gage. Or use one of those infra-red temp testers.
On older vehicles, I have just removed the thermostat, hooked everything backup, see if it helps? It will affect gas mileage or usually. I haven't had no thermostat, set a code on older vehicles.
Sunday, March 4th, 2012 AT 5:59 PM