yeah, CobraR is right, however something as simple as mix ratio of coolant to water has a little to do with it as well. Too much coolant and not enough water can cause an overheat.
Coolant has a lower specific heat than water. Therefore it takes less energy to heat it up, than plain water. Also coolant alone has a boiling point less than water, but through the miracles of chemistry adding a low boiling point coolant to water increases boiling point above that of water.
Get a hydrometer, open the cap on the coolant tank, run the engine until it reaches operating temp., take your reading, you want to see about a 50/50 mix, if not, adjust as necessary.
Best cooling however would come from just water, but without coolant it will boil at 212'F at atmospheric press. but when pressurized 15psi will boil around 220-230'F.
But with just water you loose the corrosion inhibiting properties granted by the coolant.
you can also try using a DEX-COOL coolant, better known as long life anti-freeze. Simply follow the directions on the bottle, or folllow the ones in your manual on how to flush the cooling system. DO NOT MIX DEX-COOL WITH STANDARD ANTI-FREZE AS STANDARD ANTI-FREEZE USES A SILICA BASED SOLUTION FOR CORROSION PREVENTION, AND THE DEX-COOL WILL CAUSE THE SILICA TO PRECIPATE OUT AND CLOG PASSAGES IN THE ENGINE AND RAD. SO BE SURE TO DO A GOOD FLUSH. (keep adding fresh water until water comming out of the drain is clear)
