First make sure you have enough coolant in your overflow tank and in your radiator (engine cold). If coolant is low, add coolant. If you lose coolant rapidly in the next days/hours, you have a leak somewhere: inspect all the hoses, radiator, radiator cap (any sign of coolant around the radiator neck?), Water pump (pay particular attention to the water pump as it is a common problem), heater core area (wet passenger floor?) Etc. If no external leak is detected you may have an internal leak (check for exhaust gas in coolant, pressure test your cooling system).
If your level of coolant is fine, make sure your thermostat is working okay: with engine at normal operating temperature, grab the upper radiator hose; it should feel very hot. If it doesn't feel very hot, just kind of lukewarm, your thermostat is stuck closed and you need to replace it.
If thermostat checks out okay, let's look at your cooling fan. I don't know if your car has electrical or mechanical fan. If mechanical: check your fan clutch; if fan spins freely (engine off) with no resistance when turn by hand (wheel of fortune style) clutch is no good. If electrical, make sure your fan comes on when your car gets in the hot zone. If fan doesn't come on, check relay, motor, connections, fuse. Try to put a jumper from the battery to the fan motor; if fan doesn't come on, motor is bad. If motor comes on, you have a bad relay, or bad wire, or bad connection.
More stuff to look at:
*your cooling system could be clogged: flush the system and refill with fresh coolant.
*your serpentine belt could be loose on the pulley; check belt and tensioner.
*your exhaust could be plugged
*your bottom hose may have collapsed
Let me know if you need clarification
Tuesday, September 29th, 2009 AT 1:17 PM