Flow blockage is a good place to start. What sort of time frame are you considering "immediately"? Does coolant boil and froth from the filler neck within a couple of minutes of starting?
Have the mechanic use the chemical that detects the presence of combustion gases in coolant.
Make sure the heater hoses are properly connected. Test the thermostat with a pan of boiling water and a high accuracy liquid thermometer.
Sunday, October 9th, 2011 AT 6:38 PM