Don't waste your time with the thermostat if the gauge isn't indicating overheating. Have you been filling the reservoir to the top? If so, it's overfull. What happens to the level when the engine cools down? If it stays full, the radiator cap is not sealing. That lack of sealing allows the water in the coolant to boil and expand. Your gauge will read in the normal range when coolant gets up to 220 degrees, but by that time, water will be boiling unless it is kept under pressure by the radiator cap. For every pound of pressure on the system, water will boil 3 degrees higher.
If the coolant reservoir empties back into the engine when it cools down, there is likely a trapped air pocket in the system. That could have been caused by not bleeding the system after a previous service, or is a typical symptom of a leaking head gasket. Your mechanic will use a special tool at the neck of the radiator to test for combustion chamber gases in the coolant. A special dark blue liquid will turn bright yellow if gases are present. Most commonly, you will see large air bubbles flowing into the reservoir, not coolant, although the increased system pressure could cause coolant to flow into the reservoir in some cars.
One last thing to check is to be sure the shroud around the radiator is in place and properly sealed to prevent air from bypassing the radiator. Be sure the fan is working properly too.
Thursday, February 25th, 2010 AT 11:19 PM