Bubbling without overheating suggests the possibility of a head gasket leaking combustion gases into the cooling system. That will show up very quickly with a "leak sniffer" test. Your mechanic will use a glass cylinder with two chambers partially filled with a dark blue liquid. Air is drawn through it from the radiator or reservoir. If combustion gases are present, the liquid will turn bright yellow. That bubbling will stop within a few seconds of stopping the engine.
If you hear thumping inside the engine after it is stopped, overheating should still be suspected. If the fan is running, the dash gauge can read lower than it should if there is an air pocket around the sending unit. They don't respond well to hot air, just hot liquid.
The definitive test to tell if the engine is overheating is to read the coolant temperature sensor reading on a scanner. The Engine Computer uses a different sensor than the gauge. It's the one the computer uses to determine when to turn on the radiator fan.
Overheating can also occur when the cooling fins between the tubes in the radiator are rotted away. That is fairly common in northern states where they throw a ton of salt onto an ounce of snow.
Wednesday, February 2nd, 2011 AT 3:00 AM