A sticking thermostat is caused by wear. The only option is to replace the thermostat.
the coolant goes through the radiator too fast to be cooled sufficiently. a thermostat will help get the engine to operating temp and the let only enough coolant through to keep engine at that temp. this slows coolant down enough to be cooled properly.
Mineral sludge from tap water. A good radiator flushing will help and then use proper coolant to help keep the sludge building up and blocking the radiator.
If there is no thermostat in the radiator, the coolant can flow too quickly through the system and fail to properly regulate the temperature. This can cause the coolant to boil over and be expelled from the radiator. Installing a thermostat can help control the flow of coolant and prevent overheating.
The thermostat is located at the front of the engine, directly above the water pump. now you might have to remove the coolant shroud to and coolant fan to get down there. if you need any other help/directions email me at snoboarderperson@aol.com and i will be glad to help i have downloadable directions if you would like them
Is thermostat opening? Is system airbound? Is water pump circulating coolant?
More info would help here, what year passat? What engine? Generally I recommend that flushing the cooling system with OHC belt replacement. If the water pump is driven by the toothed cam belt replace the unit while you're there. This will necessitate coolant replacement. Suggest that replacement of the thermostat be also done at this time.
For the coolant level it is usually located in the coolant reservoir. For the coolant temperature it is usually located on the engine close to the thermostat or on a cylinder head. Need more info like year, make and model to help you more.
Unplug the coolant sesor for a day or so. This will allow the sensor to reset itself (sensor is prone to sticking).
maybe try flushing the cooling system, or replace temp sending unit.
The function of a car thermostat is to ensure that coolant gets to the radiator when it should. It blocks the coolant from reaching the radiator when the radiator is cold, and then lets it flow once it is heated up.
If a thermostat is stuck open it will cause the "bubbling". The thermostat opens and closes giving your coolant time to cool off in the radiator. When stuck open the coolant will keep circulating and just get hotter and hotter until overheating occurs. You should change it right away. I hope this will help you. If by "open" you mean that it is stuck in a position which allows coolant to constantly flow then the last answer is FALSE. If it is stuck open, and coolant constantly flows, then your engine will not heat up to the proper temperature. It will constantly be cool, because guess what, that's what coolant flow does. That means your bearings will not expand properly and such causing a host of other engine problems. If it is stuck open, then you need to replace it.
The water pump forces the coolant to move through the water jackets after the thermostat is open to the manufacturer to the temp. spec. After the thermostat is open the coolant flows through the radiator to help control the cooling. When you turn on the heater the hot water flows through the heater core and back in the return line to the engin block.