Thermostat April 27, 2020, 09:11:26 PM The thermostat controls the coolant temperature by regulating coolant flow to the radiator. A thermostat stuck open will cause the engine to warm up slowly and run below normal temperature at highway speed. A thermostat stuck closed will restrict coolant flow to the radiator and cause overheating.Replacement is only necessary if the thermostat is faulty. In addition to the tools required for draining the coolant, a wrench or a socket is required to remove the thermostat housing. Use a new O-ring, Volkswagen Part No. 059 121 119, when the thermostat housing is reinstalled.Be prepared to change the water pump and thermostat housing too. The water pump is aluminium and by design spends its life filled with water. Aluminium may not rust but corrosion is still an issue, especially when aluminium is in contact with steel. There's a strong chance of at least one of the steel housing bolts shearing due to chemical reaction between the corrosion of the aluminium and steel - I swear this combination is stronger than welding!At time of writing the new water pump is about £30 so it's hardly worth the effort of trying to remove the bolt.The thermostat housing itself is usually plastic (on some models you may be luck enough to find an aluminium housing) and is likely to warp from prolonged exposure to heat. It usually poses no problem when until you come to remove it, as it's unlikely it will seal properly once refitted.It's recomended that genuine VW parts are used for the thermostat and housing, and always fit with new seals. Check the seals come with the new parts and if not make sure you get them at time of purchase, it's a pain to have it all apart on the drive only to discover you're missing a crucial o-ring.