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Join Date: Feb 2008
Location: United Kingdom
Re: 156 V6 - faulty thermostat or waterpump?
There are four possible answers to this:
Poor flow - Dodgy water pump, the temp wouldn't fluctuate dremaically though but if you still have the original pump fitted then you want to have it replaced at the next cambelt change anyway.
Poor flow control - Dodgy thermostat, if this was stuck open then the water temp would sit around 70 on the guage as you drive round and then shoot up to 90 as you sit in traffic, if in the unlikely event it was stuck closed then the flow would not go to the radiator thus no water would be geting cooled and the car would overheat.
Sudden temperature fluctuations (1) - Airlock/blockage, If the water level was low and not topped up early enough then there could be an airlock within the engine, this could be in the thermostat housing and as the air would be at a higher temp than the water the sensor reads hotter until the cooler water splashes the probe again to give the sudden drop on the guage, simply add more water/antifreeze 50/50 to your expansion tank, go for a drive and check the level again afterwards, if it drops then top up again, if your adding more than a litre though then I'd suspect the next problem.
Sudden temperature fluctuations (2) - Head gasket failure, This is where the hot gases from the cylinder pass the gasket to get into the coolant area, normally when this happens it's the rear head and you have a lot of steam from the exhaust. Sometimes you feel a little power gain too similar theory to water cooling a turbo air supply to make the air denser.