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My Volkswagen Passat V6 temperature gauge, or coolant gauge goes from normal (the needle is in the middle), then as I'm driving the gauge goes down in temperature (toward the cooler part of the gauge). There is no sign of a leak on the ground where I park it, and it's not overheating and no check engine light is on. I'm confused about what is wrong with the temperature gauge. Can someone offer me some advice as to what this problem might be please??

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It "sounds" as though from your explanation your thermostat is staying open partially when the engine is cooled to normal levels. When you're travelling down the road, there is a greater flow of air through the radiator. This means there's a greater cooling capacity (increased air = increased cooling ability).

If the thermostat is behaving correctly, it will only allow as much warm coolant out of the engine going to the radiator and cooler coolant to come into the engine as needed to allow the engine to maintain a temperature. If the temps are going down when travelling at speed, this means the thermostat is allowing too much coolant from the radiator into the engine and allowing the temps to decrease, which is actually not a good thing. It won't harm the engine, per se, but it does hurt fuel mileage overall, which will hurt your pocketbook in the end.

While I don't suggest you should just run out to get the thermostat changed, I do highly recommend you have it tested to see if it's behaving correctly.

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