Temp goes down in traffic

[2008tdihighline]

Hello

I've noticed my 2.0tdi temp gauge goes down when the engine isn't up to temp and I'm sat in traffic. For example it's slowly going up to 70 degrees, I wait at traffic lights and I can see it dropping.

It's fine once it's up to 90 degrees though. I believe even with the heater off it still does it.

I see the tdi has 2 thermostats, one listed as coolant and engine...

Thanks

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