From memory, your Webasto heater is also not running?

If not, then it's pretty normal for the car to take a long time to warm up. With everything working correctly, including the Webasto aux. heater, in sub-10Deg.C. temps it can take up to 20 minutes of city driving to reach operating temperature. With no Webasto, it will take even longer.

Higher speeds (on motorways etc.) will increase engine load and reduce engine warm-up time, but this is somewhat offest by higher airflow over cooling components.

Another possible failure that could extend warm-up time is the viscous fan coupling. If the viscous clutch is seized, then the fan will run at full speed at all times. While this should matter while the thermostat is closed and the coolant flow through the radiator shouldn't be a factor, the sheer volume of cold air flowing over the entire engine etc. can increase warm-up time slightly.

And of course, as Andrew stated - new does not equal fixed (or good) - the new thermostat may be faulty. I've heard quite a few reports poor after-market brand thermostats for our diesels (which are bytw, 3.0d not 3.0i) - the best bet here is an OE part. Worked well for me when I replaced mine 6-8 years ago, and still does.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)