Hopefully one of the diesel guys chime in. In the meantime we just had a thread about a diesel not warming up with some really good info posted so maybe have a look at that.

Even the slightest difference in the seal and bypass hole in the thermostat can make a big difference so brand of thermostat is a big deal.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)