what would be the problem?

The problem can be safely cured! swap your driving period with whoever else you drive with and investigate the fault properly!

It's 330d what year what engine what mileage what age?

But first and foremost, investigate before changing bits, and if you do change bits, make sure you bleed the cooling system properly otherwise you will have other problems.

First port of call should be a BMW dealer or BMW specialist indie.

Runninng the cr with a cool engime isn't going to do much harm-otherwise engines would never cope with winter cold starts, would they?

We all put off doing car maintainance in winter-it isn't fun after alL!

How long have you noticed the engine running cool?

Temperature has dropped this week. Big diesel engines loafing along in cool temperatures struggle to reach 90C.

I've known industrial engines struggle to achieve coolant temperatures of over 50C when left idling for extended periods in sub zero temperatures. Used to cause the dreaded phenonenom of bore glazing whereby partially burned diesel formed a varnish on the cylinder bores leaving a shiny surface which couldn't hold a film of oil.

Don't worry, this won't happen. Just get your foot down! If you want ultimate economy the 2.0d will be a better bet. Diesels like to work!