The Thielert engine has two limitations about fuel temperature: one for starting (-22F) and one to have it running.
What is important to know about Diesel engines is that about 30% of the fuel is not used in the engine and comes back to the tanks - and this is how you will keep it warm enough, even at FL100 !
If you fly the retrofitted C172 you will see that the "BOTH" position on the fuel selector is gone. The warm fuel goes back in the tank in use, so you will have to switch periodically to avoid fuel imbalance but also to keep both wings "warm".
On the good side, the engine management system will warn you if fuel temperature is too low.
More about it in
http://www.plasticpilot.net/blog/200...e-like-it-hot/