Join Date: Sep 2006
Car: 2004 Facelift 75 Connoisseur 1.8 turbo with DD Pack. And remember, the troof is out there.
The optimum running temperature for water cooled internal combustion engines is 95-100C. This is where the best fuel efficiency is noted. As the running temperature drops, fuel consumption rises. A 5C difference will have a discernable effect given a reasonable running period. The diesel is a cool running engine and would benefit (IMO) from a higher rated thermostat than the nominal 88C standard. With a new 88C stat, diesels seem to run around 88-92C, but rarely higher. In stationary traffic all engines will heat up, diesels included. The petrol engines also use an 88C thermostat, but typically run at 95-100C, eflecting their lower efficiency maybe, but they are also lighter and made from aluminium. I believe the massive cast iron diesel engine simply absorbs more heat without raising its temperature significantly. People have searched for a suitable stat of higher rating (say 90-94C), but without success thus far.
When diesel owners with a stuck open stat, running at 80C or lower get theirs up to 90C or so, a significant fuel saving is reported.