[Rocklobster]

Within the last 4 months we have experienced a drop in fuel millage with our 328d xdrive. In the past we averaged about 40 in the summer and perhaps 37-38 mpg in the winter here in cold Minnesota.

This fall we saw a mild drop in millage at about 64k miles. Then as it got colder out the milage plumeted further. We are getting an average of 31mpg.

This is my wife's car and we decided to trade for a few weeks so i could diagnose further. The car also seems to have lost performance and finally with colder temps i'm noticing all the signs of a thermstat that is stuck open. Coolant temp is not consistant and gets up to about 180 at best. When below 0deg F it never gets above 175 (read through obdii), and that's only on the freeway. I'm not sure if that's normal but in the past it seemed a consistant 200 deg+ was a common operating temp (per the dash gauge).

The car, being a diesel, always took longer to get to operating temp but would hold once there at around 205. Now it takes forever to get above 150 and when at idle will rarely get above 145. It also drops quickly from 175 to about 145 at stoplights I am taking it in to the dealership for the 2nd time with the issue, they diagnosed "all systems normal" last time we were in for low milage and power. Now with the more prominent coolant temp issue hopefully they will find the problem.

I cant find any info on how the thermostat operates, if it's more than a mechanical thermostatc valve (standard on most cars)? Is it electronically controlled in any way? What is the proper operating temp for the N47?