[popsicle_3000]

I've got an '07 E92 335i.

This winter (I'm in Australia) I noticed gas mileage was much worse than last year. Two days ago I did a 2hr highway drive in -5 to 2 deg, and oil temp gauge never started moving. On way back, temp was up to 15deg, and oil temp only rose in town, then dropped down to ~90deg on the highway. It never hit the 100-120 it normally runs at in the summer. On that 4hr drive I only was able to manage 9.5L/100km which is quite poor. I figure it's running the car rich as it's always cold, hence the hit on fuel economy.

Cabin heating works ok. Last time car was running too hot was several years ago and I replaced the water pump then. I don't notice that the rad fan is working all that much differently. It doesn't take that long in cold starts to get back to normal idle.

Earlier in the year it threw a 2EF7 code for the thermostat, but was buried in a mechanics report and I didn't notice it, nor did he mention it.

How can I work out if the problem is the oil cooler running open all the time, or the thermostat?

thanks!