This relates exactly to the lower mgp's I've been getting since winter started. I'm pretty sure my thermostat is partially stuck open. Last summer I only had to fill up about once every 4 weeks. At that time, I was reaching 85 degrees every time I was driving in the city. Since winter, I've been averaging about 75 degrees and have to fill up about every three weeks now.

I'm a constant gauge checker and last summer I always maintained 85 to 90 degree coolant temp on the interstate but this winter, I can only max at 85 and when I go from 80 mph in the interstate to 60 on back roads, it quickly drops to about 75 degrees, sometimes down to 70 (ambient temps of about 50).

I have a new 85 degree Behr thermostat and am about to install it with some zerex. I'll let ya'll know what effect this has on MPG. FWIW, I thought my temp gauge was inaccurate so I hit the thermostat housing with the IR gun and it was close to the gauge temp, then I ohm'd the temp sensor on the head and it matched the gauge so I know the gauge is reading correctly.

Willson,
85 300CD

I would test the thermostat before installation.

I can't speak to the turbo version, but the naturally aspirated version of the same engine ('97) has had a problem with head gasket coolant leaks to the outside. I doubt if 10 degrees hotter temps would have much effect, but the long aluminum head on the iron block seems to stress the head gasket at normal operating temperatures.

Make sure you get a good coolant for diesels that has a higher boiling point as well, like Pentofrost NF.

Phil Forrest

yes

I am of the opinion that you can run it as hot as you want as long as cooling system is pressurized as needed and you keep the egt below what it takes to melt a piston.

I definately plan on testing the thermostat before installing. And its actually an 80 degree t-stat. Isn't that what most people normaly use on the turbo OM617?

another issue with winter fuel economy is the fuel itself has less energy in the winter mix...