But it only heats the fuel when it's below something like 65*f.

And only heats when you have the heater turned on in the car.

The stock coolant circuit runs from the block through the heater, to the aux coolant pump, into the heater core, to the monovalve, and back to the block. In addition, the heat exchanger is only about 6-8 inches long. Double that since the fuel makes two passes. Something like 15 square inches of surface area. Nothing to get excited about.

The stock fuel flow goes through a diesel thermostat.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0AK2W821H0AK9JS&year=1987&make=MB&model=300-DT-002&category=All&part=Diesel+Fuel+Thermostat
It starts opening at 25 deg C (77 F) and is fully open by 8 deg C (32 F).

More info:

http://biodiesel.infopop.cc/eve/forums/a/tpc/f/359605551/m/5751031641?r=5251041641#5251041641

http://biodiesel.infopop.cc/eve/forums/a/tpc/f/159605551/m/898604161/r/1941012641#1941012641

I use a 26 plate Flat Plate Heat Exchanger (FPHX or FPHE for short) to bring my oil up to coolant temperature in the summer. I only run it when I know I am going to burn through most of it in one day's driving and can dilute what is left with biodiesel or diesel. I don't run vegetable oil in the winter. (OK, that is not totally true. I have been known to add 2% new canola oil when I am out of biodiesel and have to add ULSD to my tank.) I will be adding a home water heater style tempering valve to allow me to keep my temperatures at around 130 deg F going into the IP. That is hot enough for the oil to be pumped easily by the injector pump, but cool enough to hopefully not damage the IP. From there I have 25 watts worth of injector line heaters attached to each injector line.

I have been toying with the idea of modifying a second diesel fuel thermostat to activate at a higher temperature so that I could skip the tempering valve. I have not looked at it lately. Surely it will involve either putting in a stronger spring or snipping off part of the stock one, depending on how it is set up.

-Jim

__________________
1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.