From Mike L
What we have found is that the fuel is being returned to the tank at temperatures starting at 130 degrees. On hot days, towing heavy, heavy throttle applications, long road trips, and stop and go driving (aka normal driving) we have seen temperatures well into the 170+ degree range. There are those out there that have written that there is less soot production, which is true, at higher fuel injection temperatures. But there is also viscosity breakdown of the fuel which leads to poor atomization upon injection. If you will look at your fuel system from start to finish you will notice in the stock system that there are many places for the fuel to pick up heat. After leaving the frame rail it comes up by the transmission and across the lifter valley working its way in some applications to the FICM to act as a coolant. Then it is run through the fuel filter which sits directly above the exhaust manifold and on into the CP3 where it see’s compression up to 23-26K psi. Which we all know pressure equals heat. After making a run to the injectors the unused fuel is then returned via lines that run through a hot oil bath under the valve covers and through the lifter valley. On the return trip you will notice an extremely inefficient stock fuel cooler before returning to the tank. Upon replacing the stock cooler with the new Fuel Cooler all vehicles tested immediately saw a 2-4 MPG increase. Just from keeping the fuel at ambient temperatures in the tank. We have an optimal number for injection but it would be next to impossible to maintain with all of the heat adders that I have pointed out above. The next best thing we have found is ambient or as close to ambient as possible for the fuel leaving the tank.

There are things we don't know yet about the possibilities of cool fuel in diesels. This has been a study of mine for 3 years. I have called knowledgable people around the world and asked a simple question. " What is the optimum fuel temp in a diesel?" I got answeres from; the hotter the better, or it doesn't make a difference. Quite a few knowledgeable engineers said they really never looked at it that close. The basic answer was "I have no idea " Is there such a thing as optimum fuel temp? I don't know. The best down to earth information I have recieved so far is from Cummins Marine division. It shows the fuel temp coming out of the engine at 150F and into the water heat exchanger with an out temp at 75F. Is this the optimum temp? I still don't know , but it satisfies Cummins and I now use this number as a goal. I am still learning but am convinced I am on to something and I see results.
The 2 fans draw less than 4 amps so it will not draw much from the alternator. Fans can be cut off at will with a switch. As`far as winter weather; I have not tried it but the fuel in the system will still get hot and the lower the fuel level the hotter your fuel will be. I do not believe our oem fuel cooler is enough and you can test for yourself by touching both the inlet and outlet lines at operating temps coming out of the cooler. If you try same test with mine you will see a difference.
At this point I do not have an answer for my cooler protection in offroad use; sorry, not there yet.