The fan will come on when the coolant temp hits around 212* and if you are counting on the gauges to tell you when things are hot you are wrong, when they hit hot it is to late you are in a melt down. You need to get gauges like the Edge Insight to tell the difference in EOT-ECT max 15* i think 10* your cooler are starting to get plugged a coolant flush may help if it is not to bad but gauges are the only way to tell. Here is a coolant flush that may help.
This is a post by PGreenSVT
Flushing and cleaning the 6.0
Restore is for cleaning out any silicate goo. VC-9 is for cleaning out iron and scale. Restore Plus is the same as VC-9 and cheaper.
To flush drain the coolant by removing the lower radiator hose from the radiator, and removing the drain plug from the driver side of the block. There is a drain plug on the passenger side also but you have to remove the starter to get to it. I don't bother with that one. You can skip the block drains if you want except for the final drain before adding fresh coolant. You will just have to flush a few more times to get the block clear.
I also highly recommend you pull the thermostat out on your first drain and put the housing back without the thermostat. It only takes about 10 minutes and will save you 2.5 hrs or so in doing this whole procedure.
Ok now put the lower hose back on and the drain plug back in the block if you removed it. This is the procedure you will use each time to drain the system, except you will not touch the thermostat again until you're finished the whole procedure. Make sure that you set your heater to high while doing this to flush out the heater core as well.
Fill the cooling system with tap water, start the truck and let the water circulate for 5 minutes. (if you did not remove the thermostat you must run the truck until the thermostat opens + 5 minutes to circulate. This takes 15 to 20 minutes each cycle and is why you should just remove the thermostat) Stop the engine. Drain the system and repeat the flush.
Now add at least 1/2 gallon of Restore and fill with tap water. Drive truck or run on high idle for 60-90 minutes. Drain and flush 3 or 4 times.
Add 2qts VC-9 or 1/2gal of Restore Plus, top with tap water, and drive the truck or run on high idle for 60-90 minutes. Now drain and flush until flush water comes out clear and clean. When you get clean flush water, flush 3 more times using distilled water if you are draining the block or 5 times if not. This is to replace any tap water in the cooling system with distilled water and very important.
After your final distilled water drain it is time to put the thermostat back in. I recommend you install a new thermostat at this point. They are only $20 or so and it is good maintenance procedure to do so. Fill with 3.5 or 4 gals of ELC concentrated coolant and top off with distilled water. Drive the truck or let run for a while topping off with distilled water. Check the truck over the next few days and top up as required while any air left in the system works its way out.
Keep some 50/50 ELC on hand to use to top off the cooling system from here out and you are good to go.
new t/stat from Ford it should all come together (housing-t/stat-o-ring) in a box. The part number is RT-1169
Here is the Restore Product from Cummins dealer