I purchased my truck from a friend about a year ago with about 58K on the ticker. Truck ran perfect and had recently had the radiator replaced due to the Ford recall at a local dealer. My truck is out of warranty due to age not mileage. I did the dpf delete and decided to run a Spartan tuner. Every thing was awesome, gas mileage went from about 10 mpg to just over 19.6 empty and 17.5 towing my 36 foot toy hauler. My temps would stay about EOT 201/ECT 201 until recently. My oil temp would start to climb until it set off an alarm (on the tuner) at 230 but my coolant would remain about 190/196. So I start to think the worst (Oil Cooler), so I start to save some cash and get ready to pull the trigger on a new oil cooler (everyone says cab off approx $4-5G!!). Just before I get ready to do the cooler I noticed the ECT temp start to fluxuate between 196 down to 178 and continue to bounce up and down. All the while no check engine light and if you pull to the side of the road oil temps cool off to about 205/215. So I wait on the oil cooler and did some research. Turns out I had a hyper-extended thermostat!!! The 6.4 has a two thermostat system. So I purchase the thermostats and buy an “Airlift” coolant system to do the coolant flush. I flushed the system 5 times with just distilled water, no chemicals, replace the coolant with ELC coolant from local truck retail center and off I go . Now the truck runs at 192 EOT and 192 ECT!!! No need for oil cooler!!!!! I also added a sinister coolant filter and delete. Hope this helps anyone having the same issue with their 6.4!!!!