2008 F350 6.4, the temp gage will read hot at 70mph and sounds like a cooling fan or something is kicking on at 60-65mph it will read normal temps and the fan sound will go away. Two months ago I replaced the lower alternator ( Batt Light stays on ) and the ETC sensor. When this issue first happened it said to reduce engine power and the green wrench light came on. also since this has happened my heater will only blow out cold air. It pulls a p0073 code but I have replaced that sensor twice in the last two weeks, inspected the wires for that sensor and found no visible damage to the wiring the ambient temp on the disply screen seems to be correct. I went ahead and replaced the thermostats yesterday but the problem at 70mph is still there. it has been going in and really quick of the regen of the exhaust. I am kib=da at a loss at this point but thinkiing possible water pump, head gasket or something with the emissions. Any Ideas

 

how’s the coolant look
is there excessive quick /cold pressure in the Degas bottle
is the oil level normal
is oil milky in color

lowing cold air in heater is common on head gasket problems but since the 6.4 has egr which uses coolant ..not always the head gasket.

also leaks in oil cooler/ coolant exchange will have same Symptoms

 

The oil and coolant look good. The level in the degas bottle is holding a constant level for hot vs cold.

 

It sounds like your DPF is clogged.

 

Most scan software will allow you to monitor the EBP Sensor and the DPF back presure sensor as well as the regen status.

 

As you drive you can see the regen trigger when DPF back pressure is about 35% of Exhaust Back Pressure, if frequent regen then the DPF is probably clogged.

 

Thanks, I will have it checked out.