Changed Coolant & High Coolant Temp Code
 

I changed my coolant. Put 4.5 gallons back in. Nonetheless, their must have been an air pocket in there. About 5 miles after driving down the road I lost most of my power - truck was in limp mode & check engine light came on. The temp gage indicated coolant temp was just under 200 degress - where it always is. Pulled off the highway & opened up radiator cap (it was cool to the touch). Checked codes and got:

P0217 Coolant Temperature Too High
P2121 Pedal Position Sensor 1 Performance / Rationality

Engine felt cool so I drove in town & stoped engine after 5 miles. Everything felt normal. Got back on the highway and then stopped to fuel up after 15 miles. Everything still felt normal, but check engine light still on. Got back on the road and no more check engine light.

I was surprized about the air pocket & also about the P0217 code when the instrument cluster indicated engine temps being good. How do you flush coolant w/o getting the air pocket? Is there a way w/o pulling the thermostat, etc.?

What I've done when changing coolant is put in as much as you can, turn the heater to hot and fan on high, start vehicle and keep filling till its full. Put rad cap back on and put rest in the overflow tank. I have noticed on the 6.7 I have a vent tube that needs to be opened while filling and there was nothing on the 5.9.

Is there really a reason to turn on the heater in our trucks to purge the system? I can see why you'd do that on other cars/trucks that have a valve on the heater hose connected to the AC control nobs to shut off coolant flow to the heater core.

But our trucks don't have a valve like this from the factory. So, coolant flows through the heater core all the time, AC on or off.

I do it to make sure that coolant is flowing through the heater core and that there is no air trapped. Just me being OCD.

One good trick I commonly use is to disconnect a heater hose from the engine while I'm filling the system, usually as high up on the system as possible. As you pour the coolant into the rad, it pushes all the trapped air out through the heater hose. When coolant starts coming out, just re-connect the hose and finish topping it up.

Another trick is to point the nose of the truck uphill, as steep as possible, to fill the system, and then while running the engine initially after filling it. This will help to force the air pocket upwards toward the radiator and out of the engine.

Some vehicles are much more difficult to bleed than others, and these tricks can be a real necessity. The Cummins I haven't found to be too bad, with the exception of the first gen, if you don't remove the bleed plug in the head.