EGR plugged

More then likely the ports are becoming clogged up at 475 K.

 

With your egr plugged you should get better mpg.

 

Are you losing coolant? If so, EGR cooler could be to suspect

 

Well it's not really a matter of cleaning the cooler. When they go bad, they leak internally. Not good. You said pint but I'm more familiar with gallons. Half a gallon to a gallon a week I would be pretty concerned and trying to pressure test the system to find this leak (if this was the case) ...

Now, coolant could gum things up if the cooler was leaking and cause sticking of EGR valve. If the truck has over 500k mi and has the orginal exhaust back pressure sensor, it probably needs replacing. They go bad around 500-600k. Also check IMAP sensor & delta-p differential pressure sensor. Those sensors must be in good order for the emissions system to do right.

The IMAP sensor will probably be covered in soot. A tooth brush could brush it clean, but a new would be best

 

IMAP (intake manifold air pressure sensor) look on the driver side, it screws into the intake manifold itself, from/on the top of the manifold towards the middle