Possibly my first rebuild/need 2nd opinion.

[DESERTazRAT]

Currently have a 2014 Kenworth t680 under the wrench, with 875k miles. Truck has been overheating and getting bad milage. Its always gotten slightly less mpg than our Peterbilt, my guess is due to the 3.25 rears compared to the 3.55 geared Pete. However it has gotten worse, I was told it always got .5-1 mpg less then our Pete and we're up to 2 mpg less currently. I made up some caps for the turbo outlet and inlet and when I pressurize the system to 15 lbs I can hear it leaking pretty bad. Shutting off air it falls fast to zero in less than 8-10 seconds. I removed valve cover, check valve stems for leaks, nothing. I can hear it leaking air from down under, through the return passages in the head. I did a blowby test with the manometer like in Rawze youtube video. The KW has 3" of separation at idle, it moves to 3-1/4" at 1400 RPM. Compared to the Pete it has 1" of separation at idle and 1-3/8" at 1400 rpm. Is that 3/8 of change on the Pete enough to worry about? It was rebuilt approximately 150k miles ago. The KW has a significantly higher amount of blowby and from what I saw on the video, if I managed to make my manometer close enough, we are probably within the rebuild range. It was time for an oil change anyway, so I dropped the pan and applied air pressure again. Barring the engine over I was able to hear every cylinder leaking at different times. I took pictures of the liners and they are showing wear. there is a small drilled hole on the bottom of piston I assume for oil relief. The air escaping past the rings made little bubbles I could see.

For the overheating problem I have removed water pump. It is in good working order. Borescope inside the radiator. it is clean and not corroded. Temp sensor moved to different truck. Fanbelt and tensioner replaced. Thermostat and seal replaced. Old one was gouged pretty bad I imagine it hung up and was giving us problems but after replacing, didn't fix it. We aren't in our hot season yet here in the southwest. Even when this truck was in 1teen degree weather last year it never really got hot unless working hard on a grade. When I went to unhook and bring in the shop it was overheating just idling. I will check resistance of wire from plug to plug but driver states it got hot. He would open the hood to check while giving it a break and the hoses were bulging pressure was in the system, and it was definitely hot. No leaks haven't had to add coolant. Have not done a pressure test on coolant system. Should I?

Injectors were throwing driver codes 3-4 months ago. started with 3x then 4x then 5x and we replaced all 6. It had more power than before so I wonder if the old injectors were wore out and the newer ones are too much for our worn-out rings? I imagine the turbo is having to work extra hard to maintain boost levels and causing extra heat. The liners were looking worn, I believe there is a coating on new liners. I looks to me like that has worn off, will that cause excess friction? The excess blowby probably doesn't help the piston during its combustion cycle creating more pressure under the piston.

Files have been downloaded and hope to have them reviewed. I suspect another butcher job but will be happy if it is in good shape. Anything else anyone can think of to check, or should I take this old girl out back and put her down? Should I do a compression test?

Thank you.