The ol' Tahoe is down for the count - coolant in cylinder(s)

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I recorded my best fuel mileage to date with the new engine yesterday: 17.35 mpg. It had been hovering around 15.5 to 16.5 previously. I'm hoping that wasn't a fluke and that she's finally getting broken in and will keep providing better fuel economy.

Casey

That's encouraging!

I'd like to experiment with Sarah's Blazer in an effort to improve fuel economy... First up, install a DB2, though it's not quite that easy. My Banks 6.2L TD 1982 GMC would get 24+mpg at 65-mph. I was always a little disappointed that this Blazer, with 3.73 gearing, produced 5-7 mpg less. That 1982 GMC had a TH700R4 and 3.42 gearing, but I'm guessing the 6.5 would do better with a DB2 than it does with the DS4. But, it would be a costly experiment - a good DB2, fuel injector line set and electronics package (TCM/harness) for the 4L80-E... ~ A couple grand... to experiment.

From your summer post...

Fuel economy - This is still a concern area. We had one long stretch of 4 lane highway that I pushed her hard to keep up with traffic (67 mph, 2600 rpm in 3rd), and that tank came back up at only 8.47 mpg!! I was shocked. After that, I backed the speed down closer to 62 mph, and used less of the cruise control. I noticed that with the cruise on, the ECM was often commanding a lot more fuel to keep up the speed, whereas if I just let it slow down a little on the hills, I would use much less fuel. After that, I averaged 10-11 mpg on the following tanks. Overall, after 1100 miles round trip, I averaged 11.7 mpg for the whole trip. I still think it should be better than that, but again, the 3.42 gears and 33" tires hurts the towing mileage for sure.

Just as a frame of reference.... Our 6.5TD Project truck did the run from Montana to Dayton Ohio and back in 2001, while towing a medium sized 5th-wheel travel trailer - that trailer probably weighed about 7,000-8,000-lbs. The 6.5 averaged 10.8-mpg for the ~4.5K mile round trip, running mostly at 70-mph. That truck had 4.10 gearing, and always ran in OD (4L80-E) unless pulling a grade. I wish we'd had your Holset turbo, but it ran with a GM-8. It could pull grades about as well as a similarly loaded stock LB7 Duramax/travel-trailer that traveled with it. Never had a heat issue while pulling grades or on the flat, even though it was pretty warm... i.e. 105F running across South Dakota in late August. You can see pics of the truck/trailer and read more about that trip here: https://www.thedieselpage.com/finale.htm