"I think IT's done"

[00sd250]

2000 PSD 250,000 miles
Truck started sputtering with a loud knock...checked all injectors and decided might as well replace them, after all is 250,000 miles on it....checked UV wiring harness....all is well...put back together....same problem...took back down and did compression test....all fine except #5 cylinder....totally dead. Even took one of the other injectors out and traded it and still the same outcome. It's DEAD....Decided to take that side down to see if it had a crack or hole....but nothing...a few pits, but nothing bad. Bore still looks good...still shiny...
SOOOOO, I'm thinking its a burnt valve or valve SEAT...Am I thinking in the right direction?

[Laketrotter]

With the Head off spray so carb cleaner into intake valve port and the exhaust port and if the valve is bad the fluid should pour past the valves. If you get a little seapage keep looking for for the problem. Maybe the loud knock was the rod breaking and the piston is not moving?