Don't give up on your C240 engine
, they are a great long life engine.
I would bet on the problem being something in the injector pump was not secured properly, and has let the fuel rack go to the full open position instead of a regulated position.
If it was an injector problem, it would have malfunctioned on the first start up.
A good shop that did the pump overhaul
would be glad to test it again on the bench.
Also, could there be any possibility of contaminated fuel.
I have this same Isuzu engine on my boat, and it is 31 years old, with all the original paint
on the threads of the injectors and the pump. I do not know how many hours are on the engine, but it has worn out several mechanical tachometers.