Sometimes the simple straight forward solutions are the best. Not having a lot of experience with diesel water injection usage - my first recommendation is to try an air-to-air intercooler to see if that brings you to a stable level. You may be satisfied with this alone, but if you want to then optimize with water injection then you can look further into it.
I say this because I try not to recommend to others running setups that just won't work well or at all without the water injection. I prefer water injection to be used for optimization rather than pure ability to run properly.
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