Your water injection system is set up as a performance system. It only comes on at full throttle/high boost to cool the combustion chamber and lower EGT.

See if you can get it set up so you can turn it on when cruising on the freeway. Maybe a push button or momentary switch.

I would try using it at legal freeway speeds. Down shift so the rpms are between 2500 and 3000. Turn on the system. If the engine falters after a while, then it is injecting too much volume for continuous use. In that case you will have to control the volume by turning it on and off.
This water therapy will help clean the cylinders and may help loosen stuck rings. It will not help if the problem is warn off metal.

Do this at your own risk. YMMV

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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO