Back in the olden days when I was a mechanic we would have old folks complain about their engine having a strange vibration. I would take it out and blow out the system using heavy acceleration. Also I would get a spray bottle full of water and spray it down the carb with the engine running/revved up. This would clean carbon deposits off the valves, pistons and spark plugs. I would not charge the customer for the cheap ''fix'', which they liked.
This was on carbureted engines, I don't know if it would work on computer controlled fuel injected engines.
As with most things, the answer is "It depends". The general blowing the carbon out is still a thing. Less so than it was in the carbureted days since fuel mixture is much more regulated and leaded fuel with associated deposits is no longer a thing in the automotive world (granted the leaded fuel aspect mostly independent from carburetors). But, engines still benefit from some heat and pressure in them. Older fuel injected cars (where they had more leeway with mixtures) especially could benefit from it, but even new ones can. However regarding the water...
Reminds me of when I would go visit my mom and take her slant six Dodge for a quick spin. Pull out on the highway and hammer it and see that black soot pouring out the back. After a couple of those runs it was much better. No, she wasn't riding with me when this was done!
As far as running cleaner through the intake, Nissan warns about that as they say some sensors could be damaged. But, now that you mention it, water might be an idea to ponder on a bit ...
The sensors are the big thing. Water, if it gets in them, can throw off the readings as well or outright cause issues. Since water from condensation can be an issue you usually see sensors oriented inlet down so that they don't have problems, but it can still be something to consider. On a car with a hot-wire MAF sensor, those are especially sensitive.
Oils can be issues as well here, which is part of why I won't run a K&N filter on any of my newer cars and stick to paper. Some people would say that means I'm over-oiling the filter and they've never had issues. That may be correct, but I'll just stick to paper for me.
I installed water injection on the bus (for intake cooling purposes mostly) and it does seem to be running better after running a pretty decent amount of water and water/methanol mixture through the engine on the last trip (more straight water than water/meth, but both have happened). I'm sure that after >120k miles of running, even without any sort of PCV/CCV or EGR, there is/was a level of gunk that was on the valves that got cleaned up and had to have helped flow. Maybe also cleaned some gunk off of the grid heater on the intake. That engine (Cat 3126B) is old enough that within the intake path there aren't many sensors, basically just boost and intake air temp. Theoretically that water/methanol mixture should be mostly evaporated by the time it reaches the valves, but the steam cleaning effect still appears to have done something. It would be really interesting to put a borescope into the intake on that engine now and see how it looks. Would've been even more interesting to do a before and after.
(The water/methanol mixture would be more commonly known as "Windshield Washer Fluid" - but only certain kinds - when picking that out you specifically look for ones that are around 30%+ methanol)
Curious, how did you prove that it came from their fuel?
Fill up tank -> car immediately runs like crap. It was isolated to their tank of premium in the case where we complained and got an insurance payout. They'd gotten calls from everyone who'd filled up with premium that day (and maybe some 89 folks). They didn't fight it and just referred me to their insurance company.
Other times, we mostly had seen issues with having bought 87 there (it's the closest gas station to our house) and then our various tractors/mowers not running well afterwards. We just avoid buying any gas there now.
