I have never heard of using SHOUT to clean water jackets, just vinyl or cloth jackets
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Seriously, that is new to me. For one thing, Shout is kinda expensive in the bottle form it comes in, so considering how many ounces one needs to do a system flush...well, why not use vinegar instead?
Yeah, that sounds ridiculous, but so does SHOUT. Considering what that stuff can do to your paint, I certainly wouldn't run it through those exposed alloy surfaces!!!