Hi.
Rodded some tubes, applied different household chemicals to others to evaluate if some would work.
Acetone, drano, penetrating fluid, alcohol, detergent, tub cleaner, oven cleaner, charcoal starter...
All the tubes had an uniform thin layer of slimy deposit, chemicals did not dissolve/dislodge them (in only 15 minutes.) Rodding was the best way. The only chemical on hand that did a noticeable improvement was [ The Works® :: Toilet Bowl Cleaner ]

Rodded everything, ended clean and shiny now; but I suspect the block guts have such deposits too.

Those deposits not only narrow the radiatior passages but believe they are also preventing intimate contact between coolant and aluminium for heat transfer.

Is there an aggressive known-to-work chemical or flushing product that really performs in your opinion ?

 

I had to laugh when I got to "charcoal starter"! Sorry. I too have been curious about what would clean the block. I want to do it just as a preventative measure, no over heating issues here. I am a little bit afraid of "eating" some internal engine part. I use a mild acid called Alumibrite before welding. It does a great job on aluminum wheels. We need a chemist to chime in.

 

Mercedes sells packets of citric acid. Home remedy has included borax laundry booster, white vinegar, and brick washing acid (eats aluminum). My indy rad shop rodded out and hot flushed (circulated by a pump) citric acid. Did my D1 rad $75 carry in, included unsolder tank, and few spots of repair as well). D2 rad not real practical, the plastic side tanks become brittle, and the build up inside came from years of chemical process, both hot and cold. Disolving all that is a challenge. Now doing the white vinegar run and flush every two - three years would make sense as oppsosed to never touching it. $135 and up aftermarket radiators for a D2, $235 - $700 for a D1.