I have used lots of Oxalic acid, an organic chemical with two acid groups (-COOH) compared to one on vinegar, for dissolving rust. I haven't tried it inside an engine, and would bypass the radiator (both brass and aluminum) and heater core if I were to do so, even though normally it shouldn't react with them. I get it free, mostly as products originally sold as wood bleach, from the Toxic Waste Warehouse I've mentioned before. Here's one ad on EBay for a reasonable amount at a decent price; smaller quantities can be found in local 'box' stores, but usually at much greater price per pound.
Another organic, citric acid (which is edible), is used in places like water heaters (where toxicity would be a problem) to dissolve scale. I think its activity is a bit less, but being so widely produced, its price can be very low. Not sure how well it attacks the hard oxide left from aluminum anodes; magnesium anodes leave a loose floculant deposit that can be easily washed out.
That's the diammonium salt of phosphoric acid. The use of plain phosphoric acid for cleaning steel parts has been discussed here at length.
Last edited: Sep 27, 2019