What's a good source for this citric acid? Is this concoction sold as a coolant rust removal agent? What about Prestone collant system flush? What's the active ingredient?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked