My follow up answer to the dumb heater core question. After much yakin here and with other searches, I cleaned my heater core with standard 5% white vinegar: Detached both hoses from intake manifold and water pump. Blew out with pressurized air all water in the core and then raised both hoses up higher than the core and poured the vinegar in right out of the bottle until I knew the core was saturated. After about 5 min I lowered one pipe into a bucket and blew out the vinegar via the other hose. At first the vinegar was pretty clear but by the third fill, wait 5 and blow out, the vinegar looked like diarrhea (sorry about that🥴). I repeated this process about 8 times. Eventually the vinegar was reasonably clear. Then flushed out whole system with clean water, drained, added all new antifreeze and I could almost fry bacon where the heater air comes out!!! I believe the pressurized air really helped as it broke up the crud and then the vinegar could really attack the deposits. No leaks that I can see)) Someone will likely suggest just leaving the vinegar in overnight but based on some comments suggesting possible damage, I chose the route above.