I live in Germany, and bought a beater, 150K mile 1994 316i. It runs strong, and I picked it up cheap. Anyway, I changed the oil, oil filter and air filter--all was going good.
But we had freezing weather hitting, and I checked the antifreeze with one of the little prestone antifreeze gauges. It showed hardly any protection at all, so I went and grabbed a jug of Prestone to put in so I wouldn't crack the block.
I know that BMWs are supposed to have phosphate free "genuine BMW" antifreeze to prevent aluminum corrosion, but I figured in a pinch, anything was better than nothing.
I did not flush out the whole system. Instead, I just drained the radiator from the bottom when it was warm. I filled it back up with a mix of Prestone and water.
Now the problem:

What did I do? Did I air-bind the water pump? Did I knock loose a chunk of corrosion and clog up a passage somewhere? Or maybe something else?

Thanks in advance for any help or advice!