Put a bottle of Bars Leaks in my 289 and the heater core quit two weeks later
I'm not saying stop-leak ruined it for sure, but here's what happened. My 65 Mustang was seeping coolant at the water pump gasket, so I dumped a bottle of Bars Leaks in like my dad used to do, and it stopped the leak fine. About 14 days later the heater core let go behind the dash and dumped coolant on my passenger floor mat while I was idling in traffic on Route 9. My radiator guy in Poughkeepsie said the stuff can settle and plug the small passages, so your mileage may vary, but I'd pull the pump and do the gasket before I ever pour that in again. Has anyone had a stop-leak fix that actually held up long term without a side effect like this?