02/06/2026
Worth reposting.
The consequences of Stop-Leak and why it’s not to be used as a permanent repair.
Stop-leak products have been made and used for decades to temporarily stop or slow down a leak. They make a product for coolant, oil, transmission fluid and power steering fluid. For the cooling system(coolant/anti-freeze), the product is designed to fill the source of a leak by clogging it with a solution that becomes solid and plugs the hole or crack temporarily, thus stopping the leak. For oil, transmission fluid and power steering fluids that are gasket or seal related leaks, the stop leak solution is a different chemical that is added to those fluids and the Stop-Leak solution works by making seal and gasket material swell up thus sealing off the leak(if it’s not too significant).
The bad news about Anti-freeze Stop-Leak. Your cooling system consists of hoses that connect the engine(with small coolant passages within), the radiator(lots of small passages) and the heater core(lots of even smaller passages) together. Stop leak for a cooling system may seal a leak somewhere temporarily but actually often clogs the radiator and heater core passages rendering them useless and requiring replacement if engine temp issues or heater(no heat) issues surface because those components are plugged up with Stop-leak.
The bad news about oil, transmission fluid and power steering fluid Stop-Leaks. When these products come into contact with a gasket or seal that’s leaking, it causes the seal or gasket to swell slightly which often slows down or stops a leak. The Stop-Leak chemical also contacts EVERY OTHER SEAL OR GASKET on those components and makes them swell as well which can sometimes create new leaks you didn’t have before.
This is a pic of one of our team taking out a heater core that would not flush or reverse flush out to allow coolant flow again. The repair to replace the heater core costs far more than the original leak would have cost to repair but the heater core was clogged with Stop-Leak and the customer has no heat.
Just something to think about before you add it to your car for a leak.