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Cooling System Stop Leak hot engine?

Hi there, I have some small leaks in my cooling system, so I ordered the Wynn’s Cooling System Stop Leak product ( It says on that website I should bring the engine up to temperature and then pour the liquid into the cooling system via the radiator cap. However, I am always told you should never ever ever open the radiator cap while the engine is hot, let alone running. Am I reading something wrong or should I really do that?

  1. Leave the radiator cap off while you run the engine. For a short time you will be fine.

  2. These miracle in a jug fixes may or may not work. Even if this stuff works it may fail in short order. Can you locate and fix the leak?


Alright, so take the cap of before starting the engine, wait a few minutes until it gets warm, pour the stuff in, put the cap back on, let the engine run a few minutes and that’s it?

It is not one clear leak, but various small leaks i suppose, because the leaking coolant (though not dripping or flowing, just a bit wet) can be seen in multiple places under the engine and drive shaft.

Yes, you want the coolant circulating when you pour in the stop leak, it must be poured into the radiator directly, not into the overflow/reservoir tank.
After adding, run it more than just a few minutes. Take it out and drive at highway speeds. You need to get it well circulated throughout the entire cooling system.
As noted it might work, on the other hand it may clog some passages.


I’ll try that, thanks! We’ll see if it works.

It might just be one leak, and then as you drive the air movement sprays it all over. Before you add the stop leak, get the engine up to temperature and pop the hood. See if you can see steam and if so, where it’s coming from. It would be a shame to potentially gunk up passages with stop leak if you only needed to replace a heater hose.


I agree with shadowfax. This product is a last-ditch attempt to be used only when a proper fix can’t be done for some reason.


Can you see the leaks? Do you suspect that it could be the head gasket? If you think it is the head gasket, I would suggest that you use the Subaru cooling system conditioner. I’ve used it but it only works if the leak is very small. It does not clog passages but I don’t know if it will work on leaks other than head gaskets.

Stopleaks work quite well on radiator tube punctures,not on radiator seam, rubber hose or gasket leaks.

Alright, I’ll have a thorough look at everything when’re engine is up to temperature. Thanks for your comments.