Know the Pitfalls: Best Radiator Stop Leak

Click here to see examples of radiator stop leak.
Click here to see examples of radiator stop leak.
We all know that unfortunate feeling of having to pull your car over because it has engine trouble. It usually starts when we notice that our temperature gauge starts to climb despite normal driving and weather. Soon after comes the initial ominous puff of white smoke from under the hood, followed by that whiff of sweet smelling antifreeze. Before you know it, that puff of smoke becomes a cloud, and then that cloud continues to grow until starts to feel more like you have an erupting volcano under your hood rather than an engine. You pop your hood and take a look…bad news. You have a coolant leak. To get your car out of this predicament, sometimes it makes sense to use radiator stop leak.

When You Should Not Use Radiator Stop Leak

So before we get too deep into this article, we want to take a second to discuss when you should not use stop leak, or at least make you aware of the drawbacks of doing so. First of all, there is only one right way to fix problems with your cooling system and that is to remove and replace the part that has failed. This is the absolute safest way to fix cooling problems on your car because it isolates the offending component and as such is unlikely to affect any other component. Leaky antifreeze problems are also usually easily diagnosable as the source of the leak usually makes itself readily apparent.

Stop leak works a little bit differently. To use coolant stop leak, you simply take a bottle (or tablet in some cases) and pour it into your radiator fill neck (please note, if your car has been running immediately prior, you need to allow some time for it to cool down before opening your radiator cap or you risk being splashed with boiling coolant which will quickly put you in the ER) and then run your car for a few minutes. It works by plugging small holes in your cooling system.

That said, the drawback to using stop leak is that it does not discriminate. It acts on your entire coolant system and affects every component. It doesn’t know whether a hole is supposed to be there or whether it is the source of a leak. Stop leak plugs indiscriminately and as such can sometimes plug things that it shouldn’t such as cooling jackets or channels, potentially causing more overheating issues and engine failures in the future. It also tends to gunk up your coolant system pretty quickly. Furthermore, stop leak is rarely a permanent solution and while its use may grant you a few miles of leak free driving, you can reasonably expect that your leak will reappear over time in many cases.

In short, you should not use stop leak if you are in a position to repair your car the conventional way. Using stop leak does carry risks, so you must make sure that you take these into account.

When Should You Use Stop Leak

So now that you know when not to use stop leak, we can discuss when you should use it. Our rule of thumb is to only use stop leak when you have absolutely no other options. Perhaps you are stuck on the side of the road and out of cell phone service, or you need to limp your car to a mechanic because you can’t afford a tow. In these instances, it is acceptable to use stop leak.

That said, if you do choose to use a bit of radiator stop leak, this does not absolve you of the necessity to replace the faulty component. You should plan to replace the leaking component as soon as possible after filling your radiator with stop leak. We recommend waiting no longer than one week. Furthermore, we highly recommend that you do a complete coolant flush when you replace to component to make sure you get as much of the stop leak out of your cooling system as possible.

There are a couple more things that we should note. First, using stop leak will possibly make it harder for a mechanic to diagnose where a coolant leak exists. Second, some components such as water pumps are designed to start seeping coolant as they are nearing failure as a warning sign to replace them before they fail catastrophically and put your entire engine at risk. Using stop leak in this case may fix the seep and cause you to avoid replacing your water pump, but this may invite risk of greater failure as a result. For this reason it is highly critical that you get your leak checked out, even if you use a stop leak product.

The Best Radiator Stop Leak

Considering the possible consequences, it is imperative that you pick the best radiator stop leak for the job. Making a mistake can end up costing you quite a bit in repairs so it pays to get it right. Below we have provided our recommendations for the best coolant stop leak products on the market.

Bar’s Leaks Radiator Stop Leak Tablets

bars leaks radiator stop leak
By far our favorite product is the Bar’s Leaks Radiator Stop Leak Tablets. While most stop leak products come in liquid form, the biggest draw to these is that they come in tablet form. Because they come in tablet form, they are much easier to store in your car, so you can keep them on hand in case you get stuck on the side of the road. The bottles of stop leak liquid can take up quite a bit more space, and are usually round so they roll around in your trunk. These tablets can be stealthily tucked away under your trunk mat without getting in the way.

As far as performance, these tablets do better than most of the stop leak products out there. We find them to not be super aggressive, which means that while they won’t plug a big leak, they also have lower likelihood of damaging something else in your engine bay. The Bar’s Leaks tablets also are compatible with all types of antifreeze, so you don’t have to worry about matching up your coolant type with the right type of stop leak. Finally, they are inexpensive compared to some of the liquid products on the market. Overall, these combine easy storage, low trauma on working cooling system parts, and low cost to make a pretty solid all around stop leak option.

K-Seal Multi Purpose One Step Permanent Leak Repair

k-seal coolant stop leak
If you don’t intend to cart your stop leak around in your car with you so that you have it at all times, another product worth considering is K-Seal’s Permanent Leak Repair. It is capable of stopping leaks in a variety of locations including head gasket leaks, radiator leaks, freeze plug leaks, heater core leaks, water pump leaks, and even small cracks in your engine block. When it comes to carrying it in your trunk, the bottle is a little too big to covertly stash somewhere, so it’s likely better suited for sitting on a shelf in the garage.

When it comes to performance, the K-Seal is perhaps a little bit more aggressive than the Bar’s Leak tablets reviewed above. This means that it is more likely to stop leaks, and that the temporary fix is going to last longer. Yes, we know they refer to this as “permanent stop leak repair,” but we still insist on using stop leak on a temporary basis only for the sake of engine longevity. The K-Seal is compatible with all types of coolant, which makes it highly convenient. To use it, it just requires a quick shake and a pour into your radiator fill neck. Very little skill is needed to use the product so it is ideal for people with limited mechanical aptitude.

When it comes to cost, the K-Seal costs about two and a half times what the Bar’s Leak product does, but this is pretty typical for a liquid stop leak. All in all, we think that this is the best radiator stop leak that you can get in liquid form.


So hopefully this guide illustrates when the appropriate times to use coolant stop leak are and when you may want to forego using it in favor of a conventional repair. We also hope it points you in the right direction when it comes to making sure you choose a good one. While we have listed the best coolant stop leak products in our opinion, should you want to explore further feel free to have a look at some of the other stop leaks available. Happy wrenching!