This time we put our team to work on finding as many busted radiators as we could so that we could test the most highly rated radiator stop leaks. After tons of radiators, a little bit of creative analysis, and a lengthy selection process we found that K-Seal ST5501 was undoubtedly the best radiator stop leak. It works on all radiator types and did the best job at being a reliable stop-gap measure.
When setting out to find the best radiator stop leak K-Seal ST5501 consistently came forward. In practice it performed the best on the widest varieties of radiators and seemed to be able to seal larger leaks than any of the others. We recommend giving it a shop as a stop-gap measure before you get a new radiator.
Table of Contents
How We Selected Products
Radiator stop leak chemicals are incredibly controversial. Some people swear up and down that they never work, others insist they ran for another 20,000 miles on a busted radiator, but most of the mechanics we talked to when researching this article told us that the truth was somewhere in between.
The entire point of a stop leak, according to the experts, was that they’re able to help you get along for a few more miles before they’ll inevitably fail.
We questioned four professional mechanics, a retired mechanic, and grilled a few auto-parts stores in order to find out which options they recommended. Then we snatched them up in order to test them out.
Almost all of the ones that were recommended to us came from the Bar’s Leaks line of products, which are a bit more specialized than the others we found.
One thing to keep in mind: for the multi use products, such as Bar’s Leaks 1109 Block Seal Liquid Copper Intake and Radiator Stop Leak we did not test their additional capacities. We only used them to see how they would handle a broken radiator.
We also made sure to get them in a few different forms. Powders and pellets, while they don’t seem to be as effective, are much easier to carry in a car emergency kit and we thought that they’d serve as an excellent way to limp your car home if something happens on the road. At the very least it’ll be cheaper than a tow truck.
If you check the listings on our products you may notice that, on average, they have a lower star rating than most of the other guides we’ve done. That’s primarily because people simply think that radiator stop leaks are going to work miracles… they’re not.
After the products we’ve included a brief guide for how to use and when to use them. If you follow our guidelines then you’ll be covered. Always remember that stop leaks are a temporary measure and not a real fix.
How We Tested The Radiator Stop Leaks
While some of us spoke with the experts, others began to gather together what we needed for the tests.
Crawling through junkyards is always a good time, but our goal here was to find any busted radiators we could so that we could test the products.
We found that radiators come in two varieties:
- Modern ones have an aluminum tubing core and plastic fittings
- Older ones use copper in the core and aluminum fittings
We picked two common cars, a 2002 Honda Civic and a 1982 Mustang. Neither was in the hottest shape, but the idea here was simply to be able to find radiators with small to large problems and then run the cars in order to see how the stop leaks work.
All of the radiators were run for control beforehand, so we may have done some additional damage although we were careful to turn the vehicle off once the fluid level had noticeably dropped.
Armed with seven radiators for both of the vehicles we began our testing.
We made sure that two of each had some serious damage in order to test them with the best of the best once we’d finished our initial testing. For the record… neither seemed to slow down whatsoever once we’d applied the K-Seal so even the best radiator stop leak isn’t going to do much for any serious damage.
Of the dry variations that we tried out, only Genuine GM Fluid really seemed to do the job, working almost as well as the K-Seal variant that we tried out while being more convenient to use since it was in a pelleted form. We’d mostly recommend it for those who are on the road.
The specialized variants of Bar’s Leaks both worked quite well, perhaps even better than the K-Seal fluid on those with minimal damage.
Unfortunately, until afterwards we didn’t think to actually measure the damage. On the most damaged before we hit the minimum 1” rifts in the really screwed up radiators we found that K-Seal tended to work better but either of them is a good option for those who have slow leaks.
Oddly K-Seal didn’t seem to work quite as well on the pinhole leaks, but it still worked better than the Genuine GM option as well as the more generic Bar’s Leak.
So in the end, these would be our recommendations for the average vehicle owner:
- K-Seal is undoubtedly the best of the lot, although it seemed better on noticeable drips than super slow leaks.
- The Genuine GM Cooling System Seal Tablets are a good idea to stow in the trunk of your car for long trips since they should allow you to limp the vehicle home without taking up too much space
- The specialized Bar’s products are great if you have pinhole leaks which have you topping off every couple of days, but you’ll need to know what your radiator is.
- The more generic Bar’s product didn’t perform as well as the rest in our tests but it’s more common in auto-stores in our experience so if you have to get going now then it’s worth a shot.
Basically, each has their use, but for most people K-Seal ST5501 is what you’re looking for. It “fixed” the most damage and it’s reputation for longevity of repairs speaks for itself.
If you’re not already damaged then keeping some of the tablet form Genuine GM Fluid 3634621 Cooling System Seal is the way to go.
Our final measure? We inexpertly tore apart the damaged radiators once they’d cooled off in order to check for any corrosion or other problems which might have occured. We actually didn’t find anything but we felt it was important to look just in case.
For those who are connisseurs we wanted to leave a word about Blue Devil brand radiator stop leaks. We felt they were too expensive to make our list and that they didn’t offer any real advantage over the K-Seal option which we picked out.
In testing we used the Blue Devil Radiator Stop Leak but we felt the higher price wasn’t warranted as it didn’t seem to do anything special and it certainly didn’t stop the leaks in the more damaged radiators.
Top 5 Radiator Stop Leaks
|K-Seal ST5501||All||Liquid||Check Price|
|Genuine GM Fluid 3634621||All||Tablets||Check Price|
|Bar’s Leaks 1186 Liquid Aluminum||Aluminum||Liquid||Check Price|
|Bar’s Leaks 1109 Liquid Copper||Copper||Liquid||Check Price|
|Bar’s 1196-6PK Radiator Stop Leak||All||Liquid||Check Price|
1. Best Overall Radiator Stop Leak
Reviewed by: Max Perzon
K-Seal has been in the stop leak business for a long time and their products offer you a good shot at taking care of your radiator. They’re a proprietary liquid blend which is billed as safe for all radiators. In our testing we noticed that it certainly didn’t seem to degrade any of the radiators that we tested it with.
Of all of the manufacturers of stop leak they’re probably the one that you’d want to trust. While we were assured it was good for the whole coolant system both of our test cars were actually in pretty good shape until we jammed damaged radiators under the hood so we don’t know how true that is.
They’re expensive. Not Blue Devil expensive, but on the higher end of the scale. That’s why we recommend using Bar’s if you’ve got a smaller leak and you know what type of radiator you’re working with.
Our testing, albeit brief, showed that it had a lasting effect. While we wouldn’t trust a stop-gapped radiatore for an across the country trip it should be enough to keep you going until you’re able to replace the radiator itself.
K-Seal is probably the most solid option to go with in order to get a long lasting fix on a serious radiator problem. Remember that even the best is temporary, but this is your best shot if you’ve got a serious problem.
- Works on whole coolant system
- Performed the best in our tests
- Trusted brand
2. Best Radiator Stop Leak for Emergencies
Genuine GM Fluid 3634621 Cooling System Seal
Reviewed by: Max Perzon
These pellets are cheap and effective.
In fact, we found out that GM routinely adds them to all of their manufactured vehicles while they’re still on the assembly line as a preventative against hairline cracks that are bound to occur within the radiator.
While they’re primarily used for aluminum radiators, there’s nothing that will interact with a copper option here and it’s a good measure to take. We didn’t find any damage in either after using these tablets in our test vehicles.
You’ll have to figure out how many liters of coolant are in your system, however, and add a precise amount which can be a bit cumbersome compared to a liquid product. We didn’t have much trouble after the first test, but it can be a pain. Try to calculate how much you’ll need before you have to use them so that you’ll be able to just drop them in.
This isn’t for a serious leak, however, but instead for hairline fractures where you’re having to top off every week or two. It’s a semi-permanent fix but you’ll still want to see a specialist at the first opportunity.
We found that it did work on slightly larger leaks in the radiators we tested, but we’re not entirely sure how long it will hold up. Once it’s been flushed through the system you should at least be able to get home, but we wouldn’t trust it much farther.
For small leaks, these pellets can work wonders but they’re not really useful for serious problems.
- Easy to store
- Perfect for hairline radiator fractures
- Not trustworthy on larger leaks
- Can be cumbersome to use
3. Best for Aluminum Radiators
Bar's Leaks 1186 Liquid Aluminum Stop Leak
Reviewed by: Max Perzon
Bar’s Leaks is another top brand which has been in business for a long time. They make a variety of formulas, many of which are designed for a specific type of radiator.
This stop leak is created specifically for aluminum radiators, so give it a miss if you’re working with the copper variety. This one is also supposed to be able to seal outside leaks within the hoses and other parts of the cooling system and has a reputation for actually doing it, but we didn’t manage to fit that part of things into our testing.
It’s no K-Seal, however, and is best used for slow leaks instead of a problem where you’re having to top things off every couple of days. We found it sealed hairline fractures quite readily and worked on slightly bigger problems as well.
It failed with the most damage radiators that were sealed with the K-Seal.
If you’re using it appropriately, however, you should be able to get a couple more weeks of regular use in your vehicle before you need to replace the radiator which is where it shines.
Give this one a shot if you’ve got an aluminum radiator and a slow leak problem. If you follow the instructions, you’ll be good to go and might be able to stall the fix for awhile.
- Designed for aluminum
- Treats whole coolant system
- Not for larger leaks
4. Best for Copper Radiators
Bar's Leaks 1109 Block Seal Liquid Copper Intake and Radiator Stop Leak
Reviewed by: Max Perzon
Older vehicles often have copper radiators and this fairly reactive metal can cause some complications if you’re not using the right product.
This is a great temporary fix for copper radiators and treats the whole coolant system as well.
Keep in mind that this product is primarily designed for pinholes and hairline fractures, anything big enough you’re seeing coolant leaking on the driveway is going to be a bit much for it to handle. It can prevent smaller problems from growing for quite a while, however.
While it’s billed as fixing blocks… don’t trust it for that. None of the mechanics we spoke to recommended using any kind of chemical fix for your block unless it was strictly a last ditch effort where you’re stranded and can’t afford a real fix.
Use it as a radiator stop leak, however, and you’re unlikely to run into any trouble for at least a couple of weeks until you can afford a new radiator.
If you’ve got a slow leaking copper radiator, then give this one a shot. It’s your best bet to prevent further damage until you can make it to the shop.
- Designed for copper radiators
- Treats whole coolant system
- Designed for small leaks only
5. Best Common Stop Leak
Bar's 1196-6PK Radiator Stop Leak
Reviewed by: Max Perzon
Bar’s also offers a fairly generic product which seems to work as well as their more specialized blends. This one is billed for all radiator types and does a good job at handling small leaks and problems which occur over time.
We quickly found it to be usable, but inferior to our favorite. It is available in the vast majority of auto-stores right off the shelf though, so if you’re experiencing problems and can’t find K-Seal then you may want to give it a shot.
This blend is billed for handling the small damage which occurs over time, and if you catch a slow leak quite early in its development you’ll be able to take care of the problem by using it carefully. It’s unsuitable for later damage, however.
The nice thing is that it’s easy to find and you can use it on any radiator. It’s not the best, but every single automotive shop that we walked into happened to have it in stock, in bulk too. Just remember the brand name and if you’re in a hurry you’ll be able to pick it up.
It’s not as good as our favorite or the more specialized products on our list, but it’ll do the job for really slow leaks and you’ll be able to find it pretty much anywhere if you’re looking for a stop leak.
- Works on all radiator types
- Available pretty much everywhere
- Affects whole coolant system
- Not as good as our favorite
When to Use a Radiator Stop Leak
Before you consider using one of these products, keep the following in mind: chemical solutions to radiator problems are not intended to be a permanent fix, no matter what the bottle says.
In general, you’ll find these products useful primarily for tiny “pin hole” leaks, rather than wanting to use them on anything which can actually be seen with the naked eye. You also want to use the least aggressive product possible for the best results, since stop leak products can cause problems in and of themselves.
For the most part this will take the form of particles entering your cooling system as a whole, which can cause damage. Liquid products are better for not causing this, but there are no guarantees over time.
We found that there wasn’t any damage during our testing, but our tests were relatively short lived and probably most closely aligned with limping a car home rather than trying to drive on a damaged radiator for another few months.
It can be hard to tell just how aggressive a product is from the marketing around it, since almost all of them are billed as a permanent solution to the problem, which isn’t really true.
Basically, the more coolant you’re losing over time, the more aggressive of a product you’ll want to use.
If you get something too gooey, however, the product can also cause clogging in the radiator which will render everything ineffective and you’ll have to replace the part immediately anyways. We didn’t see this with any of the ones which we used in our test, but we’ve heard cases of it happening with some stop leaks.
If you’re using a stop leak product, it should only be as a gap until you have the time and money to invest in a new radiator.
How Does a Radiator Stop Leak Work
Rather than mystifying the products, like the manufacturers tend to do, let’s discuss exactly what makes these products work.
Radiator stop leaks work by adding a fiber or other mass to the coolant system. Naturally, these larger particles will tend to be attracted to the lower pressure areas, namely where the leaks are, and clog together to form a gap.
Some products will use a low melting point metal additive as well, forming a temporary weld when the radiator cools down after usage. These can be great for some applications, but it can be hard to tell which is which when you’re just looking at the bottle.
Basically: the companies are loathe to say exactly what their product does since it runs the risk of their formula being stolen. We’re pretty sure that specific products use this method, however, since the only real difference between a fiber and a temporary weld is that you can’t use the same materials to weld copper and aluminum together.
If this sounds kind of dangerous… it can be. Never use more than recommended. You run the risk of fouling your entire coolant system and causing damage to more than just your radiator.
How to Use a Radiator Stop Leak Effectively
Make sure you follow all of the instructions on the bottle in addition to the following general outline in order to make sure you run the best chance of fixing things. Some products will need to be shaken and pellets and powders will have specific directions for how much to add.
Follow the amounts to the letter. Too much is definitely not a good thing.
First, you’ll need to let the car cool down and remove the radiator cap. Don’t try to remove it when the car is still hot for obvious reasons, a face full of steam definitely isn’t a good time. Even if it’s cool be careful, slowly crack the pressure rather than just ripping it open. Pressure differentials can do some weird stuff.
Afterwards, you’ll want to add the product that you’ve chosen and allow the engine to warm up to normal running temperature. Keep an eye on the ground under the vehicle to figure out when the leak stops. Once everything stops, let the engine run for a few more minutes and then you can turn the key.
Even if the product doesn’t mention it, we recommend flushing the system while it’s still hot. The last thing you need is excessive amounts of fibers or low melting point metals running around in your radiator. The build-up can cause serious problems throughout the system, and while it’s not guaranteed you’re already looking at the expense of replacing a radiator so it’s probably best to err on the side of caution.
If you’re particularly paranoid, you can flush the whole system twice with water before adding the manufacturer recommended antifreeze back into the system.
From now until your radiator is replaced or repaired you’ll want to keep an eye on the temperature and fluid level. Get things replaced as soon as you can, you might be looking at anywhere from a couple hundred to more than a thousand miles before you have to actually take care of things but remember that a radiator stop leak is not a permanent fix.
We did our testing following the amounts off the bottle, so there’s no guarantee in results if you’re not doing the same.
How to Pick a Radiator Stop Leak
While there’s a common perception of stop leaks being automotive snake oil, the fact is that some will work in order to let you avoid minor repairs for anywhere from a short time to an extended period.
We’ve done the work of digging through the reviews of this extensive part of the market in order to bring you the best and try to ascertain the situations they work best in. The main thing you’ll want to do is check to make sure the product is compatible with your radiator material.
Those products which are compatible with only one type of radiator may be the best to go with, but it can be hard to tell. This limited capability usually indicates a more precise chemical reaction however.
For the most part, pellets and powders will be a cheaper option. They’ll also work just as well as most fluids, since the solvent matters less once they’re dissolved. They’re also more easily stored within your vehicle for emergency situations than liquids which tend to require more volume.
Fluids on the other hand are a bit more foolproof as most are made to fit a specific size of cooling system.
Whatever you choose to go with, you should be able to get back on the road pretty quickly but get that radiator fixed properly as soon as you can. You wouldn’t just tape gauze over a cut that needed stitches, but it might let you get to the doctor without losing too much blood. Pretty much the same principles apply here.
If you’ve been looking for the best radiator stop leak, we still recommend K-Seal ST5501. It’s a bit pricey but not prohibitively so and you’ll be able to leave it in your garage for when your radiator starts to go. Remember to pull it out if you’re going on a real trek, however.
There’s no reason not to have some around, just remember that even the best is only a temporary fix and your next step should be shopping around to get the part replaced.