How to choose the best radiator stop leaks 2023

(Last Updated On: March 13, 2024)

A leaking radiator is akin to a time bomb waiting to explode. Sooner or later, it will cause serious damage to your engine if you don’t take appropriate action.

So if you are getting a leakage problem and looking for a solution that doesn’t involve expensive repair cost? Radiator stop leak should be your first try.

In this article, you’ll get to understand:

  • How radiator stop leaks work
  • How to select the best suited for your car with a review of the top 6 products available in the market
  • How to do this step completely from the comfort of your garage

So, read on to put an end to all your radiator leak worries.

What is a radiator stop leak/sealant?

A radiator stop leak (or sealant) is a chemical that treats minor to moderate leaks in your car’s cooling system by preventing the coolant from leaking out. Thus, it protects your engine and all of its moving parts from getting damaged due to very high temperatures.

Note that radiator stop leak is not a permanent solution. You should always figure out why the leakage appeared and make a plan to fix that root issue. 

However, using a radiator stop leak is a quick and effective way of plugging leakages yourself. You can avoid any breakdowns before you get the chance to address the underlying problem.

How does radiator stop leak work?

The stop leak is a chemical formulation that consists of small particles to ‘fill’ the leaks inside your cooling system. These particles deposit inside gaps and bond with each other using the heat provided by your engine.

Depending on what product you buy, the particles could be ceramic fibers or sodium silicate. Some modern formulations also use chemical compounds that react with oxygen to bind with the Aluminum, steel, or copper inside your radiator.

In most products, you will get stop leak formulations that can run inside your radiator along with the coolant. This helps to prevent any future leaks that show up without damaging the performance of your coolant.

Benefits of radiator stop leak

Increase car safety

If there’s an untreated leak in your radiator, the coolant will slowly empty from your car’s cooling system. After that, it’s only a matter of time before your engine overheats and leaves you stranded in the middle of a road.

This is also not safe for your car because it can cause permanent damages to your engine. The moving parts will face increasing resistance and will get worn out at a rapid pace, reducing the life of your engine significantly.

Moreover, it can also put you in danger. If your engine ceases in the middle of fast-moving traffic, you could easily get hit from the back by an unsuspecting vehicle. By making sure you’ve installed the radiator stop leak fluid, you will improve your car’s safety and yourself while on the road.

Boost car performance

Over time, minor leaks in the radiator can result in a significant decrease in your car’s performance. Since the engine isn’t being cooled enough, it continues to work at much higher temperatures than it is designed for. This leads to more wear and tear in the engine parts and reduces the power your engine can produce.

Radiator stop leaks work particularly well on minor to moderate leaks. They will help you keep your car running smoothly and your engine performing at its best.

Prevent malfunctioning

Your car’s engine needs the radiator to do its job whenever the engine is running, especially if you’re driving at moderately high speeds. However, with no coolant inside the system to cool it down due to a leakage, your engine will likely end up malfunctioning.

You will notice the engine temperature light flashing on your dashboard. The engine will also make loud noises of parts scratching against each other because of excessive friction.

Let’s see how a radiator stop leak can help you prevent this. It will keep the leaks plugged and keep the coolant at a critical level. Since then, it can prevent the engine from getting heated up too much. It will give you enough breathing room to safely get your car to a repair shop without worrying about engine malfunction.

Professional advice: Radiator stop leak is an effective long-term solution for minor leaks. However, when you find a major one, it is best to head over to a repair shop as soon as possible because the stop leak will not hold on for very long.

Prevent future leak

An important property of radiator stop leak is that it can run inside the radiator without reacting with the coolant or affecting its function. Because of this, you can have a set amount of sealant installed inside your car at all times.

This acts as a strong defense against future leaks – whenever a minor leak shows up, the stop leak fills it in immediately even before you notice the leak, making sure no coolant can escape the radiator.

Note: Even though radiator stop leak products are designed to be installed inside the cooling system, you should avoid overusing them. In excessive quantities, they can end up interfering with the function of the coolant.

Save repair cost

If you let your car run with a leak inside your radiator, you might end up with a breakdown of your engine. This will force you to make several repairs. You will have to replace your leaking radiator by spending anywhere from $500 to $800. If your engine gets damaged beyond repair, a new engine will cost you at least $3000-$5000 based on your car’s model.

You can avoid this heavy repair cost by using a radiator stop leak at the right time. As long as you react proactively by applying the sealant at first sight of a leak, you’ll be sure to save a lot of repair costs.

benefits of radiator stop leak
Choosing a right sealant brings a lot of benefits to your car

Buying guide

You must have understood the importance of radiator stop leak products by now and maybe ready to make a purchase. However, it is not as simple as walking into a store and selecting the first product you see on the shelf.

You need to consider several aspects to make the right choice and pick the best product for your car. This section will guide you through those considerations in detail.

First considerations

Before you get down to buying the right stop leak solution for your car, you should consider several things.

Are you sure there is a coolant leak? 

If you are mechanically-savvy and know your vehicle fluids well, this should not be a problem for you. However, if you’re not well aware of what coolant fluid looks like, it’s best to consult your car owner’s manual or seek an expert’s opinion and make sure that you’re not being misled. A car has several fluids running inside it, and the liquid you saw on the ground beneath your car could have been any one of those. You need to check for coolant leak symptoms to make sure if it is coolant or some other fluid. 

What is the size of the leak?/ How large is the leak?

If you find that the leak is large and the coolant is dripping out quickly, a radiator stop leak will not solve your problem. In this situation, it will be best to seek professional advice (if you are not experienced) and head over to the repair shop. You can only turn towards a stop leak fluid if the leak is a minor-to-moderate one.

Is it compatible with your car?

Every vehicle does not have the same kind of cooling system. Therefore, a particular type of radiator stop leak may work effectively for one vehicle but damage another’s radiator. You should check your car’s manual to learn about the types of radiator stop leaks it is compatible with.

Types of radiator stop leaks

Once you’ve decided that you are going to use a radiator stop leak to treat your coolant leak, you now have to choose the type of product you will be using. Radiator stop leak products come in two forms:


Liquid-based formulations are widely available and commonly used to treat radiator leaks. They contain several additives in a pre-mixed form that fight rust, prevent the accumulation of dirt and grime, and increase lubrication. All you have to do is pour the liquid sealant into your car’s cooling system to make it work.

K-Seal Coolant Leak Repair is one of the best liquid radiator stop leak products recommended by consumers. 

Check Price at Amazon


Tablet-based sealants are easier to store and handle but usually require an extra preprocessing step before application. You will have to dissolve a tablet in warm distilled water and then pour the solution inside the cooling system to fix leaks.

Bar’s Leaks HDC radiator stop leak tablet is among the top tablet-based sealants available on the market.

Check Price at Amazon

Note: If not used correctly, some products can clog the insides of your cooling system. Therefore, always read the instructions given on each product carefully when applying the stop leak.

Top brands

Several brands have been making radiator stop leak products for a long time. They have built a strong reputation for the effectiveness and reliability of their products. By buying sealants from these brands, you will have the assurance that you will get your desired results.

Bar’s Leaks

Bar’s Leaks has been making radiator stop leak products since 1947 and at it for more than 70 years now. It has cemented its place as one of the top brands that make advanced both liquid and tablet formulations. Their most popular product in this field is Bar’s Leaks 1186.


ATP Automotive has been famous for making aftermarket parts for the automotive industry since 1955. It is one of the leading names that produce highly effective radiator stop leak products. One of their popular product in radiator stop leak is ATP AT-205 re-seal stops leaks.


K-Seal is another top radiator stop leak brand that has academically-tested products. Its sealants are suitable not only for stopping radiator leaks but also for other issues like head gasket, water pump leaks. K-Seal coolant leak repair ST5501 is considered the most common product of this company chosen by many DIYers for fixing a leakage.

Liqui Moly

Liqui Moly was established in 1957 in Ulm, Germany. It got its name from its very first product, liquified molybdenum disulfide. Over the last 60+ years, Liqui Moly has established itself as a top producer of additives and other oil products. Its radiator stop leak products are renowned and used all around the world. Hence, the Liqui Moly 3330 radiator sealant proves the credibility of this brand with its many exciting features.

Genuine GM

Genuine GM is a brand owned by General Motors, one of the largest manufacturers of automobiles. The company was established in 1908 and has a long heritage of automobile expertise. Its radiator stop leak products are considered widely reliable. Their most popular product in oil stop leak is Genuine GM 3634621

Best radiator stop leaks review

Comparison table

K-Seal ST5501
Liquid$$Best overall
- For people who have little knowledge about the car’s cooling system and its components
- Helps fix leaks in all the cooling system parts
- Not suitable for trucks
Bar's Leaks
Liquid$Best value
- Used for truck and large vehicles
- Suitable if your engine regularly gets very hot
- Work for all materials leak
Liquid$$Best solution
- Used for the leak is caused by a rubber seal or gasket getting too hard or worn out
- Suitable for vehicles of all sizes
Liqui Moly 3330
Liquid$$Best protection
- Treat minor leaks in car’s cooling system if it is water-based and does not have a water filter
- Work effectively on cylinder head gasket leaks
Bar's Leaks HDCTablet$Best function
- Work well for coolant tanks on all vehicles
- Improve the electrolysis and pH of your radiator system along with fixing some leaks
Genuine GM 3634621
Tablet$Best budget
- Work well with all minor leaks
- Organic-based

Best overall: K-Seal ST5501


  • Do not need flushing
  • In case something goes wrong, the company will provide technical support to resolve your issue
  • Highly effective for older vehicles


  • Not suitable for trucks
  • Tend to clog the heater core
  • Work for a short amount of time

K-Seal coolant leak repair ST5501
K-Seal coolant leak repair ST5501
This product is suitable for people who have little knowledge about the car’s cooling system and its components. It works on more than just the radiator and helps fix leaks in all the cooling system parts.

K-Seal’s stop leak product is suitable for people who have little knowledge about the car’s cooling system and its components. This is because it works with all water-cooled engines and does not need any extra steps other than the pouring step itself. So if you’re searching for a product that requires a minimal number of steps, this ‘pour and go’ product is a good choice.

Moreover, if you’re unsure whether the leak is inside your radiator or somewhere else, such as in your head gasket or water pump, K-Seal is a good option to try. It works on more than just the radiator and helps fix leaks in all the cooling system parts.

Notes for usage

  • Do not pour the entire bottle inside your container all at once. Add it slowly in batches so that you don’t end up clogging the system.
  • For better results, replace your coolant before installing K-Seal.
  • After installing it and switching on the engine, let your heater run for a while so that the sealant can circulate through it properly.

Best value: Bar’s Leaks 1186


  • Fix small leaks permanently
  • Detailed documentation and user guides make the process easy to complete
  • Work equally well for trucks and other large vehicles


  • Not suitable for water pump leaks
  • Do not work on head gasket leaks

Bar's Leaks 1186
Bar's Leaks 1186 Liquid Aluminum Stop Leak
This productis is based on liquid Aluminum, which solidifies inside the leaky gaps in your cooling system. It is also suitable for the engine that regularly gets very hot since it has an additional cooling component called ‘Xtreme Cool’

Bar’s Leaks 1186 is based on liquid Aluminum, which solidifies inside the leaky gaps in your cooling system. If you are not sure if the leak is from a metal, plastic, or aluminum part inside your cooling system, you’re better off using this product since it works for all materials.

This product is also suitable if your engine regularly gets very hot since it has an additional cooling component called ‘Xtreme cool’. It reduces the coolant’s surface tension, decreasing the amount of heat the coolant absorbs, and so helps the engine temperature stay low.

You will find user guides and other documents related to safety measures and application methods in the product description in the buying link. Make sure you go through each document and understand how to use the product safely and effectively.

Notes for usage

  • If your cooling system is clogged or dirty, it is best to flush it properly before applying this sealant.
  • The sealant can be irritable to the skin and eyes, so it is best to wear protective gloves and goggles while applying it.
  • Follow the dosage recommendations given in the ‘specification sheet’ in the buying link for the best results.

Best solution: ATP AT-205


  • Work on most fluid systems (power steering, transmission, engine)
  • Can be used to improve and preserve all kinds of plastic and rubber parts (not just those in your radiator)
  • Suitable for vehicles of all sizes


  • Can permanently damage rubber seals if overused
  • Cause problems in Toyota transmission and power steering systems
  • Require a change of fluid in high-mileage vehicles

ATP AT-205 Re-Seal Stops Leaks
ATP AT-205 Re-Seal Stops Leaks
This product is targeted specifically for rubber seal and gasket leaks. It contains a ‘plasticizer’ compound used in rubber manufacturing industries to change rubber’s hardness

ATP’s AT-205 is targeted specifically for rubber seal and gasket leaks. It contains a ‘plasticizer’ compound used in rubber manufacturing industries to change rubber’s hardness. So you can infer that ATP works by making your rubber seals and gaskets less hard and more ‘plastic’, reducing the amount of oil that can leak through them.

Therefore, you should use AT-205 whenever you know that the leak in your car is coming from a hardened or worn out rubber seal or gasket. Unlike other products, you also can use this sealant for most fluid systems in your car (such as power, hydraulic, and differential systems)

Notes for usage

  • Do not use AT-205 with the brake fluid.
  • One 8oz bottle is suitable for a 6-quart capacity.
  • Do not run the engine immediately after adding AT-205 to your car. It will react with the paint and clog your system. Instead, just let it work on its own for at least an hour before you start driving again.

Best protection: Liqui Moly 3330


  • Remain effective in cold (winter) temperatures
  • One of the few products that work effectively on cylinder head gasket leaks
  • Do not clog the cooling system


  • Slow to act and fix leaks
  • Do not fix porous hoses or cracked connections
  • Work only for tiny leaks

Liqui Moly 3330 Radiator Sealant
Liqui Moly 3330 Radiator Sealant
This product is used to treat minor leaks in car’s cooling system if it is water-based and does not have a water filter. It also can be used for both Aluminum and plastic radiators, as well as with an antifreeze agent

Liqui Moly’s 3330 is not meant to be used as an all-purpose radiator seal leak. As is mentioned clearly on the product page, this product is best used for treating minor leaks in your car’s cooling system if it is water-based and does not have a water filter.

Other than that, you can use it for both Aluminum and plastic radiators, as well as with an antifreeze agent. If you have an old cooling system that is prone to regular leakages, you can leave the 3330 inside it for preventative use.

Notes for usage

  • Make sure to switch on the engine and let it run idle for at least 20 minutes after installing the 3330.
  • Use 7oz (one bottle) per 10 liters of coolant.

Best function: Bar’s Leaks HDC


  • Work effectively for water tanks of all vehicles
  • Performance not affected by rust and build-up inside the radiator
  • Detailed documentation and user manuals for easy application


  • More difficult to use than the liquid alternative by Bar’s Leaks
  • Can get stuck inside the cooling system if not dissolved properly
  • May cause overheating

Bar’s Leaks HDC
Bar’s Leaks HDC radiator Stop Leak Tablet - 60 Grams
This product is the top-recommended tablet-form radiator stop leak. It can improve the electrolysis and pH of your radiator system along with fixing some leaks

Bar’s Leaks HDC is the top-recommended tablet-form radiator stop leak. Although there is no remarkable difference between using a liquid or tablet-based sealant, you will get several conditioning advantages by using the HDC tablets. This is because these tablets will help prevent rust in your cooling system and lubricate your water pump’s seals.

Moreover, if you want to improve the electrolysis and pH of your radiator system along with fixing some leaks, using HDC will help you tackle all of those issues. Make sure to go through the provided user guide and other documentation before you start using this product. 

Notes for usage

  • The tablet has to be dissolved in warm water and should not be added directly to the coolant.
  • Drive the vehicle (or leave it on idle) for at least 15-20 mins after adding the HDC for maximum effectiveness.
  • Use two tablets for each gallon of coolant.
  • If the system is dirty or partially clogged, you should flush your engine before usage.

Best budget: Genuine GM 3634621


  • Can be dissolved directly inside the coolant and/or antifreeze
  • Works very well on minor leaks
  • Does not have adverse reactions with coolant and other components


  • Can clog your system if not crushed before dissolving in the coolant
  • Can cause overheating (especially in trucks)
  • Muddies the coolant and water in the radiator

Genuine GM 3634621
Genuine GM 3634621 Cooling System Seal Tablet - 4 Grams
This product is organic-based. It is estimated to work well on minor leaks. It is suitable for those who are skeptical about using “synthetic” chemical formulations into cars.

Genuine GM’s 3634621 tablets are organic-based, so if you’re skeptical about introducing ‘synthetic’ chemical formulations into your car, you should buy these. However, note that these are not to be depended on as solutions for moderately large leaks. At best, you should use this product if you have a minor (barely noticeable) leak in your cooling system.

Another thing you should keep in mind is that these tablets will only help you stave off engine and radiator failure in the short term. If you’re using these organic tablets, you should be prepared to plan a proper check-up and repair from a professional mechanic.

Notes for usage

  • You must flush out all the cooling system contents before you replace your radiator or install a new coolant.
  • Add the entire pack of 5 tablets to a 1-1.5 liter engine.
  • Can be irritating for the eye and skin, so wear protective gloves and goggles during handling

Final thought

Radiator stop leaks can save you hundreds (and even thousands) of dollars in repair costs. However, it depends a great deal on how quickly you act. 

If you use a radiator stop leak immediately after noticing a leak, you might be able to fix it permanently. The longer you wait, the more the chances that your stop leak will not work effectively and leave you with a damaged engine.

Finally, take good care of your car by understanding its compatibility with and picking the best-suited products. Even if you’re buying something other than a radiator stop leak, your priority should always be to prioritize compatibility and quality.


1. Why does the radiator stop leak not work for me?

There are several reasons why it may not be working for you:

  • There is no coolant leak in the first place. The liquid you see building up beneath your car may be coming from some other system such as the engine fluid or transmission system. It is best to seek an expert’s opinion if you are not experienced and check what that liquid is.
  • Your car is not compatible with the type of product you used. Check your car’s manual and research online forums about the most compatible product you should be using based on your car’s manufacturer and model.
  • The leak is too big for the product to handle. Radiator stop leaks are suitable for moderating minor leaks only, so it may be that your coolant is leaking more than the product can deal with. In this case, you should take your car to a mechanic and get it repaired.

2. Is a radiator stop leak permanent?

A radiator stop leak may only be permanent if the leak is very small. In most cases, the stop leak will prevent your radiator’s coolant from leaking for a long period. But, there is no guarantee that the leak will not come back if you don’t do the regular check or maintenance of your vehicle frequently.

3. Do I need to drain the system after using the sealant?

This depends on the type of radiator stop leak you are using. Some brands have formulations that do not need to be flushed and can safely remain inside your cooling system, protecting it from future leaks. However, some need to be flushed so that your system does not get clogged. Read the instructions given on each product’s label carefully to determine if you should drain the system.

4. Can a stop leak clog the radiator?

If not used correctly (or with an incompatible car), a stop leak can clog the radiator and force you to flush out the entire system and start again. In the worst cases, you might even have to replace the radiator entirely.

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