Do you suspect that your radiator is leaking? Have you seen the temperature gauge start to rise above normal while driving? Is there billowing white smoke coming from the hood of your engine? Are flames coming out? (okay, this might be beyond just a radiator leak now)

Symptoms of a Radiator Leak:

  • Overheating Engine
  • Leaks under the vehicle (red, yellow, or green fluid)
  • Fluid discoloration in coolant overflow tank
  • Heater stops working

Radiator stop leak is a type of additive made of ceramic fibers, sodium silicate, aluminum, copper, iron, or ground-up almond shells. It’s designed to seal and fix minor damage to your radiator. Your radiator works by moving coolant from the front of the vehicle to the coolant passages in the engine. There are a number of hoses where a small leak can develop.

The criteria of this guide is based on if the stop leak is fast, cheap, and effective. There are certain stop leaks that require a ton of work or are expensive. We’re looking for something that’ll get your car back on the road ASAP. That said, stop leaks can’t fix everything, and are mostly suitable for minor damages.

1. K-SEAL Coolant Leak Repair, ST5501 8oz, Multi-Purpose Formula

K-SEAL Coolant Leak Repair

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Verdict: Safe and effective, this stop leak repair can seal leaks up to 1/32 of an inch. Once cured, the repair withstands the high pressure of the combustion chamber, all without failing or clogging the cooling system.

Usage: Shake the bottle and add the fluid to the coolant system

How it works: Microfibers in the fluid mesh together when a gap is found, and a seal is built fiber by fiber. According to K-Seal, is is designed to repair holes up to 0.635mm in size yet it is small enough to not block the passageways in your radiator.

2. Bar’s Leaks 1186 Liquid Aluminum Stop Leak

Bar's Leaks 1186 Liquid Aluminum Stop Leak

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Verdict: The key ingredients in this fluid are what Bar calls “Liquid Aluminum and Xtreme Cool”. It was designed to be directly added without having to flush anything.

Usage: Add fluid and drive/idle the vehicle for 10 to 20 minutes.

How it works: Liquid Aluminum stops the leak and Xtreme Cool conditions the system to stop overheating and reduce water temperature.

3. Prestone AS145 Radiator Sealer Stop Leak

Prestone AS145 Radiator Sealer Stop Leak

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Verdict: Uses a corrosion inhibitor to protect against pitting and corrosion. Has the ability to seal leaks up to 0.020″ x 0.5″. Acts similar to K-Seal by using fibrous particles to seal leaks. By inhibiting corrosion, it can help prevent future leaks.

Usage: SHAKE well and add entire contents of bottle into radiator.Fill with Antifreeze and close cooling system. Run engine with heater on high for 10 minutes after reaching normal temperature.

How it works: Uses the same durability fibrous materials as bulletproof vests

4. AlumAseal ASD24/2-24PK Radiator Sealer Tube Dispenser

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Verdict: This is the easiest and fastest choice. This is designed to work with rubber, metal and plastic as well as compatible with antifreezes and coolants.

Usage: Pour powder into radiator, turn heater to hot, then drive.

How it works: Finds and seals the cooling system as well as lubricates the water pump.

5. ATP AT-205 Re-Seal Stops Leaks

ATP AT-205 Re-Seal Stops Leaks

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Verdict: This product is designed to seal not just radiators, but also stop leaks in engines, transmissions, power steering, and hydraulic systems. The common denominator in these systems is the rubber parts. It does not contain petroleum distillates so it does not breakdown seals.

Usage: Pour fluid into cooling system

How it works: Restores worn or dried out seals in the cooling system

6. Zerex Super Radiator Sealer

Zerex Super Radiator Sealer

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Verdict: A bit pricier than the rest. However, this fluid uses all natural materials and is water-based so it is lower risk (plant fiber, clay and water). This product is authorized by Cummins (legendary engine manufacturer) and is proven safe.

Usage: Seals leaks in about an hour

How it works: Uses all natural materials to seal cooling system leaks

7. Irontite All Weather Seal

Irontite All Weather Seal

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Verdict: If your vehicle undergoes extreme conditions (hot or cold weather), this product is a good choice. This also works with all types of materials in your radiator (aluminum, cast iron, plastic, rubber). It’s been designed not to clog heater cores.

Usage: Flush cooland if it is old and dirty. This is not required but recommended. Shake bottle, add into radiator. Fill coolant system with coolant and start vehicle. Leak should stop in 2-5 minutes.

How it works: Micro Fibers find leaks and stop them.

8. J-B Weld 2120 Radiator and Plastic Repair Kit

J-B Weld 2120 Radiator and Plastic Repair

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Verdict: This is an affordable solution from a very well-known brand. That said, this is a kit and not a pour-in solution. The kit contains epoxy putty, fiberglass cloth, sand paper and an application brush. It can fix holes up to 3/8″ in diameter or cracks up to 4″ long.

Usage: Cut out the piece of resin. Sand the crack. Mix liquid and use brush to apply. Place resin on top and add coat on top.

How it works: The kit contains a chemicals which become an epoxy which you apply to the crack or damaged area of the radiator.

9. Liqui Moly 20132 Radiator Stop Leak

Liqui Moly 20132 Radiator Stop Leak

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Verdict: This German made product is made for smaller leaks and is safe for aluminum and plastic radiators.

Usage: Add contents to cold cooling system, start up the heater, then drive for at least 10 minutes

How it works: Secures the combustion chamber from possible damage caused by coolant leaking in

10. Blue Magic 1008 Liquid Glass Metallic Seal-Up

Blue Magic 1008 Liquid Glass Metallic Seal-Up

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Verdict: Ideal for sealing up cracks and is able to withstand up to 2,000F

Usage: Pour into cooling system

How it works: Forms a copper-ceramic seal to fix cracks.

Why Should I use Radiator Stop Leak?

If the radiator needs to be replaced, the costs range between $300 and $1200 for parts and labor. The products on this list are much cheaper and can save you a lot of money if it works.