Rick's Free Auto Repair Advice

Radiator stop leak products — Do they work?

Radiator stop leak products don’t work on large leaks and they can cause much bigger problems

Just about every day I see a question from forum participants asking if stop leak additives work to stop cooling system leaks. Without fail, all the other contributors jump on the poster and tell them not to use it because it clogs up everything. That can happen if you don’t follow the direction to a “T.” But there are many cases where coolant stop leak does work. So let’s talk about when radiator stop leak products work, when they don’t, and how to use them properly.

Radiator stop leak products only work on small leaks

Stop leak products work on small pinhole leaks in the radiator, and heater core. They never work on large leaks and they rarely work on hose leaks because the rubber hoses flex too much.

Evaluate the external leak before you even think about using a stop-leak product

An external coolant leak will leave some evidence. You may see a puddle on your driveway after the vehicle sits for a while. You may smell a sweet coolant smell when driving or when shutting down the vehicle. If you pop the hood, you may see wet spots in the engine compartment or on or under the radiator. These are all signs of an external coolant leak. If you find an external leak, you may be a candidate for a coolant stop leak product.

If you don’t see any external leaks or puddles, rent or buy a cooling system pressure tester. Pressurize the system and then use a flashlight to look for the leak. If you see coolant drops or a very small stream while the system is under pressure, you’ve found the source of the leak.

Learn the leak size that might be fixed with a stop leak product

Grab a bottle of spray cleaner in your home. Turn the nozzle to stream versus spray. That stream is a fairly large cooling system leak and it’s doubtful any product can plug it. Turn the nozzle back to spray. THAT’s the size a leak product works on.

spray bottles

If there are no external leaks and you’re losing coolant, chances are you have an internal leak

An internal coolant leak is a completely different animal. Here are the signs of an internal coolant leak: Your coolant reservoir is always low (at least once a week you have to add coolant) and there are no signs of an external leak. You see white smoke coming from the tailpipe even after the engine has warmed up. An internal coolant leak is usually caused by a head gasket or intake manifold gasket failure. You can buy head gasket stop leak sealers, but they rarely work. See this post on head gasket sealers.

What to look for in a coolant stop leak product

First, will it work with your coolant?

The coolant stop leak product must be compatible with the coolant type used in your engine. Some stop leak products require you to completely drain the system and use the sealer with water. Forget that! You don’t want those because you can never really get all the water out and the remaining coolant will contaminate the coolant stop leak product.

How long does the coolant stop leak product take to work?

Some stop leak products require a long drive, while others claim they can seal a small leak in as little as 15 minutes while idling. If your vehicle is losing a lot of coolant, it doesn’t make sense to use a product that requires an hour’s drive, since you’ll be losing coolant for most of the trip.

How do you install the stop leak product?

Coolant stop leak products must circulate through the entire coolant system to work. And, the engine must be at operating temperature for the sealant to plug the leak. So most manufacturers want you to pour the stop leak chemical directly into the radiator instead of into the reservoir.

In many cases that means you will have to gain access to the radiator cap or even remove the upper radiator hose, remove some coolant, pour in the sealer, and then reconnect the hose.

If you don’t know how to do that, don’t buy that type of product.

However, if you buy a product that says you can add it to your coolant reservoir, don’t believe the times the manufacturer posts for the product to work. That’s because only a small portion of the sealer enters the coolant system and then, only when the engine cools down.

What’s in coolant stop leak products?

They’re made with several different chemistries:

• Ceramic particles
• Metallic particles (aluminum, copper, iron)
• Ground-up almond shells
• Ceramic fibers
• Sodium Silicate

How does coolant stop leak work?

It’s pretty simple. Coolant stop leak plugs leaks with a gel or a pulp-like material. A leak site is a low-pressure area and when the engine is at operating temperature, it runs about 7-16-psi. So the high pressure forces the sealant into the leak to plug it.

Filler products like ceramic particles, metallic particles and ground-up almond shells work by circulating through the cooling system while it’s under pressure. When those particles circulate near a leak, the cooling system pressure forces the particle into the gap to plug it. The particle has to be larger than the leak for it to work. It’s really that simple.

Unfortunately, filler sealers may not form a permanent seal. You may think you’ve stopped the leak, only to discover that the leak returns when you flush the coolant and add new coolant. In that case, you must add the sealer again.

Ceramic fiber, or micro-sphere nano-technology (as it’s sometimes called) works a bit differently. The extremely small fibers or spheres circulate just like other filler products and embed into the gap. As more fibers are attached to the first fibers, they build a mesh that hardens and cures with engine heat. This type of sealer provides a more permanent seal, even if you flush the system later on.

Sodium Silicate, or Liquid Glass is sometimes used in stop leak products and it also circulates through the cooling system. When the particles reach a leaking gap in a hot portion of the engine, like a metal ring on a head gasket at the top of a cylinder or the metal ring around a coolant passage in a head gasket, the particles melt to form a plug. Sodium silicate melts at 212-221°F. However, once it melts, its re-melting temperature rises to 1,490°F, so it forms a more permanent seal.

Sodium silicate is often used in conjunction with metallic filler materials in head gasket sealer products.

Can coolant stop leak damage the radiator or heater core?

Absolutely! It happens all the time. BUT, it’s

plugged radiator

Plugged radiator fins due to an overdose of coolant stop leak

usually caused by operator error when DIYers add too much sealer to the cooling system. You want just enough sealer in the coolant so it can plug a leak when it approaches a low-pressure area, but not too much so it settles out of suspension and plugs up the small tubes in your radiator or heater core. See this image of a radiator that’s been plugged up with a coolant stop leak.

When you shouldn’t use a coolant stop leak product?

If you have corrosion in the system

Keep in mind that ALL radiator, heater core, and steel tubing

stop leak product can fix a leaking radiator

This is what neglect looks like

leaks are caused by owner NEGLECT. You haven’t changed coolant according to the manufacturer’s recommendations and the system has corroded from the inside out. The pinhole leak you’re trying to fix with a leak stopping product is just a symptom of a much larger problem. If you add a stop leak product and it stops the leak, don’t think for a second that you’re out of the woods. The system will fail in another location. This is the price you pay for neglecting proper maintenance. No magic  in a bottle will fix leaks in a corroded cooling system.

Where stop leak products NEVER Work

Water pump shaft seal

Stop leak products will NEVER stop a water pump seal leak. Never. It’s a rotating part and it’s physically impossible for any plugging material to fill the gap while the parts are spinning. Got a leaking water pump shaft seal? Don’t even think about using a stop leak product.

Leaking Plastic Radiator side tank

Aluminum radiators with plastic side tanks are made from a nylon/glass formula that doesn’t adhere well to stop leak products or adhesives. You may get a plugging coolant stop leak product to stop a leak temporarily, but once you shut off the engine and it cools, the plug usually lets go and fails.

Leaking coolant reservoir

Stop leak products will NOT seal a leak in a coolant reservoir. That’s because the reservoir is subject to heat expansion and contraction and because of the plastic materials these reservoirs are made from — nothing sticks to high-density polypropylene or HD polyethylene.

Can a stop leak additive fix a head gasket leak?

Head gasket leaks are a whole different animal, so the stop leak additive products work differently. Since head gasket leaks are usually pressurized, you can’t rely on a “plugging” type of fix. Most head gasket sealers are made with a heat-sensitive compound like sodium silicate that hardens when it comes in contact with high heat. In a blown head gasket situation, the high heat is usually near the gasket breach. However, head gaskets can also leak in low-pressure areas like a coolant or oil passage. Neither of those gets as hot as a leak from the cylinder.

Also, some head gasket sealers require you to drain all the coolant and replace it with water before adding the head gasket sealer. In that case, you dump and refill and then run the engine for the recommended time. Then most require you to empty the coolant and expose the sealer to air for 24 hours. Other head gasket sealers can be added directly to the existing coolant. Keep in mind you CANNOT add those products to the coolant reservoir—it must go poured into the actual radiator.

Head gasket stop leak additives sometimes seal small breaches in the gasket area. At best, it’s a 50/50 chance, and then only if you follow directions to the letter and have a small leak. Any brand that says they have a higher fix percentage is selling snake oil and charging a very high price to make up for all the refunds they have to give out.

Can a stop leak additive fix an AC refrigerant leak?

Several companies make a refrigerant stop leak additive product. These products work when exposed to moisture, so they literally leak out of the system, come in contact with outside air and form scab over the pinhole leak. Do they work? Well, sometimes they do. But there are other issues, like:

• AC stop leak additives will NEVER stop a leak around a compressor shaft seal because it’s a rotating part.
• AC stop leak additives rarely stop leaks around O-rings and metal-to-rubber transitions.
• AC stop leak products may stop a leak in a condenser or evaporator as long as the holes are pinhole size.

HOWEVER, the minute you add an A/C stop leak product to your A/C system, it is contaminated. If you ever take it to a shop for professional service, they will charge extra to remove and dispose of the contaminated refrigerant. A/C stop leak products can damage the shop’s refrigerant recovery machines so they must be extracted by other means. In other words, you can try an A/C stop leak product as a last ditch effort on a vehicle that you never intend to take to a shop. If you’re willing to give up your A/C if the stop leak additive doesn’t work, or you’re willing to pay 3X more to have the system cleaned out, then you’re a candidate for an A/C stop leak product.

Can a stop leak additive fix a power steering leak?

Power steering stop leak products never work. The power steering system can generate upwards of 2,000-psi and that’s way too high for any plugging materials. Plus, most leaks occur at the rotating pump seal or in a rubber hose where stop leak products don’t work.

What goes wrong with stop leak additives

When forum participants say that stop leak additives gum up the works, they’re talking about the lame-brain users that ignore the bottle directions and figure that more is better. Keep in mind that stop leak manufacturers advise you to add just enough to circulate the plugging material through the system. If you ignore that advice and add more, it will settle out of suspension and clog the very narrow passages in your radiator, heater core, and engine oil galleries.

If you use stop leak additives, follow the friggin’ directions to the damn letter!

Add more than the recommendation and guarantee you, you’ll have problems—and they’ll cost far more to repair than the original problem.

©, 2015 Rick Muscoplat




Posted on by Rick Muscoplat

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