Rick's Free Auto Repair Advice

Do stop leak products work?

Should you use a stop leak product

Just about every day I see a question from forum participants asking if stop leak additives work. Without fail, all the other contributors jump on the poster and tell them not to use it because it clogs up everything. That’s not quite true. Stop leak products work in some cases when they’re properly. So let’s talk about where and how you can use them to stop a leak.

These leak stop products are used far more often than you think. Engine rebuilders routinely add a small cube of leak stop additive inside the cooling jacket. That reduces the risk of leaks on a fresh install. Car makers add these products at the factory because modern cooling systems are much smaller than in the past and even a small leak can cause an engine to run hot and overheat. All it takes is a pinhole to create a leak.

Cooling system leaks usually occur at the water pump shaft seal, in the seams of a radiator where the plastic tanks are joined to the core, in the heater core, at the intake manifold gasket and cylinder head gaskets, and in hairline cracks in the cylinder head and engine block. Sometimes, leaks can occur due to casting porosity right from the factory.

Leak stopping products can sometimes work in all of those locations with one exception; no stop leak product will ever work on a rotating shaft seal. That’s pretty easy to understand when you realize that the shaft is spinning and there’s usually fluid pressure behind the seal.

What’s in those products?

They’re made with several different chemistries:

• Ceramic particles

• Metallic particles (aluminum, copper, iron)

• Ground up almond shells

• Ceramic fibers

• Sodium Silicate

How do stop leak products work?

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. 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 attached to the first fibers, they build a mesh which hardens and cures with engine heat. This types of sealer provides a more permanent seal, even if you flush the system later on.

Sodium Silicate, or Liquid Glass also circulates through the cooling system. When the particles reach a leak 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.

When can you use a stop leak product?

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 manufacturers 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

stop leak products can't fix a leaking water pump

This water pump is so corroded that the impeller actually disintegrated

buy you out of this.

Cooling system leak stopping additives work by circulating around the cooling system and plugging pinhole leaks in the radiator, heater core, and steel heater tubing. Some stop leak products are made from pulverized wood fibers, or aluminum, or a sticky substance.

You can use a stop leak additive to stop a radiator leak–see the caution below.

You can use a stop leak product to stop a heater core leak –see the caution below.

You can use an additive to stop a

corroded water pump

No stop leak product will ever fix this kind of damage

leak in a steel heater tube –see the caution below.

You CANNOT use an  additive to stop a water pump leak. Stop leak products never stop leaks at seals around rotating parts.

You CANNOT use an  additive to stop a leak from a crack in a coolant reservoir

Can stop leak additive fix an engine oil leak?

Engine oil additives work by softening seals and gaskets so they swell and stop the leak.

Engine oil leak stopping products MAY stop a leak in a rubber oil pan gasket

Engine oil leak stopping additives May stop a leak in a rubber valve cover gasket

Engine oil leak stopping products will NEVER stop a leak around a front seal, rear main seal, or any kind of cork gasket.

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 blows 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 get as hot as a leak from the cylinder.

Also, some head gasket sealers require you to drain all the coolant and replace with water before adding the head gasket sealer. In that case, you dump and refill and then run the engine for the recommended time period. Then most require you to empty the coolant and expose the sealer to air for 24-hrs. 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, some times they do. But there are other issues, like:

A/C stop leak additives will NEVER stop a leak around a compressor shaft seal because it’s a rotating part.

A/C stop leak additives rarely stop leaks around O-rings and metal-to-rubber transitions.

A/C 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

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