Rick's Free Auto Repair Advice

Do stop leak additives work?

Should you use a stop leak additive

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.

When can you use a stop leak product?

Fix a cooling system leak

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

fix leaking radiator

This is what neglect looks like

leaks are caused you 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 stop leak 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

radiator stop leak

This water pump is so corroded that the impeller actually disintegrated

buy you out of this.

Cooling system stop leak 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 a stop leak additive to stop a

radiator stop leak

No stop leak product will ever fix this kind of damage

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

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

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

Fix an engine oil leak

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

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

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

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

Fix a head gasket leak

Head gasket leaks are a whole different animal, so the stop leak 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 that hardens when it comes in contact with high heat. In a blow 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.

Fix an AC refrigerant leak

Several companies make a refrigerant stop leak 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.

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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