Rick's Free Auto Repair Advice

Heater core leak

Does heater core stop leak work?

Heater core stop leak products work sometimes fix a heater core leak. They work best on a recently flushed and cleaned systems because the chemical components in the stop leak product can reach and bond to clean surfaces instead of building up and clogging on top of the existing corrosion. You’ll see lots of warnings online about how stop leak products plug up your radiator and heater core and destroy your engine. Well, they can if used improperly. So let’s talk about what you can do to avoid clogging up your radiator and cooling system by using a heater core stop leak product.

First, understand what caused the heater core leak

Heater cores are designed to last the life of the vehicle. That’s why car makers install them deep into the dash of your car—they don’t expect them to ever need service. So how do they develop a heater core leak? Owner neglect. Not replacing engine coolant at the recommended service intervals is No. 1 cause of heater core, water pump and radiator failure.

Engine coolant does more than prevent your engine from freezing and cracking. Coolant contains anti-corrosive and anti-electrolysis additives that prevent the system from corroding from the inside out. Since a typical engine and cooling system contains many different types of metals, it can exposed to galvanic action, where ions from the dissimilar metals move from one component to another—like a battery. Fresh coolant prevents galvanic action, while worn out coolant accelerates it.

What’s the point? Well, in most cases, where you have a heater core leak the entire cooling system is most likely corroded and possibly plugged. Worse yet, the material that’s transferred from your heater core has deposited on other portions of the cooling system. If you add a heater core stop leak product to the system without flushing and cleaning the system, that stop leak product can attach to the transferred metals in other parts of your system, causing more problems.

The second most common cause of a heater core leak is the removal of a flow restrictor. Car makers install flow restrictors to increase the “dwell” time of hot heater core flow restrictorcoolant in the heater core. That allows them to get the same amount of heat output from a small heater core. If the flow restrictor is removed, too much coolant flows through the tiny heater core tubes and causes internal wear.

Adding a stop leak product to a corroded system can clog it.

If you’ve neglected maintenance of your car’s cooling system and simply add a stop leak product without doing the steps below, chances are the stop leak product won’t work. In fact, it may clog up your entire cooling system. Imagine a plumbing pipe in your home that’s rusted through. It’s dirty, has corrosion on the outside, and you’re trying to fix it by applying an adhesive on top of rust. That’s what most DIYers do when they add a bottle of stop leak product to their cooling system without cleaning it first. So you have to remove the corrosion buildup first before you try a stop leak product. However, there’s a risk in doing that because it can actually make the leak WORSE.

So you’re facing a Catch-22 situation. Try the heater core stop leak product to patch a heater core leak, or perform a complete cooling system flush and risk removing the corrosion that’s plugging multiple pinholes, causing all of them to leak once you’re done.

My position is this: you’re into this for a new heater core at the very minimum. If you flush the system and make the heater core leak worse, you’re in no worse shape. However, if you flush the system and end up with heater core AND radiator leaks, you’ll have to shell out even more money. But don’t think you weren’t in trouble before the flush. If the flush opened up more leaks, it was just a matter of time before those other pinholes started to leak.

Here’s what a plugged and corroded heater core looks like:

heater core leak


Start by flushing the heater core

Remove the heater hoses and flush the heater core to remove the gunk. Then attach 5/8″ vinyl tubing and reverse flush the heater core

Connect vinyl tubes to heater core

Connect vinyl tubes to heater core




Flush the heater core

Flush the heater core

heater core replacement cost

Lisle 60900 Heater Core Backflushing Tool

Then flush with the entire a cooling system with a special cleaner system cleaner product


Purchase a cooling system cleaner and prepare to do a complete system flush and fill. cooling system cleanerFollow the directions on the package label to use the cleaner. In many cases the recommendation is to:

  1. With the engine cool and off, remove the radiator cap. Loosen the lower radiator hose clamp and move it out of the way.
  1. Move a drain pan into place and detach the lower radiator hose from the radiator. Do NOT empty the radiator using the drain cock. There are two reasons not to even attempt to use the drain cock. First, the plastic and O-ring gets brittle with age. If you manage to open it, it’ll never seal when you try to close it. Second, draining the radiator only removes a portion of the coolant. Removing the lower radiator hose drains more of the old coolant.
  1. Re-attach the lower radiator hose and clamp. Add the contents of the cleaning product directly into the top of the radiator according to the dosing instructions on the label. Add water.
  1. Start the engine and turn your heater control to the maximum heat setting. Run the engine for the recommended time AFTER it reaches operating temperature. Then turn off engine and let it cool. This step is important. Don’t drain hot coolant. The cooling period is important for the cleaner to work.
  1. Drain the coolant by removing the lower radiator hose. Repeat steps 3 & 4. Run the engine the recommended time.
  1. Drain the coolant by removing the lower radiator. Re-attach the lower radiator hose and clamp
  1. Refill the cooling system through the upper radiator hose with fresh coolant and a top quality nano-technology or polymer based heater core stop leak product. Follow the product’s dosing recommendations to the letter! Do not add more than the recommended fill. Adding too much heater core stop leak product can cause clogging issues throughout your cooling system!!!! Refill the coolant reservoir with fresh coolant. Make sure the heater core stop leak product is added to the radiator, not the coolant reservoir.
  1. Drive the vehicle for the recommended time.

Then will a heater core stop leak product work to stop a heater core leak?

I can’t guarantee success if you clean the system, but it will increase your chances of success. At best, you’re looking at a 50/50 shot. Without cleaning you’re looking at less than a 10% chance of success. If the heater core stop leak product works, you’re done. If it doesn’t then you’ve given it your best shot.

©, 2017 Rick Muscoplat


Posted on by Rick Muscoplat

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