Did you overfill your coolant? (What can happen and how to get rid of the excess)

Overfilling your cooling system is not a good idea. In this article, we’ll look at what can happen if you fill up too much coolant, how to dispose of excess coolant, and how to refill coolant properly.

But first let’s take a look under the hood and check the two types of coolant tanks that can be found in your car and where you would add coolant to your system if it’s running low.

Expansion Tank vs Overflow Tank

For most cars today, antifreeze will always be added to the expansion tank or overflow (coolant recovery) tank located near the engine. What is the difference?
Expansion tank

The expansion tank is the pressurized part of your car’s closed cooling system. It is connected directly to the radiator and is exposed to the high system pressure that your cooling system generates when the engine is hot.

This tank will have a metal cap with pressure, like the cap you used to see on radiators you may remember years ago. Never remove this cap or attempt to add coolant when the engine is hot. Remember that hot systems are pressurized and removing this cap can cause scalding hot antifreeze to spread everywhere.
overflow tank

The overflow tank is a non-pressure storage tank for engine coolant. It was also connected to a radiator and would have a simple plug that was vented to the atmosphere. Sometimes this type of enclosure has visible ventilation holes at the top.

Coolant Expansion Process

Every time you start your car, the engine will gradually warm up, and with it the volume of antifreeze will increase. Each type of tank provides sufficient storage space to accommodate the coolant that expands while the engine is running. When the engine is turned off and cools, the coolant will contract and return to the radiator.

Decades ago, automotive engine cooling systems did not include this additional storage capacity. From the freshly filled radiator, the expanding coolant simply overflows into the filler neck drain hole and flows onto the street below.

After this cycle has occurred once or twice, an empty air space develops at the top of the radiator which will then accommodate the expansion of the coolant. Environmental considerations influenced the development of the closed overflow systems and expansion tanks provided in today’s automobiles.

On some of today’s cars, the filling provisions on all types of tanks will include a drain hose to drain any accidental coolant from under the car.

However, many cars do not include this long exhaust hose feature or may only include a very short hose. In most cars, an accidental drain goes directly into the engine compartment. More on that in a moment.
Checking the coolant level (hot or cold engine?)

Both types of tanks are usually made of translucent plastic material. Whichever type you have in your car will be marked with two levels of coolant. On most vehicles, there will be a maximum coolant level and a maximum coolant level.

Always fill the system when the engine is cold and fill the tank up to the “Cold” line, but never higher. The air space above the fill line is necessary to accept the expanding liquid as the engine warms up.

Note: Many vehicles use the terms “Low” and “Full” or “Min.” Y Max “The same logic applies. You need to make sure the coolant is at a “Low” or “Min” level when the engine is cold.

The full line (marked “Hot”) on the side of this tank is there to allow you to check the coolant level in a hot engine. If the coolant level is ever observed above this line, the system is too full.

This overfill condition must be corrected when the engine cools down by draining some of the coolant from the tank. See “How to remove excess coolant” below.

If you’re not sure where to add coolant to your car’s cooling system, always consult your Owner’s Manual.

Is too much coolant bad?

Yes absolutely. An overfilled coolant tank will overflow into the engine compartment. Period. The three possible problems this can cause are discussed below.

Related: Leaking antifreeze but no visible leaks?
What can happen if you overfill the coolant?

You are checking the level of antifreeze in your car’s tank. And the tank seems to need more antifreeze. That is, it is not complete to the edge of the filler neck. So by mistake you decide to add coolant to the top. This can cause problems.

Remember, we said that the tank is marked with full lines for hot or cold engine conditions. Never fill above the “Hot” line. And when the engine is cold, never fill to the “Hot” line. An overfilled tank will overflow as soon as your engine warms up. And these are some of the problems that this will cause.
1 – Mess in the engine compartment

As mentioned above, many cars do not have a drain hose to carry that overflow fluid from under the engine. Therefore, the coolant will drip or spray into the engine compartment.

And if you’re driving, turbulent airflow under the hood can distribute sticky antifreeze throughout the interior of the engine compartment. If nothing else, this will be an annoying mess that you’ll have to clean up later.

On top of that, when the coolant comes in contact with the extremely hot metal components under the hood, you will notice a strong smell of burning coolant inside and outside the vehicle.
2 – Possible damage to electronics

The area under the hood is full of electronic components. Antifreeze blowing into the engine compartment could easily damage such electronic components.

Devices such as the alternator, engine sensors and controllers, ignition coils and spark plugs, etc. could be sprayed and the engine could stop running by leaving it parked by the side of the road. Correcting moisture damage to electronic components could also be expensive.
3 – Danger to animals

There are many types of refrigerants. Most of them are toxic. Any drainage under your car while in your own or another family’s driveway could leave puddles that could attract pets.

Consuming this antifreeze in the driveway could also result in the death of pets and wild animals. Be kind to animals…never overfill your cooling system.
How to remove excess coolant

So you made the mistake of filling the coolant tank to the top. Now what?

Check your auto parts store or hardware store (take a different car). These stores usually sell small hand sump pumps with lengths of plastic tubing that will work well to suck the extra liquid out of the coolant tank.

The Mityvac is a popular product, but the Pennzoil Multi-Purpose Pump is a good, cheap option, too.

You can also use a plastic turkey container available at most supermarkets or large discount stores. Don’t reuse this baster in the kitchen on Thanksgiving.

Store coolant in a heavy-duty plastic container with a tight-fitting lid. Label this container “50-50 antifreeze” for future use. Store it out of the reach of children. Please comply with local environmental laws in case you decide to dispose of it.

Is it safe to drive temporarily with too much coolant?

Doing so risks causing the refrigerant overflow problems mentioned above. While this is not itself a security risk, all three of the issues discussed above could occur.

For these reasons, unless you have an emergency situation, avoid operating your vehicle until the coolant level has been corrected.

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