Overfilling your cooling system is not a good idea. Overfilling a coolant reservoir can lead to the coolant being pushed out when the engine heats up, potentially causing leaks and spills. It won’t directly cause overheating, but the excess coolant can overflow and contaminate the engine bay.
In this article, we’ll explain what will happen if you overfill your coolant, how to remove the excess coolant, and how to properly top off your coolant.
Before we look at overfilling your coolant, let’s check under the hood and see the two different types of coolant tanks that may be used in your vehicle and where you would add coolant to the system if it is low.
Expansion Tank vs Overflow Tank
Antifreeze will be added to either an expansion tank or to an overflow (coolant recovery) tank on most cars today, and an overflow tank, but what’s the difference?
Expansion Tank
An expansion tank is part of your car’s closed cooling system and is pressurized. It is connected directly to the radiator and is subject to the high system pressure of the cooling system when the engine is hot.
This expansion tank will have a metal pressure-rated cap just like the cap often found on a radiator, just like you may remember from years gone by.
You should NEVER remove this cap or attempt to add coolant when the engine is hot. The high pressure in a hot cooling system can cause scalding antifreeze to spray everywhere if the cap is removed.
Overflow Tank
An overflow tank is an unpressurized storage reservoir for engine coolant. An overflow tank is also connected to the radiator and will have a typical cap that vents to the atmosphere. In some cases, there will be a vent hole on the top of this kind of cap.
Coolant Expansion Process
Every time you start your vehicle, the engine’s temperature will slowly rise, and with that increased temperature, the antifreeze will expand.
All radiator and overflow types of tanks have enough capacity to allow the changing coolant to remain in storage as long as the engine is running. When the engine stops and cools, the coolant in the tank will contract, and it will be drawn back into the radiator.
Years ago, vehicle engine cooling systems did not have this traveled pathway for excess coolant storage. In fact, when the expansion process started in a new radiator, there was excess and it overflowed from the filler neck drain port and drained onto the roadway.
Once this process cycle was repeated a few times, there was an empty airspace at the top of the radiator to store the expanding fluid.
It was just the desire of the car company to help the environmental impacts of an open radiated coolant tank system that led to overflow and expansion tanks in the vehicle engine cooling systems we have today.
Some people’s cars, although there would be a source of filler container, either type, that would have a drain hose to the bottom of the car to allow for any inadvertent coolant overflow to drain down below the car on the road.
However, as a comparison, many vehicles do not have a long drain hose, or maybe only a very short one. Hence, the inadvertent coolant drainage would simply go into the engine compartment. There is much more to come on this!
Checking Coolant Level (Hot or Cold Engine?)
Both types of tanks are typically made of clear plastic, and no matter which type you use in your vehicle, it will have two coolant level lines. In most vehicles, one level indicates the maximum level for cold coolant, and the other indicates the maximum level for hot coolant.
Always top up the system when the engine is cold and fill the tank to the “Cold” level line, but not higher. The air above the fill line will be required to accommodate the expanding coolant as the engine gets hot.
Note: Many vehicles will use the terms “low” and “Full” or “Min” and “Max”. This same logic applies. You’ll want coolant at the “low” or “Min” level when the engine is cold.
The full line (marked “Hot”) on the side of this tank is there to allow a coolant level check with a hot engine. If the coolant level is ever found above this line, the system is overfilled.
This overfilled condition must be corrected after the engine cools. Excess coolant must be removed 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, the Owner’s Manual is always a good manual to reference.
Is Too Much Coolant Bad?
The answer is yes, too much coolant is bad. An overfilled tank of coolant will overflow into the engine compartment. Period. The following are three possible problems from an overfilled tank.
Your engine is more likely to overheat if your vehicle is low on coolant. Because the fluid levels will be so low, it is likely that a lot of air is in the engine now, leading to airlocks. If your engine does overheat, it can create damage that could cost you thousands of dollars.
Typically, a warning light will come on your dashboard informing you that your coolant is exceptionally low.
If your vehicle is low on coolant in the tank, your temperature gauge can spike to wild readings well outside its normal range to monitor heat levels.
There can be dangers with having too much coolant, also. Too much coolant can lead to problems with the electrical workings of your vehicle.
In certain cases, the motor can handle the overload of coolant, and it can exit the motor through the overflow hose. However, you need to be sure that your coolant is not overfilled or above the maximum fill line.
What can happen if you overfill your coolant?
You’re checking the antifreeze level in the tank on your car, and the tank appears to need antifreeze. In other words, the tank shows that it’s not full right to the edge of the filler neck. So, you mistakenly decide to fill the tank to the very top! This can cause problems.
Don’t forget we talked about the tank has indications such as full lines that indicate when to fill, one for cold engine conditions, and one for hot engine conditions. So, don’t ever fill above the hot line.
Now, don’t ever fill up the hot line when the engine is cold. An overfilled coolant tank will overflow into the engine compartment when your engine warms up. And here are some of the problems this can cause.
#1. Mess in the Engine Bay.
As mentioned previously, many cars do not have a drain hose to direct the excess fluid below the engine. So it will either drip or spray out and leak into the engine compartment.
While driving, the turbulent airflow under the hood will spread the sticky antifreeze throughout the engine compartment. At the very least, this will certainly be a nuisance when it comes to clean up afterward.
Furthermore, when the coolant comes in contact with the extremely hot metal engine parts under the hood, there will be a very noticeable burning coolant smell inside and outside the vehicle.
#2. Possible Damage to Electrical Components.
The under-hood area is filled with electric components. Antifreeze could easily damage electrical components with its spray in the engine compartment.
It is possible that electrical devices like the alternator, engine sensors and controllers, ignition coils and spark plugs, etc., could be sprayed, and the engine may not run, leaving you stranded and parked on the shoulder of the highway. Fixing moisture damage to electrical systems could also be costly.
#3. Danger to Animals.
There are many different types of coolants. Most are toxic. Any drainage under your car while you are in your own or another family member’s driveway could leave puddles that could be attractive to pets.
If consumed off the driveway, this antifreeze can be fatal for domestic animals and wildlife as well. Be considerate of animals… don’t ever overfill your cooling system.
How to Remove Excess Coolant
So you’ve overfilled the coolant tank to the brim…now what?
Check for a suction pump at your auto parts store or at a nearby hardware store (just take another vehicle). These stores will sell small hand powered suction pumps with tubing that would work well to suck extra fluid from your coolant tank.
The Mityvac is a commonly recognized choice, but a Pennzoil Multi-Use Pump is an inexpensive alternative.
A plastic turkey baster available at your food store or discount, or department stores can also be used. Just don’t use your turkey baster in the kitchen on Thanksgiving.
Once it is out of the car, put it in a sturdy plastic container with a tight-fitting lid. Label this as “50-50 antifreeze” for reuse in the future. Keep it well beyond a child’s reach. Follow your local environmental laws if you decide to drop it off somewhere.
Is it safe to drive your vehicle around with too much coolant temporarily?
There is the risk of coolant overflow workings described above. This is not a safety issue, but the accumulation of all three problems discussed above may occur.
Therefore, unless you are in an emergency, do not operate your vehicle until you have corrected the coolant level.