Overfilled Your Coolant?
Overfilling your cooling system is not a good idea. In this article, we’ll explain what happens when you overfill your cooling system, how to remove the excess, and how to properly refill your coolant.
First, let’s look under the hood to see the two different types of coolant tanks your car has and where to add more coolant to your system if the coolant level is low.
Expansion Tanks and Overflow Tanks
In most modern cars, antifreeze is always pumped into an expansion tank or overflow tank (coolant recovery tank) near the engine. What’s 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 pressures that develop in the cooling system when the engine is hot.
This vessel has a flame-retardant metal lid, just like the lids on coolers from years ago. Do not remove this cap or add coolant when the engine is hot. Be aware that a hot system is under pressure, and removing this cap could result in hot antifreeze spraying everywhere and causing burns.
Overflow Container
An overflow tank is a container that stores engine coolant unpressurized. It is also connected to the radiator and has a simple cap that vents to the atmosphere. This type of cap may have a visible vent at the top.
Coolant Expansion Process
When you start your car, the engine gradually warms up, causing the antifreeze to expand. Each type of tank provides enough space to store the coolant, which expands while the engine is running. When the engine is shut off and cools, the coolant contracts and is sucked into the radiator.
Decades ago, automobile engine cooling systems didn’t have this extra storage capacity. From a freshly filled radiator, the expanding coolant simply spilled out of the drain port in the filler neck and onto the road below.
After repeating this cycle once or twice, an empty air space was created at the top of the radiator to accommodate the expanding amount of coolant. Environmental considerations have influenced the development of the sealed expansion tanks and overflow tanks found in today’s cars.
In some cars, the fillers for both types of tanks are equipped with a drain hose to direct any accidental overflow of coolant under the car.
However, many cars don’t have this long drain hose or only have a very short hose. In most cars, unintended coolant flows directly into the engine compartment. More on this later. Check the coolant level (engine hot or cold?)
Both types of tanks are usually made of clear plastic. Regardless of the type of tank installed in your car, there are two coolant levels marked. Most vehicles have a cold coolant peak and a hot coolant peak.
Always fill your system with fuel when the engine is cold, and fill the tank up to the “cold” level line, but never fill it any further. The air space above this fill line is necessary to accommodate the fluid that expands when the engine gets hot. NOTE: Many vehicles use the terms “low” and “full” or “minimum” and “maximum.” The same logic applies. When the engine is cold, you want to make sure the coolant is at the “low” or “minimum” level.
The solid line on the side of this tank (marked “Hot”) is used to check the coolant level when the engine is hot. If the coolant level is above this line, the system is overfilled. This overfilled condition must be corrected by removing coolant from the tank as the engine cools. See “How to Remove Excess Coolant” below. If you’re not sure where to add coolant to your car’s cooling system, be sure to check your owner’s manual.
Is it bad to add too much coolant? Yes, absolutely. The coolant tank becomes full and overflows into the engine compartment. Point. Three problems that can occur due to this are explained below.