What Happens If You Overfill Coolant?

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overfilled coolant

One day, I was driving my car when I noticed the temperature gauge was reading unusually high. I pulled over to the side of the road and popped the hood to take a look. It appeared that there was too much coolant in the reservoir, so I drained some out until it was within the recommended range.

I got back in the car and started driving again but only made it a few blocks before the engine began overheating. I pulled over and found that there was still too much coolant in the reservoir; evidently, I hadn’t drained out enough earlier. There was no way to fix it on my own this time, so I had to call for a tow truck.

That day I experienced the potential consequences of overfilling the radiator reservoir tank.

What Happens If You Overfill the Coolant Reservoir?

Following things can happen if you overfill the car’s coolant reservoir:

  1. Pressure Build-Up
  2. Overheating
  3. Environmental Hazards
  4. Damage To Engine Wiring

Overfilled antifreeze tank can lead to the following consequences:

1. Excessive Pressure Buildup

Your coolant system is designed to handle a certain amount of pressure. If you overfill the coolant and allow it to expand, the risk exists that you will stretch thin metal hoses and seams until they rupture. If you fill your coolant reservoir too full, the excess pressure can force the heated coolant past the overflow hose.

When this happens, it is possible to have an overflow situation where fluid will leak out of the vent cap on top of your radiator instead of the overflow hose. But you could also have a blown head gasket, cracked engine block, or a warped cylinder head. These kinds of problems can cause your engine to overheat and fail catastrophically (meaning you might need an entirely new car).

2. Overheating

When the engine gets too hot, its self-protection mode causes the cooling fans to switch on. If the vehicle is not designed to cool itself under those conditions, severe damage can result to other components, and your engine will probably need to be replaced.

Excessive heat and pressure could cause seals or gaskets to fail, which would allow engine oil into the cooling system, causing oil contamination of the coolant, and you could have a loss of oil pressure.

3. Environmental Hazards

Ethylene glycol is the active ingredient in most automotive coolants used today. It is colorless, odorless, and has a sweet taste that attracts children and animals, so it must be kept out of the reach of pets and children to avoid poisoning. Ethylene glycol is poisonous if swallowed, and it can be absorbed into the body quickly if inhaled or by skin contact. It does not evaporate easily, so once spilled, it stays on the ground for a long time.

If coolant spills, it can pose a severe environmental hazard. It is poisonous to humans and animals and will burn the skin on contact. It is important to prevent spillage when adding coolant to your car. If your car doesn’t have a cap with a pressure release, you should use extreme caution when filling the reservoir.

4. Damage To Engine Wiring

Overfilling coolant might lead to electrical damage to your car. Some automakers route hoses over the engine’s wiring harness to keep these components away from hot areas. The excess pressure caused by overfilling can damage these delicate wires or cause them to fray at their connections.

Can Coolant Reservoir Tank Store Excess Engine Coolant Fluid?

A coolant tank can store excess coolant fluid. The stored coolant fluid is then used to top off the engine’s cooling system after it has emptied due to normal usage. This prevents the engine from overheating and reduces the need for constant refilling by an owner.

Every vehicle includes a reservoir for radiator fluids filled with coolant fluid. The coolant fluid circulates through the system to keep the engine at an optimal operating temperature during use. Over time, the cooling system empties out due to normal usage, and some of this fluid gets expelled into the reservoir.

Why Radiator Coolant Expands?

If you have liquid in a closed container and turn on the heat, the liquid will expand. If you continue to add heat to the liquid, it will eventually boil and become steam. The fluid can not be compressed back into its original shape once it has boiled away, so when cooling systems operate at high pressures, they must use the highest practical boiling point fluid.

This is usually ethylene glycol which boils at about 315* F (157*C). The coolant in your car, when cold is under high pressure, so it doesn’t boil until much higher than the normal boiling point. When you start your car, the coolant temperature raises gradually over time, and the pressure in the cooling system also rises.


Cooling systems in modern automobiles are so good at what they do that most cars can go miles and miles without ever needing coolant added. But every once in a while, you need to add some water and coolant to keep things running smoothly. This often goes something like this: fill ‘er up with some antifreeze or water, check for leaks, and get rolling again.

But how much coolant should you actually add? If you fill the radiator too much one time, it’s not a big deal. But if you keep overfilling your antifreeze tank for months on end, things can get pretty costly in terms of your wallet and the environment.

And, contrary to popular belief, putting the wrong type of fluid or concentrated coolant in your car can be a costly mistake. You should never fill your system with plain water alone since it doesn’t have any additives that protect against corrosion and other problems. In fact, if you do this once, there’s a good chance you’ll end up needing a new radiator.

Frequently Asked Questions

How do you remove hot coolant from the coolant reservoir?

Draining the coolant from the reservoir is a necessary maintenance task due to several reasons. I’ll describe how exactly you can get all that nasty green antifreeze out of the reservoir.

1. Place a bucket under the drain plug at the bottom of the reservoir.

2. Lift the brake pedal all of the ways up. With your other hand, pump the clutch five times. This will force air bubbles out of fluid lines and the pump.

3. Unscrew plug counter-clockwise until it starts to get tight and then loosen 1/4 turn more. The coolant level will begin to drop.

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