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  • You shouldn’t overfill your coolant reservoir. Doing so can lead to hydrolock and pressure buildup in the coolant system.
  • Often, an overflowing coolant reservoir is the result of human error. You might have simply overfilled your coolant reservoir and will need to drain it through the drain plug on the bottom of the radiator. Remember to use proper procedure when draining coolant.
  • Other common causes for overflowing coolant include a worn radiator cap, a faulty thermostat, and a blocked radiator.

What happens if you overfill your vehicle’s coolant reservoir? Coolant is important to your vehicle’s cooling system. However, Too much coolant in the system is bad. Today, we’ll be looking at the consequences, causes, and solutions connected to overfilling your coolant reservoir.

Is It Bad To Overfill Coolant?

You shouldn’t overfill your coolant reservoir for a number of reasons. One main issue is that doing so can lead to hydrolock. Hydrolock is what happens when the engine has too much liquid inside, filling the cylinders entirely instead of serving simply to cool them. Because the liquid, or the coolant in this case, doesn’t compress, the pistons will seize as they press against the coolant inside the cylinder. The pistons end up damaged and the coolant won’t be able to lubricate or cool the other engine parts, leading to overheating and engine damage.

Pressure buildup is another issue you’ll need to worry about if you overfill your coolant reservoir. As the coolant sits in the system, it expands, building up pressure in the reservoir and coolant lines. This expansion can crack the pipes and hoses connected to an overfilled reservoir, which leads to coolant leaking. Coolant is poisonous but tastes sweet, making leaked coolant extra hazardous for pets. It can also be a pain to clean up and needs to be disposed of as a toxic waste.

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All that being said, there are things other than human error that can lead to an overfilled or overflowing coolant reservoir. While you can overfill your coolant reservoir by accident, you might just find that something else is to blame.

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Overfilled Coolant Reservoir Causes and Solutions

Tracking down the main cause behind your overflowing coolant might take some investigating. In this section, we’ll be going over some of the most common causes and their possible fixes. Bring your vehicle to a trusted mechanic if you can’t figure out the main problem though. They can investigate your overflowing coolant further.

Ignoring Recommended Coolant Levels

Your vehicle’s engine size and your vehicle’s make and model can affect how much coolant you need in the system. Your owner’s manual should tell you exactly how much coolant you need in the system at any given time. Barring that, the coolant dipstick has marks to show whether your coolant is at the optimal level. Follow your vehicle’s recommended coolant levels and add or drain coolant accordingly. You can drain the coolant through the drain plug on the bottom of the radiator. Be sure to follow the proper coolant drain safety procedures.

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Worn Radiator Cap

Before you check the other parts of the cooling system for damage, you ought to take a look at the radiator cap. Radiator caps are designed to keep the coolant at a specific pressure. A worn cap might be loose or damaged in some way, leading to coolant spilling out even if the reservoir isn’t too full. Your owner’s manual should have more about how to replace it if needed.

Faulty Thermostat

Coolant flows through the thermostat into the radiator when it hits 190 to 200 degrees. From there, the radiator cools the liquid down before releasing it back into the rest of the cooling system. The thermostat controls when the coolant flows into the radiator, but if the thermostat is faulty, it might not let the coolant through. The coolant gets trapped outside the thermostat if it’s stuck or otherwise malfunctioning. As a result, the radiator won’t be able to cool the coolant down. The liquid will continue to heat up and expand, spilling out the top of the coolant reservoir as a result. Replacing or repairing the thermostat should prevent the coolant from getting stuck.

Blocked Radiator

Like a faulty thermostat, a blocked radiator will prevent coolant from flowing through the system. Over time, rust and sediment gets trapped in the radiator and impedes the coolant flow. Clear the blockage in the radiator by performing a coolant flush. If that isn’t enough, it might be time to replace the radiator entirely.

Bad Water Pump

The water pump pumps coolant through the system, so a faulty water pump will slow or stop the coolant flow. As a result, the engine will overheat and coolant will spill out of the system. Replace the water pump if you notice coolant spilling out of the area around it.

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Leaking Combustion Gas

Combustion gas and coolant both flow through the engine, but they follow different paths that aren’t meant to intersect. Coolant flows through passages in the engine block, cooling the engine as it goes. Meanwhile, the engine burns air and gas inside the combustion chamber, and it expels the resulting combustion gas through the exhaust system.

If the engine has either a cracked engine block or a blown head gasket, the combustion gases can escape and enter the cooling system. From there, the gases will push the coolant out of the passages and back into the reservoir, which can lead to the reservoir overflowing. Inspect the engine block and head gasket for damage and replace them accordingly.

Now that you know the likely causes behind your overflowing coolant reservoir, you can take the appropriate steps to address any issues. Coolant plays an essential role in keeping your engine cool. Refill your coolant to the appropriate levels, and keep an eye on other parts that aid its flow through the cooling system.

About The Author
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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