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Summary

  • Antifreeze is kept in the coolant reservoir near the radiator. It’s usually mixed with water to create engine coolant.

  • It helps control engine temperature by lowering the freezing point and raising the boiling point of the liquid. Corrosion inhibitors keep the coolant alkaline and protect metal parts from rust and deposits.

  • Only check or add coolant when the engine is cool. Radiator-cap systems can appear full even when the radiator is low, while capless systems rely on the surge tank. Fully drained systems must be bled of air as the engine warms, though surge-tank-only designs usually release trapped air automatically.

Antifreeze is stored in the coolant reservoir near the radiator. It’s usually mixed with water in the container, creating what’s commonly known as engine coolant.

Antifreeze is more commonly called “coolant” by professionals. This is a glycol-based fluid that helps regulate the engine’s temperature. When combined with water, it lowers the liquid’s freezing point, enabling coolant to keep flowing at extremely low temperatures instead of turning into solid ice. 

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Furthermore, antifreeze raises the water’s boiling point, keeping the liquid from evaporating into steam at very high temperatures.

Antifreeze also helps prevent corrosion. It contains additives called corrosion inhibitors that stops rust and other corrosive deposits from forming in the engine. These inhibitors maintain the coolant’s alkaline pH, preventing acidity that can corrode metal.

Warning: To prevent serious burns, never open a hot cooling system under any circumstances. Always feel the radiator hose to see if it has pressure. It’s best to let the engine cool for at least 30 minutes, then check for system pressure by feeling the hose.

Tips on How to Access the Antifreeze

open radiator filler neck with green antifreeze
Some systems have no cap on the radiator, but if they do, you can let the engine cool and remove the cap to check the coolant level (see photo). | Image Source: Richard McCuistian

Systems with a thin de-gas bottle may have coolant in the reservoir but have very little coolant in the radiator itself, so you can’t just look at the coolant level in that plastic reservoir to determine coolant level on a system with a cap on the radiator.

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If yours doesn’t have a radiator cap, then looking at the coolant level in the heavy surge tank is the only choice you have, and that’s where you’ll add all the coolant to the system.

On systems with a radiator cap and surge tank, you need to understand that if all the coolant has been drained from the system, refilling the engine with coolant isn’t like pouring water in a bucket. Air will be trapped in the system and it needs to be systematically allowed to escape as the thermostat opens when the engine warms up.

coolant reservoir surge degass tank
The ones with the only fill point being on the surge tank (see photo) are slightly different in that they tend to burp air out automatically, but make sure the system is full before you close the hood and call it done. | Image Source: Richard McCuistian
About The Authors
Written By Automotive and Tech Writers

The CarParts.com 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 CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

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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