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How to Test Antifreeze With a Hydrometer

by Blaze Johnson

Using an automotive antifreeze hydrometer to check your vehicle’s coolant level is a relatively simple task. Certain hydrometers work only on ethylene glycol-based antifreeze (which is green in color). Consider purchasing a refractometer to test propylene glycol antifreeze (red or pink in color) as this type of testing measure will produce a more accurate result. Certain GM and European vehicle makes use different colors to distinguish their antifreeze types. If you are not sure what type of coolant your vehicle uses, consult your owner’s manual.

Allow your vehicle’s engine to cool sufficiently. The coolant in your vehicle must be around room temperature to avoid injury and produce accurate readings with the hydrometer.

Open your vehicle’s hood and locate the radiator fill cap. (Read your vehicle’s owner's manual if you are unsure of its location.) Touch the cap with your hand to verify that it is cool. If the radiator cap is hot, wait for the engine to cool further before removing.

Cover the radiator cap with the cotton rag and push down slightly while you unscrew the cap in a counterclockwise motion. Lift the cap off the radiator fill neck and set it to the side. Run your finger under the filler neck inside the radiator to check for metallic residue or discolored debris.

Have the radiator flushed and refilled with new coolant if the coolant is discolored or contaminated with debris particles. If the coolant is not contaminated, lower the hose end of the hydrometer into the radiator fill neck. Squeeze the rubber bladder located on the top of the hydrometer and slowly release it to draw the coolant into the tester.

Ensure that the hydrometer is completely filled with coolant and note where the indicator points on the “freeze point” scale. The freeze point of the coolant should not be above minus 34 degrees Fahrenheit. If the reading shows a higher freeze point, you must have your cooling system flushed and refilled with coolant.

Check the other side of the hydrometer for the “boiling point” reading. The coolant reading should show a boiling point of at least 265 degrees Fahrenheit. If the indicated boiling point is lower than the minimum, have the coolant flushed and refilled.

Replace the radiator cap by placing it on the radiator neck and pushing downward while twisting it clockwise until it locks into place. Be sure to squeeze the small amount of coolant in the hydrometer back into the radiator before installing the cap. You may want to check the condition of the cap seal and replace the cap if necessary.

Items you will need

About the Author

In the spring of 2008, Blaze Johnson decided to share his expertise through writing. He studied business administration at a local community college and runs his own driveway mechanic service, specializing in computer-controlled vehicles and custom car audio installs. Johnson also serves as the de facto computer repair person for his family, friends and coworkers.

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