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Why coolant has a lifespan and how to test it

December 25, 2023

You’ve likely read in the owner’s manual or heard about the specific time (X years) and mileage (Y miles) recommendations for changing your vehicle’s coolant. But have you ever wondered why?

The primary, yet often overlooked reason, is electrolysis. In the galvanic series, aluminum is more anodic compared to steel. This means aluminum will corrode preferentially when in contact with steel.

Take the BMW E30, for example. Parts like the cylinder head, thermostat housing, and heater core hoses are susceptible to this issue. The coolant in your car contains anti-corrosive additives, which degrade over time. Neglecting to change the coolant can lead to increased corrosion as the mixture becomes more conductive.

Fortunately, there’s an easy way to test the state of your coolant. You’ll need a multimeter set to 20V DC mode. Connect the negative lead to a ground point (such as one of the strut studs) and dip the other lead into the coolant (preferably in the overflow bottle). Perform this test with either a cold engine and coolant, or with the engine on but the coolant still cold (within 1 minute of starting).

Fresh coolant typically shows a reading of about 0.1V or 100 millivolts. Any reading above 0.3V or 300 millivolts indicates that it’s time to change the coolant.

Additionally, always use deionized water, or distilled water as a substitute, when mixing coolant. Tap water contains salts that increase conductivity and thus promote electrolysis.

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