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I read this article and found it quite interesting. Just thought I'd pass it on.

I know I am supposed to replace engine coolant before it degrades into a good electrical conductor that encourages electrolysis to corrode the heater core, water pump, radiator and other metal parts. However, there are more cars than drivers in my family. Sometimes the years pass faster than the miles. Repairs over the years might have necessitated replacing some, but not all of the coolant in an engine. Here is a quick way to use an inexpensive multi-meter to check to see if engine coolant (in most engines) is preventing or encouraging electrolysis.

Switch the multi-meter to one of its smallest voltage settings. Take the radiator cap off the cold radiator and stick one electrical probe in the coolant and put the other probe on a grounded metal part or the car’s negative battery terminal. A reading of 0.4 volts or above indicates the engine coolant is encouraging electrolysis and should be replaced.

Experts recommend starting the cold engine with the radiator cap off (so no pressure builds) and then allowing the engine to warm up before measuring the voltage. The thermostat opens up and coolant starts flowing through the engine and radiator, so not just the stagnant pool of cold coolant below the radiator cap is tested. This might be an especially good tip for engines with the radiator cap located in the middle of a hose or on a small reservoir separate from the radiator. I first measured voltages on the cold coolant in my cars' radiators and found the voltages changed by only a few hundredths of a volt after the coolant was circulating and heated to operating temperature.

My family’s old cars from the previous century all use the same type of (usually green) coolant. The engine coolant in my wife’s ’93 Ford is only a few months old and it generated the lowest voltage, 0.06 volts. My ’86 Mustang has the oldest coolant in the fleet, about 12 years old, and its voltage reading was the highest at 0.12 volts. Even the Mustang’s old coolant was well below 0.4 volts. Miles must degrade coolant faster than time.

Surprisingly, the second highest voltage reading, 0.11 volts, was from the coolant in our car built this century, a nearly new ’14 Dodge. It uses special purple engine coolant. It is good we now have a baseline for what that coolant’s normal voltage reading should be.

Checking to see if the coolant has a voltage reading less than 0.4 volts is just one basic test. Coolant might also be “bad” if it is so diluted with water (typically significantly above 50%) that it does not prevent freezing, if it is loaded with debris that clog the narrow tubing inside the radiator or if it fails a test recommended by the engine manufacturer. RockAuto has a coolant and battery refractometer for owners who want to more thoroughly measure acidity, concentration and other coolant (and battery electrolyte) parameters. A Father's Day gift for dads who are most serious about taking care of their cooling systems!

Tom Taylor,
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