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galvanic action between the dissimilar metals of the engine, with the water in the cooling system acting as the electrolyte is going to give you some stray, ultra low readings. are you losing metal in any of the engine parts? water pump housing showing pitting? if everything else is properly grounded, you might try a sacrificial anode, to help reduce the number, and to fight against corrosion of radiator or engine parts.

Testing for Stray Current in the Cooling System

Another BIG factor in corrosion is stray voltage in the coolant. This causes corrosion even when the coolant is in good condition. If the engine does not have a good ground connection, voltage from the charging system will flow through the coolant to ground, creating and speeding electrolytic corrosion.

One needs a voltmeter (multimeter) with a scale capable of reading in millivolts. (I use those with digital read outs.)

1. Empty the vehicle of coolant and flush with clean water.

2. Fill the cooling system with distilled or demineralized water. (A coolant filled system gives inaccurate readings.)

3. Connect the voltmeter to the battery ground and the positive terminal into the coolant making sure not to touch the metal core or filler neck).

4: With the ignition on and again with the engine running, turn on every component. While you are monitoring the presence of stray voltage in the cooling system, have a fellow worker operate the brake lights, parking, head and high beam lights and everything you can't turn on while you're monitoring your voltmeter.

5. Note any voltage and any increases in voltage and what component increased it.

Your system has failed if you get a reading above 50mV (0.05 volts) The source (s) of the current leakage should be found, as they can destroy a radiator or other components in a short period of time, depending on the level of voltage.

The system is fine if your voltmeter reads below 50mV (0.05 volts) so refill your cooling system with coolant.

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