Custom mechanical fan builder

autoxcuda said
Might be calcium deposits.

Have you done a simple voltmeter electrolysis test? Should be under .3 volts. Connect one side of voltmeter to body ground and dangle other in coolant.

Also check engine ground straps.

Do you mix tap water with coolant or water pump lube in radiator?
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Anything is possible. I have not done a voltmeter test, but will. currently using just tap water and water wetter, didn't want to use 50/50 because I assumed I'd probably have to drain the radiator a couple more times.