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I received my monthly newsletter from RockAuto.com and thought I would pass this info along. We have had numerous discussions here over the years regarding HOAT coolants for TJ's, but this should be helpful for all models.

Your Cooling System & Electrolysis
Tom's Story

I have caught myself thinking, "Why did that heater core fail again so soon?" and then discover the heater core was actually last replaced 10+ years ago. (I avoid losing track of time by using my RockAuto account to see when parts were last ordered/replaced. To create an account, go to upper right corner of the RockAuto home page or look under "Menu" on a phone.)
Electrolysis is the likely culprit when "fail again so soon" is for real, and new heater cores/radiators quickly start leaking. Electrolysis eats away metal in the cooling system and the relatively thin walls of heater cores and radiators are the first to fail. There are two primary causes for electrolysis; acidic coolant and bad grounds. An inexpensive multimeter can be used to diagnose both.
Below are some general diagnosis tips/techniques. Visible dark spots on the exterior surface or the heater core/radiator can be another sign of electrolysis. Consult your vehicle's repair manual (found under "Literature" in the RockAuto.com catalog) and/or seek the advice of your spouse, mechanic, attorney, etc. for more specific instructions.
Typical Multimeter, Radiator & Heater Core

Typical Multimeter, Radiator & Heater Core
Acidic Coolant:
Old coolant can become so acidic that it begins conducting electricity and moving metal like the acid in a car battery. Take the radiator cap off the cold radiator (so no pressure builds). Start the engine and allow it to warm up enough for the thermostat to open and coolant to begin circulating through the cooling system.
Switch the multimeter to one of its lowest DC voltage settings. Stick one electrical probe in the coolant and put the other probe on the negative battery terminal. A reading of 0.4 volts or above indicates electrolysis is likely happening.
Loose, Missing or Corroded Electrical Grounds:
Now with the engine off and the battery disconnected, again stick one electrical probe in the coolant and put the other probe on a metal engine part (ground). If the voltage is still 0.4 volts or higher with the battery disconnected then old acidic coolant is indeed the culprit and should be flushed and replaced.
However, if the voltage reading is 0.00 with the battery disconnected, then there is a bad ground somewhere in the electrical system. Corrosive electrical current is being misdirected through the cooling system.
Solutions:
Acidic coolant should be flushed out of the cooling system and replaced with the correct new coolant (find Coolant/Antifreeze under "Cooling System" at RockAuto.com). It is often good to flush the system more than once to thoroughly remove as much old coolant as possible.
To track down bad grounds, try measuring the coolant's voltage while turning electrical accessories on and off one at a time. A change in the coolant's voltage reading may indicate the accessory (radio, fog lights, etc.) needs its grounding point cleaned, replaced or relocated.
I never know for sure if it helps, but when possible, I wrap pieces of wire around both the heater core inlet and outlet nipples and then attach both wires to the same piece of grounded metal in the engine compartment. My theory is that any stray electrical current will immediately go to ground rather than passing through the heater core.
Tom Taylor,
RockAuto.com
To read more of Tom's articles, click this link and choose from story titles on the Newsletter Archives page.​
 

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Excellent info!! Thanks for the post. That confirms the advice to use genuine Mopar coolant even if it is expensive.
 
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