Every spring the subject of flushing the cooling system comes up. It is almost as if some DIY flush as a rite of spring. My opinion is that you should flush for 4 reasons.

1. Coolant is over 5 years old.
2. You have GM orange extended life coolant in you r system. If it is over 3 years old even if it is the original get it out. This coolant is affected by air infiltration into the cooling system and will turn to a gasket destroying sludge very quickly.
3. You have anything more than a slight parasitic voltage across the coolant to ground.
4. it is dirty cloudy or in any way not pristine looking.

Use a glass container to examine the coolant closely, or place some on a clean paper towel this also works with transmission fluid. The only test that requires any explanation is the voltage test. Place the negative on the negative battery cable and the positive in the coolant do not touch anything other than coolant, you should read .02 volts or less, if it is higher start the engine if the voltage increases you have a bad ground on a component. Start by turning all components on and off if the voltage jumps you have found the culprit. No problem found ground the heater core using a simple flat ground wire. Flush and recheck. With the many different metals and plastics used in the manufacture of a modern engine this is becoming prevalent on a regular basis. Owners of vehicles that have a problem bleeding out the air after a complete drain, mostly expensive European with oddly mounted engines. They will cheat the system by draining only the radiator and performing this twice as often. This will work if you never let the coolant deteriorate. When replacing the coolant stick to manufactures recommended fluids. Why we need a dozen different antifreezes I don’t know there should be a standard.Use caution when workings on a hot engine never remove the cap until cool. Dispose of your old coolant properly.