SoCal Mike

I had inadvertently added old Peak coolant containing phosphate. Should I have the coolant flushed immeidately?

S4_Cinci (I S'edTFA)

put in is a factor as well. If it was just a few ounces, I wouldn't worry. If it was like a pint or so, then I would say you should change the coolant sooner than you normally would - the G12 is supposedly "lifetime", but that really means 100k miles per car technical speak. The mixing is somewhat analogous to adding dino oil to the car when you put synth in for the oil change.

The tales of coolant immediately gelling are BS. I've mixed G12 with Prestone and others in a container (not in the vehicle) and the only change is in the color of the mix. If the overall color hasn't changed significantly, then I wouldn't worry too much about it.

SoCal Mike

about 8 to 10 oz

S4_Cinci (I S'edTFA)

than another year, personally - but I change my coolant one way or the other about every 3 years. FWIW, phosphate isn't as bad on water pump seals as silicates are, and that is the main reason that the older style coolants had a shorter life - silicates will precipitate out of solution and are abrasive to waterpump seals and bearings. I wouldn't sweat it too much.