What should I use in my cooling system after I replace my water pump

If your mix is presently a 50/50 mix of coolant and water (as it should be),

and you lose any amount of it,

All you have to do is top it off with a 50/50 mix from a jug (either bought premixed, or you mix it that way).

That is what is cool about mixing out side of the vehicle.

It does not matter if the vehicle holds a half a gallon or ten gallons, if it is mixed before added, you will always maintain a 50/50 mixture. Even if you top it off later, the mix will always be 50/50.

If you are a 'guesser'- have no idea what the capacity is when empty, and just pour in full strength antifreeze and top it off with water, you will have absolutely no idea what the percentage is in your system.

Draining it all, and mixing fresh outside of the vehicle will insure your mix is correct.

The Medic

If it is relatively new, do not replace it.

I generally go overboard when replacing mine, I replace them once a year just as a precaution, need to or not!

The Medic

No, it is not controlled by the PCM, any problem with the thermostat would show on the temperature gauge, stuck closed over heat or stuck open running cold on the gauge, so if it is new, leave it alone.