Coolant poluted with tap water
 

Had to add approx 3 liters of tap water to my coolant tank due to a leak at the water pump while driving home

As I am about to change the water pump, do I need to flush and refill with new coolant/distilled water mix, or just add some more of the proper coolant?

Drain all you can and refill with fresh coolant and distilled water. tap water is conductive and has minerals. The coolant has inhibitors but since you are doing the job, don't half do it!

You have diluted your coolant and introduced minerals with the tap water; as you are going to change the pump anyway, I would go with fresh coolant mixed with distilled water.

I was pretty sure a drain and refill was the way to go
Thanks for the confirmation
Damn, that coolant is expensive! Was hoping to save the $150 for 3 gallons

Look at the coolant cost this way: Go with fresh and distilled water and you don't have to think about it for another 5 years or so. On that basis, it is not expensive.

If you want to save a little money, buy VW coolant.

Mercedes-Benz coolant is also a good choice, IMO. Retail comes as Zerex G-05, golden-colored.

It's $35 a gallon from Sunset Imports, versus $50 at most dealers. However, if you don't do a full flush you won't need as much. You will get about 3 gallons from the coolant drain plug (make sure to replace the crush washer), the other 2-3 gallons would require you to disconnect one of the heater core lines, and the lower radiator hoses (or you could jack the front of the car up higher than the rear as you drain it).

I have many friends who run regular tap water mixed with Prestone concentrate in their cars with absolutely no issues. Unless you live in a rural area where water is pumped from a well, I wouldn't worry about it too much. I believe Lotus manuals even say to use regular tap water.

Flush and refill with OEM coolant and distilled water. Simple and guaranteed no risk.

to have to spend $150 on coolant is borderline criminal
I ain't gonna use anything except for distilled water. At Wallyworld, a buck a gallon.
For $3.00, it simply isn't worth it to use tap water

The new WP, thermostat and coolant came in today and I'm borrowing a airlift to fill her up after I replace the parts this weekend

When I am flushing my cars, after draining the old fluid I fill the system with destilled water, run the engine until the thermostat opens, let it cool, drain (repeat..) until the water comes completely clean..

Thereafter, I fill it with a mix of the OEM coolant mixed with destilled water (never tap water).

My car has had non-Porsche (green) coolant in it for the past 50k miles. I recently did the WP and everything combo too, and chose to go with Prestone. Haven't had any problem so far.