Q: Put windshield wiper fluid in coolant reservoir by accident

If you did NOT operate the car after putting the washer fluid in the coolant overflow reservoir, simply use a turkey baster, or like tool, to suck the contaminated fluid out of the reservoir, pour some water in and then suck that water out as well until you empty the reservoir. Refill with 50-50 water-antifreeze mix that is specified for your vehicle in your Owner’s Manual. However, if you already operated the vehicle, request a coolant system flush and refill - contaminated coolant and that procedure will eliminate the washer fluid from the cooling system. There is probably no substantial effect, or indeed any effect, of this contamination particularly if you just simply remove the material at once. You can contrast your circumstance with, say, adding engine oil mistakenly to the brake system master cylinder. That would be a true catastrophe as it would require disassembling the brake system and replacing all the rubber seals to get the contamination out. So, you are OK, just have the material removed ASAP. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.