My inlaws came by today cause I had what I thought was a coolant leak. They checked it out and found nothing wrong, but my mother in law put water in my coolant resevoir to see if it would run out [&:]...will that mess anything up? Do I need to flush my radiator now?

 

When was the last time the coolant was replaced in the vehicle? Also, what are we talking about here? Year, etc?

If the cooling system hasn't been serviced in a while, you'll probably want to do a full flush on the radiator, engine, and heater. Replace the thermostat and radiator cap. And clean the overflow bottle. Inspect the upper and lower radiator hoses for signs of deterioration and replace as necessary.