Is adding anti-freeze without mixing water super bad? What about going over the cold fill line?
It does not matter to much going over the fill line. When you drive anti-freeze enters the car so if you do not wait long enough to fill it back up it may seem like there is a low level however it may still be in the car. Engines are designed for antifreeze to be mixed with water at the temperatures normal for your climate. The reason is that ethylene glycol doesn’t freeze, but it also doesn’t carry heat away from your engine as well as water does. In extremely cold climates the dilution factor is less–two or three to one, perhaps, instead of one to one–but if you don’t dilute it your engine is likely to overheat.
