So when we add coolant, we need to add 50/50 water and antifreeze into the reservoir, right? The reservoir has a lot of space above the MAX line...is it bad to overfill? Is that extra space just to prevent backflow as the engine cools and the fluids drain back or something? What happens if you add just straight coolant with no water?

 

you can overfill it some ....but the extra space is there to let the coolant expand into when the eng is hot....look at the level cold and then look at it hot....the res or expansion tank will be more full when hot....so if you over fill it too much you will push coolant out of the tank....always mix coolant and water stright coolant will freeze at a higher temp than the mix....look on the bottle of mix vs temp info...if you are just adding some then no big deal to add stright coolant...it will just change the mix in the system a little

 

Put it only a 50/50 mixture and keep it below the MAX line