I know too little wont cool your car enough and generally it would overheat, but what would too much coolant cause?
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Wouldn't overheat, but parts could be damaged. The air in the system is like a little cushion once the system is pressurized. You can compress air, but you can't really compress a liquid.
Unless you filled it until it was coming out of the overflow tank I wouldn't worry about it. The cap is designed to vent excess pressure.
It will make your car overheat. Ask me how I know.
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If your cap is working as it should, If the pressure gets high enough it will just push out some coolant through the cap as it needs.
Now if your cap is bad, it can over pressurize the system and cause things like busted hoses and in the worst case, a busted heater core\ radiator.
It will not and has no way to cause over heating. If you over heat because of to much ( volume, not mix ) coolant, its because something else if causing it, like air in the system because you have no where for the small bleed to push out the air.
Now if you put to much coolant and not enough water (wrong mix) you can cause it to overheat, but again, it would mean that something else in the system is not working as it should. Coolant does not dissipate heat as well as water. In fact it makes a good insulator and will retain heat. That is why you mix it with water.