I must say, I would have been surprised if too much water in the system could cause 'overheating'. Doesn't seem logical at all. However, when my system was overfull and getting very hot I remember looking at the expansion tank (thinking the reason might be that the water level might have gone down). The coolant was right up to the top of the bottle but none was getting past the pressure relief cap. The whole system looked to be under a lot of pressure so obviously I did not remove the cap until it had all cooled down. I'm still going to bleed the cooling system, I guess there could still be air in there.

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