Ok so there are a few (more like a lot) of us 01 civic owners that have the misterious overheat problem. I have troubleshooting this problem on my 2001 civic ex with 80,000 miles since January of 2008 which was when the problem started. I went from big and obvious all the way into small and complex.
I replaced the thermostat, water pump, coolant temperature sensor, radiator, radiator hoses, overflow bottle and then came to the conclusion that I was cursed and decided since none of these fixed the problem that I would just put the old stock parts back in and get my money back.
I have figured out that the overheating is caused by too much water in the coolant mixture and a deteriorating radiator cap. The radiator cap will work good enough to keep liquid in but as soon as the system pressurizes and the water boils (due to too much water in the mixture), the steam will leak out causing your coolant level to go low and overheating your engine and also cooking the radiator cap gasket.
REPLACE YOUR RADIATOR CAP AND mix your coolant 75% antifreeze/ 25% water
I replaced the thermostat, water pump, coolant temperature sensor, radiator, radiator hoses, overflow bottle and then came to the conclusion that I was cursed and decided since none of these fixed the problem that I would just put the old stock parts back in and get my money back.
I have figured out that the overheating is caused by too much water in the coolant mixture and a deteriorating radiator cap. The radiator cap will work good enough to keep liquid in but as soon as the system pressurizes and the water boils (due to too much water in the mixture), the steam will leak out causing your coolant level to go low and overheating your engine and also cooking the radiator cap gasket.
REPLACE YOUR RADIATOR CAP AND mix your coolant 75% antifreeze/ 25% water