2000 Hyundai Elantra.
Question: While my wife was driving, she heard a loud bang under the hood then water through the top of the hood.  I looked at it hours later.  The overflow reservoir was completely full. I checked all the hoses and couldn't find where the water was coming from.

I started the car and no leaks.  It didn't seem like the coolant was flowing but the car wasn't getting too hot after about 10 minutes.  One fan came on.  I touched the radiator cap and it was still very cool.

I turned off the car and took the radiator cap off.  No pressure.  I added coolant then started to see steam coming up.  I poured more coolant and it just flowed out of the radiator as fast as I can pour it.  The next day without starting the car, poured coolant in and the same results.  I noticed that the side of the radiator was extremely wet.  It looks like the water is leaking from the top of the radiator. Does this mean I have a bad radiator needs replacing or fixing.  Or is it something else?  Thank you.
It sounds like the noise she heard was actually the radiator splitting or cracking. This can be cause by the engine overheating and building too much pressure. That will cause a radiator to split. Overheating and having too much pressure can be caused by something as minor as a bad thermostat or as major as a blown headgakset. When they fail, combustion from the engine is pumped into the cooling system which causes a lot pressure. It also will make the engine overheat. But it could also just be that the radiator failed.

Either way, it will need to be replaced first before looking at anything else. After that, it will need to be determined if something caused it to break, or it just broke. The fact that the cap or top does not get hot is just because the coolant level is too low to be at the top and is not circulating.
Hyundai Overheating But No Pressure

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