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I have a 99' honda civic cx hatch with 139,000 miles on it that started overheating...I assume that its overheating because coolant has been steaming out, the actual guage is broken so I dont know when its hot and when its not. I replaced the thermostat 4 months ago, and there's no obvious leak except when its gushing out the top. some other problems that started at the same time are intermittent heat and the smell of coolant when the heater is on. Im thinking heater core, but haven't noticed any water pooling up or dampness but its been raining for the last three weeks. anybody have any ideas or differential diagnosis for this?

 
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http://www.dummies.com/how-to/conten...ng-system.html

If you can figure out exactly where the leak is coming from then it will keep you from buying a part you don't need.