I have a 99 EX Civic. With an overheating problem. I've replaced so much on this (radiator, hoses, head gasket, thermostat, water pump) and it's still overheating, now more than ever. It'll almost get to the red zone in under 20 minutes. I recently flushed the radiator system because before we put on a new head gasket we tried Blue Devil's head gasket sealer and some of it was still floating around in the system. I read that it could be the heater core on a different post in this forum. Is there anything else it could be? Or if it is the heater core, would flushing it do the job or is bypassing it the best thing to do?

 

May sound silly and obvious,but are your fans coming on ? Mine was getting hot at idle like at red lights, drive thrus ,etc but when it was moving it was fine and it turned out to be the radiator fan switch which is a temp sensor near your thermostat. That head gasket sealer may have clogged your heater core but turning your hvac control to cold seals it off so no need to bypass it although it wouldn't hurt to back flush it.

 

My fans are coming on last I checked. Would bypassing my heater core keep my engine from staying warm in cold weather?

 

No your engine will still get hot but you won't have any heat inside. You said your radiator and water pump and thermostat are new? I've had bad luck with non Honda thermostats, it's possible even though it's new it's sticking. You can remove it and drop it in a pot of boiling water and see if it opens or you can remove it altogether to rule it out just make sure you use the gasket off of it. Maybe flush the system again and bleed it too.