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Hey guys, I have a 2000 civic si with an aftermarket rad that has an issue with overheating that I can't seem to pinpoint. Any advice would be much appreciated.

I bought the car about a month ago with a bad water pump and thermostat (that I was aware of). I fixed both of those and flushed the coolant system, then I filled it up again with new coolant thinking it would be all good after that. But that's not the case. I started driving the car after fixing these parts and everything seemed fine until it started to overheat after about 15 minutes of driving on the highway. It doesn't overheat if I drive it around town for a while cuz the fan turns on and keeps the car cool.

So why would it overheat on the highway but stay cool when driven around town? Could it be my thermostat? Or a messed up fan?

Any thoughts?
 

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Discussion Starter #3
I think the rad is ok cuz coolant seems to flow through it fine. How do I check for air in the system? Or know for sure that I have the air out? Also what's the proper way to flush the coolant so it gets the heater core flushed too?
 

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89 EF 4wd sedan
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I think the rad is ok cuz coolant seems to flow through it fine. How do I check for air in the system? Or know for sure that I have the air out? Also what's the proper way to flush the coolant so it gets the heater core flushed too?
A bad rad is hard to diagnose, often they'll allow coolant to flow through a couple of passages fine, but the majority of passages will be blocked and with the reduction in surface area it will cause the car to slowly overheat. Had this in an old Cressida and it took me fkn ages to diagnose.
 

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Pick up a cheap Denso branded radiator form rockauto.

damn fine fitment and build quality as far as I can tell, I have one in my 97 civic, but of course its the smaller neck version for D series.
 
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