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So I have a 1999 Honda Civic Lx and am having a horrible time fixing an overheating problem. I overheat when I'm idling, especially when I have my AC on. When I turn my heater on the car cools down almost instantly and when I'm driving the car doesn't get hot. I used to have a leak but after replacing a bunch of parts I dont appear to anymore. I pressure tested myself earlier and it looked good to me. So far I have replaced the radiator, the cap, all of my heater hoses, bypass hose, all hose clamps, water pump and gasket, timing belt, and the thermostat. I'm 99% its not my head gasket because my exhaust, oil, and coolant all look good and I tried a test with my radiator cap off , another test to see if fluid flew out of my cylinders, and I used an engine block test. My AC and radiators' fans relays and fuses look good and my radiator kicks on after a while (like 15 minutes or so). Any ideas? I have no idea what else to do. Thank you.

 
Rep Power: 518

What do you see that makes you believe it is overheating?

Is the radiator fan running during this event?

 
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The radiator fan turn s on, but I feel like it takes a while, like it kicks on after 15 minutes or so. Im not sure if this is normal.

The car doesnt die or smoke or anything, but in idle my temp gauge on the dash goes up slowly until I start driving again or until I turn my heater on and the AC off.

 
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I have a 05 Civic LX with the same problem. Overheating after long drive only when idle but goes back to normal after driving again. Checked the radiator, hoses, radiator fans, fuses, fan relay, and coolant level but everything is good. Took it to a shop and they couldn't find anything and they ran a block test (to check for blown head gasket) and that was normal too. Any ideas now?

 
Rep Power: 518

Ever consider that the gauge might be lying to you?
Get a computer scanner that can read live engine data.
See what the computer thinks the coolant temperature is.

Fan time could be normal, due to the location of the fan temp switch.

 
Rep Power: 518

7th gen, completely different.
Blown head gasket.

After a long drive, the radiator level will be low while the overflow bottle will have filled and possibly overflowed. That's a classic head gasket symptom.
There are always other possibilities, but.........

 
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Wouldn't the block test revealed combustion gases in the radiator if it was the head gasket? When they did the test if came back negative for a head gasket problem... I should note that the test was run while the car was NOT overheating; should it be run while it IS overheating?

 
Rep Power: 518

No, not while overheating.

Far too many head gasket failures are able to pass that test, it is too unreliable for me to use. I may use a couple of different methods --depending on the situation, they are extremely reliable for me.

Do a search in this site for keywords like overheat, no heat, and head gasket. I have described my methods many times.

Here's a method that at least proves there is a problem. Check the video https://www.civicforums.com/forums/1...ml#post4629441