I have a 2005 Civic LX that for the past year and change has been intermittently overheating. 170k miles - it's my wife's daily driver so no modifications have been made from stock. This summer it's overheating considerably more frequently. I've never let it get so hot that the needle "pegs" all the way to H, but in city traffic the temp needle will steadily climb towards H... if it gets north of 80%, I'll pull over and let it cool. On the highway the temp gauge stays normal (about 40%) and idling in the driveway it stays normal. It does seem more likely to heat up with the A/C running.

When the temp starts to climb, I can feel the A/C compressor kicking on and off, the car accelerates more slowly than normal, and I can often smell burning coolant through the vents. If I pull over and pop the hood, often there is evidence of coolant spraying from the cap of the overflow tank, and sometimes I can hear coolant boiling in the overflow. Turning the A/C off usually brings the temp down.... if I crank the heat on I get plenty of hot air out the vents, but no noticeable cooling on the temp gauge. Even on days when the car doesn't get too hot, I occasionally catch a whiff of burning coolant when I exit the car.

So far, the coolant has been flushed and replaced... mechanic said the coolant looked pretty clean so he doesn't suspect the radiator. Thermostat has been replaced twice, radiator cap replaced. One of the cooling fans was starting to go, so it was replaced as well. Belts and hoses replaced, verified both radiator hoses get hot when the engine is warm. Burped the system myself after all that work was done just to make sure the bubbles were gone. Verified that both cooling fans come on with the A/C. Radiator cooling fan comes on when needle starts to move above normal. Mechanic ran both pressure and hydrocarbon tests, both indicated the head gasket was fine.

What are we missing here??

Blown head gasket.

If your overflow tank is filling itself up and your radiator has air in it when it cools then you have a blown head gasket.

If everything is completely normal fluid level wise, and one 15minute drive sends the car past midway temperature mark then I would be looking at replacing the radiator (lpossibly clogged up ) and verifying the thermostat works.

This generation of Civic blows more head (gaskets) than Stormy Daniels.

Well, it seems I've found at least part of the problem. The "genius" mechanic who replaced the bad (passenger side) cooling fan installed the wrong fan - he put in a pusher, so it's blowing hot air from the block towards the radiator rather than pulling it in from the front of the car. I'm assuming these things can be reversed simply enough by snipping the wires, switching them and re-connecting with wire nuts?

Damage may already be done though. When accelerating I can hear a hiss-sounding 'tick-tick-tick', and if I sniff around the block I can smell exhaust coming off the front corner next to the power steering pump. Evidence of further head gasket deterioration?

Out of curiosity, would a bad cooling fan be enough to cause coolant to boil over into (and out of) the overflow, or is it more likely that the cooling fan triggered the car to start overheating which in turn cooked the head gasket?

Wired backwards. Switch the wiring.

That's a freekin amateur hack job. Those cone shaped wire nuts have no business being in an automotive environment.
No. Manifold is either cracked, or loose for some reason like broken studs or just a poor install.
Any of the above.

SO WHAT HAPPENED ? ? Another thread .... left wondering it the heating problem is solved.

i have that. replaced, fluid, thermostat,,,it was leaking out the block on my 2006. put in fiber lock. stopped the leak but then ..now have intermittenover heating.... I hope ther is still a little air in there.

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Your mechanic is defective and must be replaced! :-)
These symptoms are CLEARLY caused by a warped cylinder head and/or blown head gasket.
Use ONLY OEM parts. Follow the OEM (Honda) manual TO THE LETTER. Cyl HD surface flatness is CRITICAL. CYL HD warpage from overheating is VERY COMMON on these engines.

People be stealing my lines here!

Between the blown head gasket and the bad manifold, I opted to quit pouring time and money into it and replaced the Civic with a Pilot. Fired the mechanic too. I"d have preferred to keep and fix it, but was overruled by my wife - it was her daily driver and she puts 2,000 miles a month on it.

The block cracked on 2006-2008 1.8L engine?
It's ruined.
Needs an engine.
Period.
End of story.

2005 and 2006 have very different engines