I have a 2005 Honda civic ex with 70000 miles on it. It overheated in December, ran car up red high temp line. Honda dealer replaced hoses and radiator cap.
Car overheated again. Brought back to honda dealer where they replaced the radiator and thermostat.

After driving 400 miles the car overheated again. Brought to another Honda dealer who actually did testing. Passed compression test. Failed leak down test for cylinder no.1. Replaced head gasket (and all other gaskets), water pump, timing belt, all hoses. Also had head resurfaced and valves tested.

Drove car approximately 830 miles and it overheated again.

I am at a loss for what the problem could be. I thought I had ruled out head gasket by replacing it. I do have wet/mildew smell in my car, which leads me to possibly believe that there may be a very small leak in the heater core. This could be BS though.

I also read in one post that it could be an "rpm sensor". The guy in this post seemed to have a very similar problem.

 

Strange one, I'm basically at a loss for ideas with the exception of the temp sensor, did you forget to mention any testing/replacement (replaced /w water pump?), or has it not been looked at?

 

lol, i'm not only surprised that you brought you car in to the dealer but also let them replace whatever without even finding the source of the problem first.

if you think it could be the heater core then just look at it, get a manual so you know where to find it

radiator fan is coming on right?? but i doubt that's be a problem cuz it's winter lol

 

I would take it back to the dealership. They could have screwed up with the headgasket.

Also, recheck the moisture problem before you bring it in. It is highly unlikely you have a bad heater core but it could happen.