Rep Power: 190

I just bought a 2001 Civic Si (US model EX) a few days ago, and have begun doing a tune up. I picked up the car for $2300 Canadian, which would indicate fun things to come for my ownership period. The mileage on it is 201,500 KM (125k miles)

First off, I have noticed that my car overheats if I turn on the heater. I drove it 150 miles non-stop without issues using the AC, fan and all accessories. With the heat on, the vents give out cold air at idle and driving with low RPM. However, I have noticed that If I drive or hold the RPMs above 2800, the heat works, and the temperature gauge goes down from above halfway.

Before today, the temp gauge would only stay just under 1/2 if I didnt use the heat. The coolant is topped off to the point where I see the fluid at the top of the reservoir.

Today, for the first time, I was driving to school and the temp would start to get high in stop-and-go traffic WITHOUT the heater. If I was cruising, the temp would idle arund half because of all the air hitting the radiator. My fans WORK, I don't know if they kick in at the right time, but I have idled on my driveway and they started right up.

I don't have any smoke. I may have some water (condensation?) coming out from the exhaust when I start the car.

Tomrrow I am visiting my mechanic, what can I expect? Water pump, headgasket, thermostat? Just to point out, the car looks like it has a new valve cover gasket.

 
Rep Power: 234

Sounds like the headgasket

 
Rep Power: 190

UPDATE: Good news (for now)

I followed another poster's instructions to eliminate air trapped in the cooling system and I no longer overheat with the heater on, or for any reason.

1) Take the rad cap off with the engine cold.
2) Start the car and warm it up to operating temperature (reving higher then idle will speed this step up).
3) Add coolant to the point where you see it in at the top of the radiator opening.
4) Turn HVAC to MAX heat & fan. There should be bubbles escaping from the radiater opening now.
5) Top up radiator while this is happening so you can see if continues to bubble.
6) Once bubbling stops, turn the AC on; more bubbles will emerge.
7) Cycle between MAX heat & MAX AC a couple times to make sure all air in the cooling system has escaped.
8) Once bubbling stops, turn car off, replace cap. Done.

*Note: If your radiator continues to bubble for more then 30 minutes it is probably because of a head gasket leak, investigate further.

 
Rep Power: 190

Well, it turns out it was the headgasket. The technique I used above can help you temporarily while you are waiting for the next paycheck.

Another useful tip for anyone having this problem, have your AC on at all times. Even if you don't turn the fans up, keep the AC engaged and the button lit. This will run the radiator fans regardless of temperature. I never got my temp gauge above 1/2 with the AC on.

Good luck!