I have a 2003 Honda Civic and the engine temperature will sporadically raise above normal. Using the heat (even a slight mixture, anything but full cold) seems to worsen the problem.
I’ve discovered that the coolant is almost always low in these situations, and so I’ve just been filling the coolant up in the morning before I start the car, but it usually takes about half a liter. There are no puddles or wet spots in the garage or inside the cabin, and I’ve had the radiator put under pressure for a weekend and it maintained operating pressure, so it seems that there is no leak.
I’ve had the water pump, timing belt, spark plugs, and O2 sensors replaced. It’s been like this for about a year, but it’s more frequent in the winter when I use the heat more. Where is my coolant going?