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Coolant disappearing, but no pressure loss

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?

Out the exhaust pipe? Have your mechanic test for a leaking head gasket.

There are only two places coolant can go.

One is an external leak, and if you have to add half a liter each day you should be able to see the leak, or at least drips on the ground.

The other is out the tailpipe, due to a bad head gasket.

You said you can’t see any evidence of an external leak.

It seems like you have the situation well under control, since it’s been like this for about a year.

That’s a lot of coolant loss.

As McP said, you should get a pressure leaksown test performrf to see if you have a bad headgasket. If it passes, I’d recommend temporarily (as a test) connecting the “in” heater hose directly with the “out” heater, eliminating the heater core and the flow valve from the loop. If it stops using coolant, you’ll know the leak is in the heater core or the valve.

Thanks for the information. I’ll have the head gasket looked at and try bypassing the heater core to try and narrow it down. I’ll report back if I get it figured out.

Is it possible that the leak is from the cap on the overflow tank when you’re on the road?

So far so good!

When I described my problem to a mechanic over the phone when scheduling an appointment, he asked if I had tried Stop Leak (AlumAseal). I didn’t even realize something like this existed, but for a $2.50 tube of the stuff at Autozone, my car has been running like a champ for the last 3 weeks! It turns out my head gasket was only 10,000 miles old, as it was replaced with my spark plugs.