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Mysterious Coolant Leak

Last week I drove my 2007 Honda Odyssey with about 100k on a 4 hour trek in the cold, and all of the sudden the heat stopped working. The fans were blowing, but warm air was not coming out. It was Christmas eve and nothing I could really do about it, so let it be. Drove another 10 minutes that night with no heat to my destination, and many hours later still Christmas eve I drove another 15 minutes. Heat didn’t work, but car operated just fine.

Christmas day, drove the car for about 20 minutes with no heat, and all of the sudden the temperature gauge skyrockets, and the engine light goes on, the car is clearly overheating (could smell smoke from the engine). Let the car cool for the rest of the day, check it and there is no coolant at all. Had a local mechanic check some other things like the thermostat but found nothing, so refilled the coolant, and drove 4 hours again with no problems–heat works and everything. I was expecting to have to stop all the time to refill the coolant, but didn’t have to because it never leaked on the entire trip. I took the car to the Honda dealer and they check it and find no leaks and nothing else wrong with the car, and offer no explanation on how the coolant magically disappeared, only confirming that they cleaned coolant which had leaked/spilled all over. The only possible explanation was that I had some work done on the car a few months ago (new belts) and the mechanic I used didn’t flush the coolant properly. I’ve put plenty of miles on the car since though with no problems.

Any thoughts what this could be? Not that they know all or necessarily checked every single thing (though of course I assume they did), but the dealer had no real explanation so I have to assume it is something mysterious that all of the sudden the coolant emptied and has had no issues since. Am I safe driving the car around?

If the leak still cannot be found, your mechanic can drop a UV dye into the coolant. If a leak is external, the dye will highlight it. If it is internal, it will show up in the exhsust or the oil.

It could be as simple as a faulty radiator cap (plunger stuck open). When the radiator cap fails to maintain pressure, the coolant will boil at a lower temperature and eventually disappear.

Radiator caps are so inexpensive that you might as well put on a new one and see if that solves the problem.

When you lose your heat it is usually a sign that your coolant is low. Your temp gauge will stop working when the coolant is below the sensor. Do not keep operating the car when the heater blows cold. It will overheat and can damage the engine. When it cools down refill the radiator and find the leak.

Was the coolant that had spilled out near the radiator cap? If so, I second the opinion from jesmed.