Coolant question, reservoir empty...

Re: 1996 Honda Civic, 238K Km
Two days ago I've noticed smoke coming out of the hood near the coolant reservoir. At first I thought it is because of the cold
winter (its below freezing temperature in Ontario). Then I smell something different and pulled over.
I found the coolant reservoir near empty and so is the radiator (as far as I was able to see). The spongy area near the radiator top is full of coolant and smoke is coming out from that area. I did not notice any change in temperature gauge while driving, though.
The area of the hose connecting the radiator seems to be a suspect in my view although there is no apparent sign of leakage. There is no residue under the car whatsoever.
Note: Yesterday and today I drove about 100km and noticed that it cost me about 18-20 liter of gasoline which is very very unusual on my car as the fuel efficiency is between 6 to 10 L/100km (Summer and Winter, respectively).
I stopped driving it for now until I take it to a mechanic. I hope I haven't damaged the engine by this time. Any thoughts on this? Thanks.
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All the evidence is, as you suspect, that something failed in the cooling system and you have a bad leak. Repairing this could be cheap (like a new hose) or expensive (like a head gasket).
But since it's Ontario and winter, and these Civics have a tiny, roughly 1.6L engine, you may have lucked out as far as damaging the car further, particularly since it was still driving without (as far as you say) other indications, like odd noise from the engine. The temperature gage may indeed be reading accurately.
The whacko fuel mileage is likely due to engine control system sensors not reading correctly, since many require proper cooling from the rad system to work right.

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Thanks Elle
Yes, there has been no other noise from the engine or anywhere. Vehicle has been maintained (oil changed) at Honda dealership for the last 2 years. In July this year, the mechanic suggested a coolant change in Winter (now). All other areas including engine head area (top area), transmission, etc are without any sign of leakage. And the engine starts in 1-2 seconds in the morning (as it used to).

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Wait, are you saying your mechanic did in fact change the coolant recently? Wager there's a connection then. Hopefully just an honest mistake. Preferably to no great detriment to anyone. :-)
A head gasket leak BTW would not necessarily manifest itself with obvious leakage, other than reduced levels. The coolant can get blown out the exhaust pipe in such situations.

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No, the mechanic did said to change the coolant in this Winter. It has not been changed. But now that the coolant is empty, it needs to be changed/filled.
Ya, I booked an appointment for tomorrow to do some testing (pressure test, head test and/or another test I forgot).

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