The heater in my 1996 Mercury Sable didn't work right. The heater would blow hot air for about 30 seconds and then the air would turn cold. I checked the fuses. They were fine. I checked the external coolant reservoir and it appeared to be dry. So poured a gallon of antifreeze into the reservoir. (I knew I should add a 50-50 mixture of antifreeze and water, but I was traveling in the mountains and the store where I was still had insulated covers on the outdoor faucets.) The heater worked perfectly and the temperature gauge stayed near the center of the dial all the way home. At home I added about a quart of water and brought the level in the reservoir to the maximum cold-fill line.
I was amazed by what I read in the car's manual about coolant. Here's an excerpt. "Have your dealer check the engine cooling system for leaks if: you have to add engine coolant more than once a month, or you have to add more than a quart (liter) at a time". I have had the car since 2003 and I have never added coolant. Apparently losing coolant is acceptable in a Mercury as long as you don't lose more than a quart a month. Amazing.
In my defense for not checking the coolant, I was spoiled by my Toyota. I drove a Tercel for 19 years, from 1984 to 2003, and I never added coolant. I checked it periodically and it was always to the top of the radiator and my antifreeze tester always showed that I was protected to a reasonable temperature. Of course the coolant was flushed and replaced a few times at recommended intervals based on the maintenance schedule, but other than that I never had to worry about it. When I gave the car away in 2003 it still had the original radiator hose, and the hose was still in good shape. I drove the car for 19 years and it never broke down, never stranded me, and never even had a flat tire. I guess I'm about to learn that Mercurys aren't made to last like that.