Dear Tom and Ray:

A group of us get together at a diner for coffee most mornings. Recently I made a critical error and told the guys that I had put a piece of cardboard in front of my radiator for the winter. So, when it's 20 below zero, my Toyota warms up quickly. And instead of the temperature gauge sitting near the "cold" mark all day, it comes up about a quarter of an inch on the gauge. Well, the responses ranged from "Have you lost your mind?" to "You're going to ruin the engine," to "It won't do any good." My friend -- our resident pseudo-engineer -- explained that the thermostat in the cooling system handles all that stuff, and that any participation on my part, by adding cardboard, is completely unnecessary. He convinced the group that he was right. But is he? -- Joe

TOM: No. He has his head up his radiator hose, Joe. In extreme cold temperatures, like when it's 20 below zero, your cooling system may work too well.

RAY: Here's a basic description of how the system works. Most engines run most efficiently at about 200 degrees Fahrenheit. So the coolant just stays inside the engine -- and does not get sent through the radiator -- until that temperature is reached.

TOM: Then the thermostat opens, allowing some of the hot engine coolant to flow through the radiator, where it gets cooled off by the cold air blowing through it. By opening and closing like that, the thermostat keeps the engine in its most efficient temperature range, and allows you to have hot air blow on your tootsies.

RAY: But here's the problem. When the ambient temperature is very low, it takes longer for the engine to reach operating temperature. And then, when the thermostat finally opens and allows the coolant to flow into the radiator, the ice-cold coolant that HAD been sitting in the radiator then gets pushed into the engine, lowering the engine temperature far more than necessary. So, the engine spends a lot of time BELOW operating temperature, fighting to get warm enough.

TOM: So when it's bitterly cold out and you cover up the grill with cardboard, you're preventing the frigid outside air from blowing through it, and keeping the coolant inside the radiator from dropping to the temperature of the outside air. You're also allowing some of the radiant heat from inside the engine compartment to warm it as well.

RAY: That's why you see a lot of big diesel trucks with roll-down shades on their front grills. It's not because the engine is doing something private. They're blocking the airflow to the radiator, just like your piece of cardboard does. And when the temperature rises, they roll up the shade and let the thermostat do its job.

TOM: And by the way, if anybody's looking for cardboard for their grill, I still have the box that my most-recent ex-wife left me to live in.

RAY: On another note, we made a dumb mistake a couple of weeks ago. We were trying to help a woman whose mechanic had put coolant in her windshield-washer reservoir, and it was greasing up her windshield. We told her to drain and wash out the washer reservoir, and then add some windshield-washer concentrate and run it through the lines.

TOM: But we apparently wrote "coolant reservoir" instead of "windshield-washer reservoir," leading some readers to wonder whether WE were the knucklehead mechanics she went to in the first place! Could be. We apologize for any confusion -- in addition to the usual confusion we cause!