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Just what is that coming out of your car’s tailpipe?

Published February 11, 2013
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Eric Lai answers readers’ auto questions every week for Wheels.

Q: I’m 8 years old. Why are cars and buses so smoky in wintertime? It smells so bad it makes me sick breathing it.

A: Peter Gerson of GB Auto Service in Thornhill replies:

Actually, what you’re seeing coming from vehicle tailpipes is mostly water vapour (steam). This would normally be invisible in warmer temperatures but can be seen in the cold. You can tell that it’s water vapour because it will disappear fairly quickly, whereas smoke would remain white, blue or black as it drifts away.

Water vapour is a by-product of combustion. That is, it’s produced when the fuel (gasoline, propane, natural gas) is burned, so that’s why it comes out the tailpipe.

More from Eric Lai:

Here’s why I’ll be avoiding drive-thru lanes from now on

Is it safe to drive on only three winter tires?

In cold weather, vehicles are programmed to run rich (more gasoline) on start-up to prevent stalling. Immediately after a cold start, the engine is running under “basic operating conditions.” That means it is burning more gas as all the sensors and fancy electronic controls have not yet come online to allow the onboard computer to run the engine at maximum efficiency given the conditions.

The exhaust may smell strongly during the warm-up period, which will vary with temperature, because the engine is running rich and there may be unburned gasoline vapours in the exhaust.

Once the onboard computer becomes active, it should adjust the fuel mixture to prevent this. In any case, after the catalytic converter heats up sufficiently, it will eliminate any unburned gasoline fumes from the exhaust.

Eric Lai adds:

With diesel vehicles, like school buses, the black smoke coming from the tailpipe is indeed smoke and I agree it smells terrible. It seems to linger more in the cold, damp air.

Q: How can I check my speedometer’s accuracy?

A: The easiest way is to find a radar-equipped sign board, if used in your municipality. They’re typically located in school/residential zones, or check local online forums. These units display your speed as you approach the sign, which you can then compare to your car’s speedometer reading.

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