It’s -6 degrees this morning and I needed to drive about 40 minutes to our local store. I’m driving a 2015 Toyota 4Runner if that matters. As I’m driving I notice a lot of steam coming out of my tailpipe. It’s particularly noticeable when I am stopped at a light, but I can see it even when I am driving at 55 mph.
As I look around I see the same thing happening to other cars and trucks, but some don’t have any steam coming out of their tailpipes at all. I know that water is a byproduct of internal combustion, and since it’s coming out warmer than the ambient air it comes out as steam, just as our local lake has steam coming off of it because even though the water is cold it’s still warmer than the ambient air.
So why don’t all cars and trucks exhibit the same phenomena as mine, instead of just some of them?
What you see is not steam but water vapour - ie. molecules of water produced by evaporation at ambient temperatures rather than by boiling. Steam is not visible and when your engine and exhaust system are fully warmed up, it will be steam coming from the tailpipe and you won’t see it.
It depends partly on the temperature of the exhaust stream too, could just be that other cars are either fully warmed up or the design of the exhaust system is cooling off the exhaust stream enough to where you don’t see any visible water vapor.
Just for the sake of completeness though, if your car is burning engine coolant, that tends to make a white smoke that looks kinda like steam. Engine oil burning makes a black or dark gray smoke. So if you see either of those coming out of your tailpipe, that might indicate a head gasket problem or worn rings or similar problems. Coolant leaks (like a leaky head gasket) tend to blow a lot of white smoke when the car first starts up, as the engine burns off coolant that leaked while the engine was off. Once that is blown out of the engine, the white smoke may drop off to almost nothing if the coolant leak is relatively small.
With a coolant leak, you’ll get the white smoke even when it is warm out.
When it is cold, what you are likely seeing is water vapor, as was already posted. In normal winter conditions, the exhaust won’t be visible once the car warms up, which takes maybe 5 to 10 minutes for a typical car. If it’s colder out, it might take longer. Larger, hotter engines will heat up the exhaust pipes faster, where smaller engines that don’t produce as much heat might take longer for the exhaust pipes to get hot enough to flash all of the water inside into steam. On the other hand, larger engines may take longer to get up to temperature due to the thermal mass of their larger engine blocks. So there’s lots of variables here, including how much airflow there is around the exhaust pipes, muffler, etc. that may tend to keep them cooler for longer as well.
One important note is that the same thing happens even when it’s warm, just not as visibly. If you only drive your car for quick 5 minute trips to the store, you might find your exhaust system rusting out and dying an early death due to the water vapor condensing inside the exhaust. If you typically drive the car long enough for the engine and exhaust to fully warm up (10 to 15 minutes or so) then the hot exhaust pipes will prevent the water from condensing and will just blow it out the exhaust pipe as steam and you don’t need to worry about it.
The normal cloud you get is probably a “mixing cloud” similar to the cloud you get from your breath on a cold day. It happens when you take a warm air mass and mix with a cool mass. The temperature is somewhere midway between the two masses as is the humidity. The thing is that the saturation point vs temperature isn’t a straight line, it’s a curve and if the combination of temperature and humidity is above the curve, a cloud forms. I’m probably mangling this but it’s in a great book, Clouds In a Glass of Beer which everyone should read.
That said, if you are going 55 and you can still see the cloud coming out of your tailpipe, that’s a lot of steam. Maybe you should get that checked.
<nitpick> Technically, water vapor is invisible … what we see are droplets of liquid water … the exhaust gases are condensing in the cold exhaust system until they exit … where the condensed water droplets are then reabsorbed in the air once it disperses in the atmosphere … </nitpick>
It has to be pretty darn cold for any vehicle at highway speed to emit condensed water vapor once the engine is up to temperature, generally below 0ºF from what I’ve seen. Of course an engine that’s not up to temperature will spew a decent cloud, but if you follow them you’ll notice it quickly dissipates as everything warms up.
When idling you can see vapor even in temperatures in the 50s or even near 70º if it’s real humid out. Because of that you can usually tell if the car in front of you is a stick shift as there will be a little puff of vapor as the engine RPMs drop when they shift from first to second gear. I’ve also noticed that cars with dual exhaust tend to produce more vapor, I assume because the exhaust is moving slower through the two separate pipes that have more surface area in contact with the surrounding air, so it cools off the exhaust a bit more before it reaches the outlet.