I was at a stoplight, the outside temp was around 35° F but only one car had steam coming from their exhaust pipe. Every car is “hot” as the engine is running and I don’t understand why every car doesn’t.
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I can think of two causes:
1. that car hasn’t been running very long and it’s not done evaporating the water left in the exhaust from the last time it ran. A hot engine with a hot exhaust shouldn’t expel visible water vapor unless it’s super cold and condensing imediately
2. the car has an issue with the engine
Some cars have an auto-off feature, where the engine temporarily turns off at red lights when the brake is pushed and held. When the brake is released, the engine turns itself back on. This is a feature in more and more cars over the last several years, intended to help with emissions and fuel saving. Perhaps that’s what you saw?
Mine starts each morning with that stream. I start driving before it goes away completely, but by the time I’m at work it’s gone. Just because an engine is running didn’t mean completely hot yet
Dew condenses in the exhaust pipe when you leave your car standing overnight. As you start the car in the morning, the engine heat converts that to water vapor and out the tailpipe. That vapor immediately starts to condense as it touches the cold air outside. That’s why you don’t see it in hot weather.
After you’ve driven for a while, all the water is gone from the exhaust and there’s nothing left to evaporate. The cars you see at the light with vapor coming out might have driven just a block before you saw it and the other cars have been driving for longer.
Cars parked in garages don’t have the same dew accumulation as those parked outside. That’s another reason why some cars have steam and others don’t
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