You’ll most likely notice this on your car’s first startup of the day.
If you start up your car and let it idle for a bit, oftentimes you’ll see black or dark gray flecks of liquid spitting out your car’s tailpipe.
As the name implies, you probably think only hot exhaust is supposed to come from your car’s exhaust pipe.
You also probably heard somewhere to look out for unusual hues of smoke emitting from the rear. So, are black and gray droplets a cause for concern?
Here’s what OP saw below, notice the splatter of dark water a foot or so from his exhaust.
You’ll be happy to learn that your car spitting out gray and black droplets of liquid is completely normal.
First, the combustion of gasoline produces both carbon dioxide and water.
Ideally, this would look like C8H18 + 12.5 O2 → 8 CO2 + 9 H2O
Of course, air is not pure oxygen, so you get other gases like nitrogen as a byproduct.
In addition, combustion isn’t perfect (no gas car combustion is) so, on top of CO2, N2, and H2O, you also have unburnt fuel that presents itself as carbon deposits in your exhaust system.
Swipe your finger in the tailpipe of an older car or one that’s turbocharged and often you’ll find a layer of black soot on your finger.
That’s carbon.
When you first start your car, your exhaust system (and everything else) is cold, cold, cold, so, while that water would normally burn off before it got close to your exhaust pipe, on first startup, it makes its way out your tailpipe.
And, if there’s a bunch of unburnt carbon lurking in your exhaust system, it picks up some of that unburnt deposits along the way and spits out onto the ground.
If you want to see this closeup, and you want to confirm this for yourself, grab a paper towel or white piece of paper and hold it about an inch from your exhaust shortly after starting up.
If it’s just black or gray-ish droplets, you’ve confirmed what it is, completely normal car things.
Source: reddit.com/r/MechanicAdvice/comments/14vx27y/car_farts_this_stuff_out_everytime_i_start_it_up/
