Black soot from tailpipe

[Kenneth Gear]

My car was in winter storage for several weeks until I decided to take it for a ride one nice winter day.  

I noticed when I started it that a black "soot" was emmitted from the tailpipe.  I noticed it a couple of weeks after the fact on the garage floor.

Any ideas of what may have caused this?

Ken G
1971 280 SL
Silver/red

[Mike Hughes]

Put on your way-back hat and think back to high school chemistry.  Combustion consists of combining a hydrocarbon (HC) with oxygen (O), yielding carbon (C), carbon dioxide (CO2) and water (H2O).  The water is usually in the form of steam, due to the heat of combustion.  When you first start up your car the injector pump and the CSV enrich the air-fuel ratio for easier starting.  So what you end up with is an excess of carbon mixed with the carbon dioxide and steam in the exhaust.  The cold exhaust pipes condense the steam to water and the carbon, in solution with the water, is spit out the tailpipe and coats your floor.

- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havanna Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)

[tdun824259]

Sooooo...I did enjoy the chemistry lesson just as I enjoy the Super Bowl commercials.  However, I've never noticed they have a huge impact on the game. You're saying that everyone should have soot on their garage floor, as it's part of the the cold start process, or is it possible Ken could be running on the "rich" side.  If so, have others experienced this soot issue and is there help?

Tim Dunagan
1971 280 SL

[Kenneth Gear]

Thanks Mike,

Having never mastered chemisty in High School, I knew I could count on those of you who are smarter than me, to give me an explanation.


Actually, I believe my car has been running rich lately so that makes sense that I would have more carbon build up than woudl be normally expected.

Ken G
1971 280 SL
Silver/red


Ken G
1971 280 SL
Silver/red

[Ricardo]

Having soot spots on my floor and the garage door, I can attest to the info that this is more or less normal....our cars run rich at cold idle especially and prolonged idling can completely foul out spark plugs, to the point where the car becomes difficult to start. Last winter my car didn't get moved to it's winter storage and so I was frequently moving it out of the garage while I worked on the rest of the fleet. Over the course of a couple of months of these short running sessions it became more and more difficult to start, until it simply refused. The plugs were a mess and yet with normal driving my plugs are a nice light tan color. I found the soot is particularily bad if you rev a slightly warmed engine, which is how I got it on the garage door  
Avoid prolonged idling....

[JPMOSE]

Ken,

When my new 250SL arrived in Atlanta three weeks ago, I started the car and let it run for a minute.  As I moved into the driveway it stalled.  I restarted it, rev'd it slightly in N, then put the car in the garage (then made myself a second Martini ).  I noticed later there was a trail of black spots on the driveway.  Funny...I didn't think I had any leaks?  Upon close inspection I saw it was carbon spots.  My car is a perfect example of a "carbonized" SL that has sat for years.  It is very typical to have this happen.  My car has only had 3k miles since 1978 and 1k since 1996.  You should have seen the spark plugs....covered in black velvet!  The exhaust system is original (as I have all the receipts).  I would expect that carbon build up in a 38 year old muffler is high.  How old is yours?  Have you only started the car and run it at idle for a long period of time?  I think your concern is not a problem...only a sign.

Best Regards,

J. P. Mose
1968 250SL

[George Davis]

Lesson learned the hard way: don't set your luggage on the ground behind the pipes of an idling Pagoda.  My car spits those dirty drops a couple of feet at least.  Maybe this is the real reason MB advises not to warm the car up by letting it idle?

George Davis
'69 280 SL Euro manual

[Mike Hughes]

Don't walk around the rear of your idling Pagoda to open the passenger door for your wife while wearing khakis, either!  ;^)

- Mike Hughes  -ô¿ô-
  1966 230SL Auto P/S
  Havanna Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)

[waltklatt]

Many years ago I asked my dad about the two black circles on the lower end of the garage door.  He said those were from the various MB's that they owned.  So it's not uncommon to see the blakc soot.  Try a diesel now and that will give a lot more dirty pants.
Walter Klatt
1967 220SL-diesel
1963 230SL-gas