[I posted this question today but it just disappeared for some reason}
2011 Mini Cooper S, auto transmission. I have only owned for 3 weeks.

Lately, I have been putting my car into the Sport mode. Whenever in sport mode, I am hearing Afterfire in the rear muffler as the car downshifts with my foot off the throttle. It is VERY prevalent. I would say anytime coming up to a red light, in sport mode, as it downshifts I will hear backfire or afterfire pops coming out of the muffler.

[b]Why is this? I have been told this is very bad for the engine.

I have new plugs, new coils, new fuel pump, new fuel filter so it is none of those things. I do currently have fuel treatment in (seafoam) and fuel injection cleaner and 93 octane gas in it (highest you can get in my area). I don't know if that would cause the problems or if this is indicative of something more severe. If I take the car out of sport mode, it is fine. No backfire or afterfire.

It sounds like this:

 

It's supposed to do that! Nothing to worry about

 

Really? Everything I read said "VERY BAD"
https://axleaddict.com/auto-repair/E...res-Diagnostic

"The backfire is produced when unburned fuel ignites inside the intake or exhaust manifold instead of a cylinder. You can hear the combustion as a mild, cough-like ignition or a loud bang. A strong explosion, though, can cause severe damage, like cracking an exhaust manifold."
...A strong explosion, though, can cause severe damage, like cracking an exhaust manifold."

 

I don’t use sport mode much,
but my 2009 JCW R55 pops
nicely on throttle lift with sport
mode on or off. I love it.

 

called 'pop and burble'

the car in S version and in sport is tuned to do this on purpose

it has come and gone over the years

google it and you'll find discussions and videos . . .

https://www.mini2.com/threads/exhaus...burble.148858/

 

Ok, as long as I'm not going to blow out the exhaust manifold. Based on the soot coming out the exhaust pipes and sound, it has to be unspent fuel exploding in the pipe or the exhaust manifold. It can't be good for the O2 sensors either way.

 

My 79 does it a bit ..... it is a classic 'sports car' sound ......

You're right ... a backfire is NOT good ... this is a bit different. My 79 had a backfire and run on / dieseling at one time ..... very different than a bit of burble.

 

It is harmless. My JCW does this in normal mode and really does this in sport mode. In sport mode the car sounds like a rally car.

The engine controller just injects a bit of fuel during the exhaust stroke which lights off at some point in the exhaust -- which is capable of "containing" this without any damage -- and generates the backfire. The amount of fuel injected and when is carefully controlled. It is when there is something wrong with the fuel system and excess fuel is supplied to the engine in high levels and at the wrong time that is bad. In this case the backfires would probably be much louder and have other characteristics along with the engine manifesting untoward behavior tha indicates there is a problem.

The soot is the normal byproduct of combustion. As I like to say: Soot happens.

Some of the gasoline doesn't burn completely -- at least at and shortly after cold start -- and soot is the result. Since the engine has a converter and exhaust gases are "burned" in the converter soot at the exhaust tip is probably from the converter anyhow not the engine.

 

My car has an extra ordinary amount of "Soot" happening.

[It is new used so this weekend, I will finally get around to cleaning it and chrome polishing those black pipes. I know there is chrome under there because I had a gen 1 originally.]