Have you ever noticed a lot of smoke coming out the tailpipes if you're really accelerating hard at high (5500+) RPM? I seem to notice quite a cloud of smoke being left behind by the XKR, especially during the 1-2 or 2-3 shift (I don't think I've ever done a WOT 3-4 shift) where there's a bit of RPM overrun and a little bit of a backfire.

Is this normal? It looks and sounds sweet when it does it, but I wonder if it's a sign of abnormal engine wear or not.

 

By the way, from what I can tell, it's black/sooty smoke, which to me would seem to indicate excess fuel. In conjunction with the backfire, I think this makes sense.

 

Mine does the exact same thing. It's a little embarrassing. Maybe too much fuel (I hope to lean mine out with an ECU tune)? Maybe also carbon deposits from driving the car too light for too long. Any ideas?

 

I watched some Motorweek video reviews of the 2012 XKR-S and it does not exhibit this behavior on quarter-mile runs! That's a brand new XKR-S, though!

 

As you surmised, your probably running a tad rich. This could be by design, as "too rich" is far safer on an engine than "too lean".

Don't know the XK well enough to speak of first hand knowledge, but the last car I owned that did this had a bad O2 sensor. The cars ECU "fail-safe" was to run rich to prevent engine damage.

Vince

 

Wouldn't a bad O2 sensor trigger a warning light of some sort (check engine)?

 

Overall, I'm not convinced I trust a computer to troubleshoot itself! That's why dealerships use a different computer to troubleshoot the onboard systems!

I suppose a sensor could be riding the edge of the tolerances allowed? MAS (Mass Airflow Sensor) could be dirty as well.
When's the last time you ran a proper fuel cleaner through the system or changed the air filters?

Just thinkin "running rich" thoughts out loud...

Vince

 

BTW.
Terminology we all grew up with can be misleading. We use the word backfire for any bang outside the combustion chamber.

"Backfire" actually refers to a combustion process that occurs in the intake runners before the combustion chamber. It is most often related to valve timing issues, or an overly lean mixture.

"Afterfire" refers to a combustion process that occurs in the exhaust system, after the combustion chamber. Often related to rich mixture.

Just an FYI

 

Good to know, I had never heard the term!

Well anyhow, interestingly I didn't notice this happening today.

 

Turbo- and supercharged engines are usually programmed to inject a rich mixtureat WOT, to prevent detonation. As was previous said, too rich is safer.

Backfire after WOT is caused by the management system retarding the ignition. Consequence is that the ignition of the mixture is still going on when the exhaust valves open. This will also ignite any unburned mixture that was thrown into the exhaust after WOT. That's why you see flames in the exhaust after closing the throttle.

This is not something related to defects or a specific car (brand). It's by design.

 

Whoa! This thing spits fire out the rear!? I'd love to see that!

 

How about running some Techron in your next tank of gas? How's the air filter look?

 

Re-reading your post today was soothing after I was a little surprised by the amount of smoke coming out of the back on a hard acceleration run.

 

hmmm, doesn't happen to me.
I'd suggest getting this checked... fouling a catalytic converter can be bad!

 

It really only happens when I rev it hard if I haven't revved it to redline for a couple weeks or so. It must just be worse because of carbon buildup that occurs when I'm driving it very gingerly. Also my commute is only 2 miles so that's probably causing more carbon buildup as well.

After I "clear it out" there is no more smoke.

 

Mine does the same. Usually if i have been doing town and sedate driving for a while. Once i have floored it once or twice its cleared its lungs out. Maybe a cleaner put through every now and then can't hurt. Most fuels nowadays have additives in them but i still run a cleaner through every 3 or 4 mths.

 

My father taught me: 'redline = shift indicator'. ;-)
Of course, he grew up in Torino...

 

Carbon deposits tend to make a brownish/grayish hazy emission from the exhaust right at the shift. Seems to be a common affliction after slogging around town for too long or driving like grandma.

As was said before, a few hard pulls and they usually straiten up.