My '05 Cooper S w/6sp tranny and 77K miles has a slight miss. When I bought the car this year, the dealer who happens to have an excellent mechanic, had the car gone over completely i.e. new water pump, plugs, wires, seals, fixed a slight oil leak on the side of the motor, new clutch and all that jazz, brakes and some other things.

But lately I am noticing when I back off the gas just a bit at highway speeds there is that slight miss/hiccup. It seems to do it with and without the a/c on. So I don't think the a/c cycling is causing it. I use premium high test gasoline--Shell. There is no smoke from the exhaust whatsoever. The tailpipe tips right at the point of exhaust exit have some slight carbon build up, but that seems to have been there a while. The car pulls strong through all the gears without a hitch.

What should I look at first?

 

Are you referencing an engine misfire or an exhaust backfire (or am I off base on what I think you're asking)?

 

Sorry. The engine seems to miss when I back off the gas and then get back into it. It is for sure coming from an engine mis-fire and nothing to do with the exhaust. It's like a plug is not firing for some reason. It is weird b/c when running up the gears, the car pulls like a champ. But cruising along and you have the occasionaly reason to lift off the gas, that's when it misses.
I'll describe it this way: it is like pulling the plug wire off the plug and putting it right back on lightning quick. And only when lifting off the gas. And generally it only does it once and only once when you lift. Under constant gas pedal pressure, it never misses. And when lifting, I mean just barely, not ripping your foot off the gas pedal, but just slightly like when you are at speed limit and need to back off just a hair.
Does this help?

 

Yep, that clears it up. If it is an engine misfire, the sensors should pick it up. If it's really bad, the engine will actually feel like it's trying to rip itself out of the car (actually felt like that when my Challenger threw that code due to two bad plugs on cylinder 8).

I don't see the correlation of lifting off being related to the plugs though. A vacuum leak is a small possibility, especially with the supercharger -- but you should see that throughout the spectrum and at idle. Might be a belt slipping as what you're describing resembles more of the turbo lag (which superchargers shouldn't have) and could be either the belt needs replaced/tightened or one of the pulleys.

 

The 2005 is tuned to have a bit of a pop/burble....
If you end up with a code for a misfire, get a compression test, dry and wet...many cars that have been run on low octane fuel end up with a burt valve...
and it gradually worsens till it is undrivable.
Simple thing to check...look at the coil..
Most mini's have a corrosion issue on the terminals...steel wool to polish, and some dielectic grease to keep it clean...see if it helps.

 

+1 on pop/burble.

 

Possible coil pack issue? Pull the plugs and see what they look like. Try referencing a chart for different plug conditions.
Are you sure the plugs are all gapped correctly?
The situation you describe is a time when you are putting high load on the motor likely at lower RPMs in 5th or 6th gear. This is the time when poor spark shows itself.

 

Yes, check your plugs for thier color and are they all the same.

But it sounds like it may be running slightly rich and what you're feeling is the ecu simply making a slight correction for it, it should eventually correct itself if that's what it is. It could simply be caused by the warmer weather effecting things, also check to see that your air filter is clean and the car isn't just compensating for having to work harder to breathe.

Once you've checked these things, if it still doesn't change things you might ask your dealer to simply clear your adaptations for you and let the car "re-learn" some things, it won't hurt anything to do it and may be all it needs. (Again if this is your issue, the car will probably take care of it all on it's own, getting the dealer to reset things would just speed the process along, so if he wants to charge you, you'll have to make the call on that.)

 

Thanks for all the great feedback. I think everyone on here just about hit the nail on the head. The plugs, coil/wire, plug wires, air filter etc etc were all changed when I bought the car in Feb 2012. This past weekend some club members took a nice drive in the WV mountains. I had put in a bottle of Lucas fuel injector cleaner and then filled the tank with high grade gasoline. The little hiccup seemed to all but disappear during the drive of not quite 100 miles. So I will see how it does from here on out.

One forum member mentioned driving in 5th & 6th gears on the four lane. He was right b/c this is when I saw this issue crop up more when I was just crusing at 70mph in top gear and showing lower rpm's. So it may have been a combination of things. I'll keep you posted on how it does on my next long drive with Twiggy.