A stumble on acceleration has been developing for a few months. A couple of weeks ago, it finally threw a "cylinder 2 misfire" code, and we limped into the dealer. The cylinder 2 plug was extremely fouled, and a new set of OEM plugs was installed, but didn't solve the problem. I added a bottle of Techron III to the fuel, which smoothed out normal running but didn't affect the stumble. Then back to the dealer, where they tried new MAP sensors along with other part-swapping trial-and-error (see below). They also gave it the Sea Foam treatment (intake and crankcase), letting it sit over a weekend and then driving it; it produced the expected impressive cloud of white smoke, and the oil was pitch black when changed afterwards. The car passed the running tests in the shop; but on the road, the problem was still not fixed, although it has maybe rolled back to where it was last month. It occurs intermittently under 2/3 to full throttle; under normal acceleration from rest; and when staying in gear and increasing the throttle. It will almost always occur after irregular throttle; e.g., backing off the pedal, then downshifting and rapidly accelerating (such as when carving through I-95 commuter traffic). Then during the same drive, it might run right up through the gears without a stumble.

The car is a 2004 MCS, late November 2003 build. 83,000 miles.
Here are the related mods:
- JCW CAI w/ITG foam filter
- 15% Helix supercharger pulley (installed @ Helix)
- GIAC flash (installed @ Helix)
- One-ball exhaust (modified stock parts, done at local Midas)
- Detroit Tuned Bypass Valve (installed @ Helix)
- Helix's "duct tape" mod, w/2005 factory plastic hood plugs
The car is always fed 91 (Sunoco) or 93 (Shell, Sunoco or Citgo) octane, which conforms to GIAC recommendations.
Weather here (Philadelphia PA area) has been mild and dry.

The possible causes I dug up, mostly on NAM, are:
- Fuel pump
- Water in tank
- Fuel filter
- Throttle body
- Detroit-tuned bypass valve sticking
- Vacuum leak (hole in hose, separated connection)
- Carbon build-up (intake and/or valves)
Dealer checked every item on this list, in many cases by swapping in new parts.

Attempted solutions to date:
- New plugs: problem returned within 24 hours
- Bottle of Techron III in gas tank, 1/2 can Sea Foam in intake and and 1/2 can in crankcase: slight effect
- MAP sensors: no effect
Solutions not attempted:
- Coil
- Ignition wires
- Who knows?

This is making me and the mechanic crazy. They always do a great job at MINI of the Main Line, and this is the first time they've been stumped for an answer. If I've left out anything pertinent, please let me know. Your input is greatly appreciated!

-- Rick

 

Search on "stumble". This is an issue as old as time itself...

 

The usual suspects have been ruled out. I'm hoping to find someone who's been through a similar process and found an obscure cause.

The stumble is characterized by a burp or two through the exhaust before responding normally to throttle input. When the code was thrown, there was a series of about five burps. (This occurred in highway traffic -- big fun!)

It also intermittently misfires when idling.

Starting around mid-summer, the problem has evolved from barely noticeable to the point where it interferes with normal driving. The car ran great after the pulley/GIAC mod in April, curing a powerband flat spot in the upper 2Ks.

 

That does sound more serious than the usual stumble. Sounds like you are not getting spark to some/all cylinders.

 

I would certainly check the coil pack - there is a known issue that the connectors can become corroded.

 

They tried a new coil pack, and that wasn't it. They're now searching for a wiring short.

 

I had something similar with my 05 MCS but never got a code. When I took a closer look at the ignition wires I could see a shiny spot. That shiny spot was a place where the wires were arcing and shorting out. Replaced the wires (the coil pack too as an electrode was fried) and the problem went away.

 

I also had a stumble with my '05 MCS. I threw codes as well. I went with a DT BPV which solved one of the problems--the spring on the OEM throttle was shot--at 45,000 miles or so. I replaced the ignition wires after noticing that they were corroded. That cured the random misfires for awhile--for about 7,000 miles anyways. Then it all began again. Hard acceleration would throw me into limp mode. I ran 104 octane racing fuel which seemed to nearly eliminate the problem with only a slight bit of hesitation on hard acceleration. It seemed that when the engine was under load--AC on max, and I was accelerating hard, I would go into limp mode. After much research and no good answers, I replaced my plugs to the Iridium cold plugs, and like magic the problem was solved.

Even though the plugs were replaced 15,000 miles ago-I had assumed they were fine, and because the history of my MINI--I got it because the number one cylinder blew it's spark plug--I wasn't eager to replace the plugs!

 

There are some lazy mechanics out there. When ever there is a hard misfire fault to a specific cylinder I always do a compression and leakdown test. Why people think it easier to guess than quantify I will never know. Just a hint, if it were the coil pack, it would affect two cylinders.

 

The cylinder 2 ignition wire was split. Not sure where; the mechanic finally found it, and I didn't ask to see it.

Everyone's input is very much appreciated.

Rick