Final update, I think. Using an OBD scan tool, I saw that the fuel trims were still very low to adjust for the vacuum leak. I reset the ECU and the trims went back to around 0, and the engine is running much more smoothly. I think we can call this one fixed!


Okay, cracked air intake replaced. The problem has developed into a very rough idle when the engine is cold and/or wet. If I let it run (misfiring and shaking badly), it will run normally after a minute or so. If I press on the throttle while this is happening, it will run fine above 2k RPM. Sometimes pressing the throttle makes it worse and I get a fuel smell. The ECU throws a random misfire code.

Bad coils? Incorrectly installed plugs? Bad wires running to the coils? It definitely seems electrical at this point.


Figured out how to check it without starting the engine. The coolant temperature matched the ambient temperature, so I think the sensor is good.

While I was looking around the intake area, I noticed that there is a crack in the tube between the MAF sensor and the turbo. This could definitely be letting extra air in and messing with the computer's fuel mix. Currently investigating what kind of tape could be used to patch the leak until I get a new intake tube.


Started the engine and read the coolant temperature. It read 39C/102F, which sounds too hot to me after a minute of idling. Ambient temperature was 12C/54F. On my drive to work, temperature climbed smoothly from there to operating temp of 105C/221F.

I'm going to check the MAF sensor next. I have a can of cleaner.


Plugs are replaced. They were not the Bosch's used by BMW, so they had been replaced at some point in the past. Assuming the previous owner(s) folllowed the maintenance schedule, that would have been at 30k miles. I replaced them with the OEM plugs and took it for a test drive. Much better response and much better power.

My cold engine problem remains, however it's not nearly as pronounced. I am getting more convinced that it's a misfire. The coils were very rusty on the outside. My thought now is that they aren't making full contact and thus not always firing. Once the engine warms up, everything expands and they get better contact. Does that sound feasible? I tried cleaning off the rust with some steel wool but I couldn't get it all off and I didn't want to remove too much material. Time for new coils?

Original Post:

I have a 2008 MINI Cooper S w/ JCW package (dealer-installed at time of purchase; includes chip tune, cat-back exhaust, and bigger air-box) with 71k miles.

The car gets parked outside overnight. When starting up in the morning, the engine starts up normally, and can be revved normally, but when I pull out of the driveway and start up the road, it loses a lot of power and I have to feather the clutch and give it extra gas to keep it from stalling. The road is slightly uphill.

As the engine warms up, the problem gets significantly less, until disappearing completely once it reaches operating temperature (i.e. warm air comes out of the heater).

Could this be a sign of carbon buildup on the intake valves? This engine is known for this, but the walnut shell cleanup is $600 at the MINI dealership, so I'd rather not do it if it isn't needed.

I recently replaced the air filter (it was filthy).

I don't know when the spark plugs were last changed, if ever. I bought the car used around 60k miles. MINI recommends changing every 30k for JCW, every 60k for S, and every 100k for base Coopers with this engine. Could it be plugs?

Fluid levels are fine and no smoke is coming out of the exhaust. No CEL. It really feels like a limiter kicks in.

EDIT: I should also mention that it feels like overall power is lower (at any temperature) than when I bought it, but this could be my imagination. It's almost like it takes longer to reach 3000rpm.

EDIT 2: Now that I'm paying more attention to it, it does seem like it's misfiring a bit at idle. I also had some hesitation yesterday under load, followed by a puff of gray smoke from the exhaust.

EDIT 3: Okay, socket arrived from Amazon. Here are my plugs. Looks like normal wear and time to replace! Plug 1 Plug 2 Plug 3 Plug 4

  • Is the chip tune a factory tune or some aftermarket job? Absolutely sounds like plugs/coils/wires except for the "does better once it warms up" part
    – Zshoulders
    Apr 10, 2017 at 15:49
  • It's a dealer-installed package. 2008 was the last year before "JCW" became its own trim level.
    – Spivonious
    Apr 10, 2017 at 20:11
  • 1
    Have you pulled the plugs yet? They would also be able to give you an indication of carbon levels to a degree. If they were changed at the 30k mark then they'd have somewhere around half the carbon buildup that could be expected on the valves. Also, did you change them since you bought it or were you told they had been changed around 60k? It could be that they are malfunctioning all the time and that you do have overall power loss, but that it's is just most noticeable when the engine is cold. Apr 10, 2017 at 20:39
  • Have not pulled them yet...planning for this weekend. I may need a special 12-point 14mm socket for the job, because BMW. I have not changed them since I bought it, and the used car dealer did not have any service records.
    – Spivonious
    Apr 10, 2017 at 20:44
  • 1
    Have you tried a Seafoam treatment? May take care of a carbon buildup issue if that's what it is. A heck of a lot cheaper than getting a "walnut shell cleanup" as suggested by the dealership. Just a thought. Apr 10, 2017 at 23:14

3 Answers 3


Start with plugs and wires. Especially if they've never been replaced, they are almost certainly causing your current problems. You can test the wires with a multimeter and visual inspection of the spark plugs can tell you a lot(you can give that a google, or we have a question here about what plugs are supposed to look like).

  • Yep, planning on checking the plugs this weekend. The MINI has the coils right on the plugs, so no wires to worry about (unless you mean the wires running to the coils...not sure how I can test those though).
    – Spivonious
    Apr 12, 2017 at 19:02
  • 1
    Oh sweet. Yea, don't need to worry about the wires then. I mean I suppose technically the wire that runs to the coils could bad, but you have no reason to suspect it at this time
    – Zshoulders
    Apr 12, 2017 at 19:11
  • the sides of the coils/tubes were varying levels of rusty. Will that affect the spark? I imagine rain water is getting down into the spark plug holes as there's no cover over them.
    – Spivonious
    Apr 18, 2017 at 15:58

It sounds to me as though the temperature sensor is giving out a valid but incorrect reading so that the ECU of the car is incorrectly calculating throttle position against fuel requirements.

I'd personally start by investigating the voltage when cold of the temperature sender unit.

  • There is a hidden menu on the MINI that will let me see what coolant temp it's reporting. I imagine that should be ambient temp on a cold engine. I will check and report back!
    – Spivonious
    Apr 21, 2017 at 14:52
  • Readings coming from the temp sensor are what I'd expect (reading 100F after sitting for 3 hours after a hard run, engine warm to the touch).
    – Spivonious
    Apr 24, 2017 at 16:19
  • What reading does it give when the car is stone cold (i.e. After being left overnight) Apr 24, 2017 at 17:01
  • I can only get into the debug menu when the engine is running. It read 39C after a minute of idling. Ambient temp was 12C. Sounds a bit high to me? I kept the coolant temp display on during my drive to work and it climbed until reaching normal operating temp (which is 105C for my car).
    – Spivonious
    Apr 25, 2017 at 14:14
  • 1
    Thanks for this. It led me to think more about the fuel mixture which led me to inspect the air intake.
    – Spivonious
    May 19, 2017 at 12:53

The turbo air intake tube was cracked almost completely around, letting in lots of extra air after the MAF sensor. In short, this led to the engine running too lean for conditions.

I replaced the stock tube with a cheaper after-market tube that won't crack in the future (silicone versus plastic). So far so good, but we've had very warm mornings (upper 70s) since the replacement. I'm waiting for another cold morning to confirm that the problem has gone away.

Update: The problem returned with the cold weather last Fall. I bit the bullet and had the dealership do their $600 walnut shell blast to clean carbon off the intake valves. WOW what a difference! Cold starts are much smoother, I have no hesitations under load, and acceleration is noticeably increased. I hope this helps any other drivers out there with similar symptoms in a GDI engine.

  • 1
    Just wanted to share a personal experience with you real quick. Had a '93 Dakota for a couple of years that suddenly started dying after putting too heavy of a load, but ONLY when it was very cold out. I suspected something ignition-related immediately. Used a tester that lit up for spark, pulled my coil wire, it lit up fine. Pulled a random plug and nothing. Decided to replace everything (it was all the original stuff and needed replacing badly anyway) EXCEPT for the coil (only 1 coil on that truck). I mean everything- wires, plugs, distributor rotor and cap, etc. (Continued) May 19, 2017 at 13:59
  • 1
    But NOT the coil because my 'tester' showed 'spark' from it by lighting up. Took some cranking, but eventually started. But I was concerned because I knew something was still wrong, even after the ECU relearn. A couple of days later, I was driving on the highway and she suddenly died. It would crank and crank but I could tell she just wasn't going to turn over. So I decided to skip the spark tester, pulled the coil wire again and just tested it on the chassis. There was spark alright, but VERY weak spark. Tested the coil with a meter, sure enough it was toast. Ran perfect after swapping. May 19, 2017 at 14:08
  • 1
    Just thought I'd throw that personal experience out there. Maybe it means nothing for you but you never know. But I sincerely hope what you found was the problem and you're good to go. May 19, 2017 at 14:10
  • 1
    Thanks for sharing. Your symptoms do match mine. The coils are next on my list if I continue to have problems.
    – Spivonious
    May 19, 2017 at 14:52
  • 1
    So was this the answer or "update 8"? Also are there any additives to get rid of the carbon maybe?
    – rogerdpack
    Oct 29, 2018 at 18:09

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