It's me again. 2008 MINI Cooper S w/ dealer-installed JCW package (chip tune, larger air intake, larger cat-back exhaust). I thought everything was fine after resetting my ECU last week, but had bad misfiring at idle again this morning which set the random misfire code. Since it's happening across all cylinders, I'm a bit stumped.

Here's what I've done:

  • Replaced spark plugs
  • Replaced cracked air intake
  • Cleaned MAF sensor
  • Reset ECU

The engine has the coils on the plugs, so no chance of a bad coil or bad wires.

I've also purchased a USB scan tool with some software, so I can retrieve any diagnostic info the ECU is putting out. My long-term fuel trim (as of last week) is slightly negative (-2.5%), short-term is right at 0%, fluctuating up and down as the engine speed increases and decreases. This is after a week of daily driving, mix of highway and city conditions.

The misfiring only happens in open loop mode. Once the car has warmed up and switched over, it runs perfectly. It seems that it's running too rich, as I get a fair bit of gray smoke out of the exhaust as it's stumbling.

Failing O2 sensors? Failing idle-control valve? Time to give up and take it to the shop? What should I look for in my scanning software?

(previous question: Lack of power when cold under load)

edit: plugged in the scan tool when I got home. Random misfire plus misfire in 2 and 4. LTFT is back up to 0%.

  • 2
    Just because the coil is on the plug does not mean it cannot go bad ... I doubt this is what's happening to you, so just pointing this out to you. It's not going to be the O2 sensors as they are involved after the engine goes into closed loop. If there was a problem with the IAC, you'd see it all the time, not just when the engine is cold. There must be a sensor which is providing bad information to the ECU, then once things get warm enough for it starts working correctly ... either that or your ECU itself is bad. I really don't know, though, so just throwing it out there. Commented Jun 1, 2017 at 20:31
  • try watching/graphing ECT voltage during warmup. check your coolant level and maybe pull a plug after getting the engine hot and letting it cool down.
    – Ben
    Commented Jun 1, 2017 at 22:34
  • @Paulster2 - my coil comment was just to mean that the main coil wasn't bad, as there are four of them :)
    – Spivonious
    Commented Jun 1, 2017 at 23:37
  • @Ben - coolant temperature sensor is working properly, at least according to the scan tool. Starts at ambient temp and works up to operating temp.
    – Spivonious
    Commented Jun 1, 2017 at 23:45
  • @Paulster2 - could it be fuel-related? The engine surged this morning, almost stalling out between surges, like the ECU was trying to fix it. It would drop to 400 RPM or so, then surge up to about 1100, then fall back down and almost stall. Repeated for about 30 seconds. I was able to give it gas then (before it behaved like it was ignoring the gas pedal) and it ran fine with a few misfires as long as I kept it at 1500 or higher. The high pressure fuel pumps on these cars are known to go bad.
    – Spivonious
    Commented Jun 2, 2017 at 16:44

4 Answers 4


It was the high pressure fuel pump. After letting it sit overnight, the dealership was able to reproduce the issue and replaced the pump under the extended warranty. The car is driving like it did when I bought it two years ago. No more trouble starting, no more stumbling when accelerating when cold.

Thanks to everyone for your suggestions!

  • So it paid going to the dealership. I did recommend it initially. Told you we'd all be guessing here. Commented Jul 10, 2017 at 21:59

2011 R57 S cabrio. I had identical issue. Misfires all cylinders. Replaced HPFP, coils, walnut blast. ALL FAILED. Engine limp mode returns. Finally checked fuel filter (75k miles). Very dirty. Replaced. Runs like new. $30 for kit. 30 minutes DIY.


Clean the IACV. Sometimes it does the trick. Which grade and type of fuel does it run on? After cleaning the IACV, check the TPS, reset/restart the ECU try reprogramming the idle after that.

  • If all else fails go to BMW for a second opinion on what's wrong. Commented Jun 4, 2017 at 9:10
  • Surprisingly, BMW doesn't service MINIs. I'd have to go the MINI dealer, which is 90 minutes away. :/
    – Spivonious
    Commented Jun 5, 2017 at 12:47
  • I always put 93 octane gas in it. The manual recommends 91 or higher, but most stations around here offer 87, 89, and 93.
    – Spivonious
    Commented Jun 5, 2017 at 12:48
  • Dump a bottle of STP Complete fuel system cleaner, not the octane booster or the fuel line cleaner nor the injectors cleaner. Go for the big bottle and dump it in let your mini rip open on a track or a freeway. Commented Jun 5, 2017 at 21:14
  • Ha, I let it rip every day. :) It's going into the dealer next week. I'm 95% sure it's the HPFP. My boss's MINI had similar symptoms and that's what it was. MINI extended the warranty on them to 10 years/100k miles so the work should be free.
    – Spivonious
    Commented Jun 6, 2017 at 13:33

Had the same problem. After compression check found no4 cylinder had low compression at around 165. Thought the worse but the pressure was holding so no leak down. Turned out to be an easy fix. Removed the rocker cover and inspected the valve springs for breakages none found. Removed the exhaust tappit rail. This can be done without removing the camshaft 5 simple bolts. Removed the tappits for no4 cylinder and on inspection found some wear and pitting on the roller. Replaced the tappit erased the fault code all now solved a cheap and easy fix. Ps the traction control light also was also coming on that too is now fixed. I hope this helps someone it only takes around 45 mins to achieve.

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