Problem with my Mini Cooper misfiring and my mechanic
November 8, 2010 8:49 AM   Subscribe

My 2003 Mini Cooper (base) is misfiring/stumbling when idling and soemtimes when braking. A recomended mechanic has made two repairs and the problem has persisted. What else should I be looking at or asking about?

First time I brought the car in, I complained that the AC didn't work and that the car had a code for a misfire (P0304) that came on intermittently. When the SES light came on, it would be at idle (though somtimes when braking), with the AC off and the car already warmed up. It didn't happen a lot because it was summer and I had the AC on almost all the time.

Fast forward to October and it's getting worse. This is when I figured out that with the AC on and a higher idle RPM, the problem wouldn't happen. As the weather got colder and colder, putting the AC on went from making no sense to very unconfortable. It was 38*F when I woke up!

First issue: What else could it be? They've replaced the plugs, coils and wires and now the valve springs on the offending cylinder. Are there tests I should be asking them to run? Vacuum/compression/fuel injectors? I don't think it's the head gasket since I didn't have an overheating problem in the summer and it doesn't look like the

Second issue: It's been back there twice now, am I out of my mind to ask that they fix it this third time for free? Or should I forever throw good money after bad because they seem to be just taking guesses at the problem. I need to drop the car off tonight after hours, I typed up this note to leave with the keys in the drop. Should I be approaching this from a different prespective? What recourse do I have if they say that $2000 worth of work is needed to finally fix the problem? Should I just get a second mechanic? (this one was highly reccomended) This garage is also part of a Getty station, should I contact their corporate office? Or am I making a big deal out of nothing?
posted by Brian Puccio to Travel & Transportation (12 answers total)
With a misfire problem they are going to be unsure what the issue is. I would be looking at the distributer cap. I would contact the head office and depending on their response find a new shop to have a look.
posted by Felex at 9:00 AM on November 8, 2010

I didn't think that a 2003 mini would have a distributor cap, I'll research that later, thanks!
posted by Brian Puccio at 9:02 AM on November 8, 2010

Nah, they should be able to diagnose or fix it or not give it back to you until it's fixed.

However, when I had similar issues in a Honda, the culprit ended up being a bad battery combined with a bad Main Relay. Electrical systems in cars are really hard to diagnose.

If you don't get any resolution, try a different shop.
posted by SpecialK at 9:03 AM on November 8, 2010

It might most likely has an electronic distributor. I would still pull suspect it first (after the wires and plugs).
posted by Felex at 9:23 AM on November 8, 2010

They really need to hook it up to a scanner and watch it while it is idling and misbehaving.

This could be any one of many different sensors that are failing or have intermittant wiring - oxygen sensor, MAP, MAF, etc - anything that can cause the ECM to calculate a bad fuel pulse width. I don't know much anything about Mini Coopers, but is could even be whatever their idle air actuator is.
posted by rfs at 9:42 AM on November 8, 2010

I suggest you go to MiniForums and check there for a reliable mechanic in your area and/or a possible solution.
posted by JohnE at 9:47 AM on November 8, 2010 [1 favorite]

Does your daily commute have any big atmoshperic changes? such as elevation change (like more than 1000', big changes in tempature or humidity? such as coast to desert (say san francisco to central valley). This things can really confuse a cars computer and if there is a marginal sensor it just makes it more likely. If you are at the random stuff to change and see what happens stage I would make my next change new O2 sensor(s)-the mini may well have more than one. They usually aren't terribly expensive and really need changing every 75k or so in my experience. I would also make sure my brake booster is holding good vacaum-this could also, maybe, cause the rough idling and if the vacaum leak is on an intake runner might screw up that cylinder enough to read as a misfire to the computer.
posted by bartonlong at 10:40 AM on November 8, 2010

I'd suggest having the EGR and PCV valves checked. If they're stuck, it can cause a rough idle
posted by bodaciousllama at 11:29 AM on November 8, 2010

I think you might have a problem with the electronic throttle body.
First of all, this Mini doesn't have a distributer. It has a pair of coils, each of which is responsible for providing spark to two cylinders, respectively. If a coil was failing, you almost certainly would have misfire codes for more than a single cylinder.

I've seen this on MANY other vehicles that employ an electronic throttle body. Before electronic throttle, the throttle was cable operated and the idle was controlled by an idle air control valve that allowed air to bypass the closed throttle plate. Nowadays, there's no physical connection between the throttle plate and the gas pedal. Instead the computer takes the input from the pedal sensor and outputs it via an electric motor that operates the throttle plate. The computer can also control the idle speed via this same motor. At ide and under braking, the computer sees that there's no input from the gas pedal and it closes the throttle to the desired idle position. When the AC operates, it opens the throttle slightly more to accomodate the added load from the AC compressor.
What's most likely happening is that the throttle body has some gunk build up inside it. It's really common for some carbon or other dirt to build up in the throttle and change the tolerance between the throttle plate and the bore in which it's housed. This can cause problems for electric throttle bodies since the computer is programmed to correlate a certain throttle position to a certain engine output given a set of conditions. Simply, what's happening is that at ide, the throttle is more restricted that the computer expects it to be due to the carbon build up. Turning on the AC causes the computer to open the throttle more, eliminating the condition.

I can't tell you how many cars with rough running, misfire, poor acceleration, and even stalling concerns that I've fixed with a simple throttle maintenance. If a simple throttle cleaning fixes this, don't hesitate to ask for you money back on the previous repairs. If they had spent any time testing or observing (ie, vacuum, compression, or ignition testing) they never would have wasted your money on ignition components or valve springs.

Check out to find a GOOD European specialty shop in your area and get this car fixed right.
posted by Jon-o at 1:34 PM on November 8, 2010

Awesome post, Jon-o! And thanks for the link to iatn for finding a mechanic.
posted by exphysicist345 at 7:17 PM on November 8, 2010

Got off the phone with the garage today. They brought up their logs on the past two visits and went over all tests previously run.

They also said that I needed a valve replaced so the head needs to come off. They said that if this didn't fix the problem, and they were sure it would, that I wouldn't have to pay for the repairs. Hopefully I'll have it back soon and will be back on the road.
posted by Brian Puccio at 12:54 PM on November 12, 2010

For future searchers, I've had the car for over two weeks now, runs awesome and the place knocked a large chunk off the bill to make up for the previous labor that didn't fix it.

Lesson learned, if you must have a Mini, you must have a warranty.
posted by Brian Puccio at 6:15 PM on December 8, 2010

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