Hi all,

I recently bought a mini cooper 1.6 N12 for a good price. It had a misfire code on cylinder 1. Everything else seemed in perfect condition and it didn’t have any other codes than the misfire code and a lambda sensor code. I drove it home for about 100 miles. It didn’t breakdown, and honestly did it pretty good. At home I have had a look at multiple things. Starting with the coils, spark plugs and injectors. I swapped the coil, spark plug and injector from cylinder 3. Cleared the codes and tried again.. this unfortunately didn’t help as the code was still present at cyl 1. I then had a look inside the cilinders with my endoscope, and I saw that cylinder one looked completely clean! Like shiny clean. Meanwhile the working cylinders looked black. I then stuck my endoscope into the injector ports of cyl 1 & 3. I did this to see if the owner was right about the valves being cleaned professionally (Not just an additive in the fuel). And to my surprise they were all really clean. Not a spec of carbon. I also couldn’t see any problems with the valves them self. My endoscope doesn’t allow me to tilt the camera up so I cannot inspect the valves from the bottom. I asked the original owner for a summery as of what happened and he sent me this:
Hi Niels,

The problems started when the check engine light came on and it turned out that the second lambda (oxygen) sensor needed to be replaced. After that, the light kept coming back on and then going off again briefly after a reset. Then the first lambda sensor was also replaced; the light stayed off for about two weeks, but after that it came back on again.



I kept driving the car for a while before making another appointment, but one day when I fully pressed the accelerator, one or more cylinders “blew out” and the car started running very rough and jerky.



After that, the car was given a cleaning, along with new spark plugs. That didn’t solve the problem either, so they removed the catalytic converter and had it professionally cleaned. In the meantime, they also cleaned around the valves / the outlet of the engine block and the inlet of the catalytic converter. After that, injector cleaner was used as well, and that was basically it haha.



Something I thought of later, but which was more or less ruled out, was that maybe an ignition coil wasn’t functioning properly, although according to the service book the coils were replaced in 2024, I think.

he also said that the garage has had a look with a camera and they said it looked pretty good. I hope someone on this forum can help me out/ guide me through it! Thanks in advance.

 

I'll throw in my two cents to try to help. I dont know much about the N12, so bear with me.
For the testing of the ignition coils and spark plugs. How did the spark plug look in cylinder 1? Clean like the cylinder? Do you have any way to monitor the misfire on that cylinder to see if its just dead or misfiring under certain conditions? Did you look at the wiring for the ignition coil? Make sure it has power and ground? Next, I would check compression. Maybe that cylinder is very weak and not firing. And lastly, you say you remove the fuel injectors. I know the problem was there before, but I know on the N14, the injectors have Teflon seals that are one time use only. They should be replaced everytime they are taken out and put back in. They could cause a minor leak that may be causing it. So maybe the shop he took it to had taken the injectors out to clean things and then it started it happening?
Good luck!

 

Post all of the current fault codes.

 

Niels, let's take it in the order you presented your problem.

1. Misfire and Lamda sensor code. The N12 engine uses a wideband upstream O2 sensor and a narrow band downstream O2 sensor. When you see a misfire and O2 sensor code. You'll want to connect a diagnostic tool to the OBDII port and look for the B1S1 (Bank 1 sensor 1) current (not voltage). Voltage doesn't change like it does with a narrow band O2 sensor, but you will be able to see the current switching. Typically, when the engine computer tells you you have a bad wideband O2 sensor (apart from the wiring being eaten by rodents), you have a bad sensor. You have to use an OEM sensor (no Amazon O2 sensors) or you will have problems. If I remember correctly Bosch made the upstream O2 sensors for MINI. As for the downstream sensor, I wouldn't worry about it when it comes to a misfire because it's mainly used for catalyst monitoring. You can also introduce propane into the intake and see if the upstream sensor current reading drops when propane is applied, and rises when the propane is removed. Once you have verified the O2 sensor is switching; you'll want to look at your STFT (short term fuel trim) and (LTFT) long term fuel trim. Excessively high or low fuel trim numbers can cause misfires.

Your #1 cylinder piston is washed; a very accurate way to confirm this is to take an oil sample and send it to either Blackstone Labs or Speed Diagnostics and explain to them your problem; you'll be looking for fuel dilution in the engine oil. Even if the injector is functioning electrically, you are probably unaware of what the spray pattern looks like or if the injector is closed (not leaking). N12 injectors aren't very expensive. Continental is the OEM supplier to MINI for the injectors. Best price is at Rock Auto. If your injector is allowing too much fuel into the cylinder, that can cause a misfire from a fuel mixture that is too rich. I've seen it happen with a stuck open injector, and the wash of your piston makes that a high probability. On a second note; you'll want to change your engine oil yesterday. Oil with gas in it is not a very good lubricant.

Operation: FYI; when you have a secondary ignition misfire (like you described under full throttle), the engine computer will deactivate that cylinder until the ignition is cycled. When you start the car back up, the injector will fire again until the computer senses another misfire. This to prevent damage to the catalytic converter. This is why the car will run on 3 cylinders until you shut it off and the run normally again when you start it back up.

Lastly, and I don't think this is your problem, N12's are notorious for dropping valve seats!

Here's an example: