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Discussion Starter #1
OK first of all I apologize if my terminology is not 100% accurate as I am not a mechanic. So I was driving my 2009 323i on the 401 highway in Toronto on the way home Friday night, felt a vibration and then the engine light came on. I pulled off the first exit at which point the car definitely felt like it was struggling for power. I towed it to my mechanic and was told I had misfire codes showing up on the OCD. I know when I previously asked for a diagnostic report I received the misfire code below so I am assuming it still existed today:

n54 - cold start rough running with misfire results

diagnosis for dme fc29dc/29f2,2fbf


Mechanic told me two of 6 coils were bad and said all needed changing. After changing all coils I was told the piston ring on cylinder 6 was not expanding causing oil to shoot onto the sparkplug. All other sparkplugs were operating at 195psi but piston 6 only 90 psi. He then mentioned to fix this issue it would be significant engine work or possible replacement in the thousands of dollars cost estimate range and it would be better just to trade in the vehicle for something newer. Rather than give up on my 09 which I bought new of the lot and to date only has 160,000 km are there any more options and what could have let to this issue anyways? The car always burned a lot of oil but I didnt follow the 1 year oil change interval from BMW and did my changes every 6 months (6000km or less). Any feedback or information that might lead to salvaging my baby would be greatly appreciated. Thank you.

Piston rings causing oil spray into spark plug chamber
 

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I'd like to know how your mechanic came to that conclusion without stripping the engine down. Is the plug from the suspect cylinder just wet (i.e. fuel) or oily? It could be an injector issue. But if a piston ring has failed then he's right, you're looking at a big bill...
 

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If after a simple pressure test cylinder 6 is only 90psi when the other 5 cylinders are 195psi then that would tell me there's something quite seriously wrong like piston ring failure, as the combustion chamber isn't pressurising properly on the compression stroke.

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Could be either piston rings or a valve sticking slightly open or a cam timing issue causing the pressure loss. Get the engine pressure tested again and for the suspect cylinder check the pressure and them squirt some oil into the cylinder [not a huge amount but just enough to cover the crown of the piston - trial and error but a tablespoon or two perhaps]. Pressure test again - of the pressure is a but higher its definitely rings, otherwise its something else. An old trick but works well
 

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Discussion Starter #5
I'd like to know how your mechanic came to that conclusion without stripping the engine down. Is the plug from the suspect cylinder just wet (i.e. fuel) or oily? It could be an injector issue. But if a piston ring has failed then he's right, you're looking at a big bill...


So I spoke to the mechanic today and he mentioned the plug was covered in oil so it appears to be oil and not fuel (according to him). Any suggestions? Also trying to determine an RCA (Root cause analysis) as to what might have caused this to happen. Any help is appreciated. Thanks!
 

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There are two kinds of rings on the piston. Oil control and compression. Both would have to have failed to cause the symptoms. (low compression and oily plugs) I think it's equally likely the problem is elsewhere.
At least do the test with putting some oil in the plug hole and do the compression test again.
 

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Discussion Starter #7
Could be either piston rings or a valve sticking slightly open or a cam timing issue causing the pressure loss. Get the engine pressure tested again and for the suspect cylinder check the pressure and them squirt some oil into the cylinder [not a huge amount but just enough to cover the crown of the piston - trial and error but a tablespoon or two perhaps]. Pressure test again - of the pressure is a but higher its definitely rings, otherwise its something else. An old trick but works well
Hi Alltorque and jonbill. Thanks for the info I will definitely ask them to try the pressure test again. In your experience what do you think may have caused this issue in the first place? Whether it was piston ring or not Im just curious how I got to this stage consdiering the mileage is still somewhat low (160,000km). Any help is appreciated.
 

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First, quick caveat - I don't know anything about this specific engine - I'm just going on general knowledge!
so - if both oil and compression rings are shot on cylinder 6 it would be through loss of oiling.

normally, (based on general knowledge!) cylinder 6 is the furthest away from the oil pump so is the first cylinder to show signs of oil starvation.
you said it uses a little oil - so maybe it ran too low on oil. maybe it was a little low on oil from oil burning and you went round a long bend that sloshed the oil away from the pickup for a while.
I would think a momentary problem like that wouldn't be enough to ruin the rings, and I was going to suggest the bearing on #6 might have momentarily run dry and spun, permanently blocking the cylinder wall spraying. but it seems the N54 cylinder wall isn't oiled via the rod journal so that theory doesn't work.
I don't know how the cylinder walls are oiled on this engine so I don't have a guess, other than it being caused by lack of oil. sorry.

still, this is all moot pending that wet compression test.
 

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Discussion Starter #9
Appreciate the info. The car did burn a lot of oil as mentioned, I would occasionally add 1 litre of oil here and there to ensure it didnt go too low but it totally is possible that what you described above may have happened. Appreciate the info.
 
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