Quote:
Originally Posted by iseck1
After weeks of troubleshooting my rough idle E93 N54 I managed to solve mine. Tried everything discussed on these threads including walnut blasting the intakes, coils, cleaning electrical connectors, etc. and nothing worked. As a last resort I decided to put back the old spark plugs and voila - car runs good as new much to my surprise. Here's what I learned through the process that I thought i'd share with the forum in case it can help others. I had a bit of a rough idle a few weeks ago for which I started with new plugs from Amazon. The plugs were new OEM BMW BOSCH 4 electrode plugs and were exactly the same part number as what was in the car - even the stamping on the side was exactly the same including the "made in germany" part. There was no reason to suspect any issues with these plugs. During the change I also added a catch can and replaced a spark plug heat shield tube on #3 cylinder which had what appeared to be corrosion on it i.e. crusty green deposits. What I later learned is that corrosion is linked to spark arcing through the coil insulator to ground through the spark plug heat shield. Not all BMW's use this heat shield but it appears to be common on Turbo cars - down side to them is that they provide a good ground very close to the coil stem which can allow the spark to jump out. Once it starts the insulator deteriorates and issue gets worse. The corrosion is ionized air on the aluminum sleeve. It also deposits on the coil stem so keep a watch for these and clean (or preferably replace) these coils as needed. I suspect this was the initial reason for my rough idle but by putting in new plugs I managed to move the problem and cause weeks of headaches trying to troubleshoot. Now back to the plugs - after close inspection of the new plugs the only physical difference appears to be the gapping of the 4 electrodes. One or two electrodes on the new plugs appear to be closer to the center pin than the old ones. Remember - these are factory gapped plugs and not intended to be re-gapped by the user. I figured the larger gaps was wear on my old plugs and decided not to address it on the new ones. After about 500Km of horrible idling and lots of troubleshooting I swapped back to the old plugs as a last resort. I examined the new ones and found the ones with the small gaps had burn marks only one side of the porcelain insulator. That a sign that three other electrodes were likely not firing. All that to say - I believe short of an internal issue with the new plug (i.e. manufacturing defect) the root cause is the smaller gap on some electrodes causing an insufficient spark for proper detonation especially on idle. That likely explains why the computer does not detect a fault and why certain conditions like load from the AC compressor or engine/ambient temperature vary the symptoms. Since switching back to the old plugs the car is running perfect. If I find some time I plan to pull the old plugs again and measure the gaps then regap the new plugs somehow to match and try them again. If that works then this will tell for certain the gap is the root cause and not a plug manufacturing defect. Another key lesson here is to pay close attention to the details and part conditions. You could replace all coils, plugs, injectors and hope for the best but in my case that would have addressed the arcing coil issue and not the faulty plug gap. I would have thrown out lots of good parts and money in the process and I hate doing that because you never get to actual root cause and you waste lots of money. If the rough idle returns, I would keep a close eye the coil stems and plug heatshields. If it happened to one, it's likely going to happen to one of the other 5 and it will leave a crusty deposit as a telltale sign. Will also need to be much more careful with spark plugs - I might try NGK's as I'm told some newer BMW's come with those from factory now. Check the gaps! Hope this helps in case you are dealing with the same issues.
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No-one is going to read that, far too wordy and long.
Use some paragraphs ffs.