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Old 10-04-2018, 12:00 AM
cn90 cn90 is offline
Join Date: Jun 2014
Location: Omaha, NE
Posts: 1,609
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- My 2004 X3 2.5i M54 engine with 84K miles, stock Bosch spark plugs.

- Interestingly, after I posted the above message, I got the codes of "misfire cylinder #3 and #5".

- Initially though to be ignition coils, replaced with good known coils, same codes: misfire in Cyl #3 and #5.

Replaced with six (6) brand-new Iridium single-prong plugs and all is well now.

Then I looked at the Factory Bosch 4-prong plugs, I realized that there is wear at the tip of the prong. I guess the spark keeps looking for the SHORTEST distance, so once a prong is worn down a tiny bit, it looks for another prong. After millions of cycles, all 4 prongs wear evenly...

NEW: about 1.5 mm thick.
AFTER 84K miles, about 0.8 mm thick, enough to cause misfire.

The bottom line is: no spark plugs last more than 80K (well they do, but fuel efficiency goes down), so no matter if you use factory Bosch/NGK 4-prong or NGK Iridium, check for gap (on the Iridium) every 30K and replace every 60K...That is what I will do from now on...

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT
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