2011 E70 N55 misfire after changing spark plugs

[davidwnv]

This is my first post, so I apologize in advance if I mess it up.

Spent the better part of a week chasing this problem down, did a lot of web searching and never really found a solid answer, so thought I would post my experience and hope it can save someone some time if they run into the same issue. Here's my tale of woe:

Bought this 2011 X5 used with just under 50k miles on it a year ago October. Car has been flawless. Changed oil and filter about 10 days ago, at 70k miles. While recording the event in the service log, noticed recommendation for spark plug change at 60k miles. Ordered and received 6 Denso FXE22HR-11 Iridium plugs, because I couldn't get the Bosch OEM's or the recommended NGK's in a timely manner. Searched the web for correct gap, the best I could come up with was 0.040" (1 mm). Measured the new plugs, and they were all right on, so installed them. Drove it to town, ran fine until I started back up a long hill to the house. When I put my foot in it, it bucked like a rodeo bronc. No codes, no mil light, just bucked like crazy. Let it sit overnight, started up next morning, and it immediately idled rough and threw the MIL on. Read the codes, and it was misfire on cylinder 3. Swapped the coils between 1 and 3 (because they're easy to get to), cleared codes and started again. Misfire on cylinder 1. Bought a new coil, installed, cleared codes and test drove. MIL light on and misfires on 5 of the 6 cylinders. At this point, I know it's not coil problem so started thinking what else it could be. Checked the gap on the old plugs, determined most were in the 0.030" to 0.032" range, with 70k miles of wear on them. Finally found some info on the web indicating turbo engines generally require smaller plug gap at high boost. Removed all the new plugs, gapped VERY CAREFULLY AND PRECISELY to 0.025" and reinstalled. Test drove and VOILA! Car runs like new.

Long story short:

The N55 engine apparently likes a smaller plug gap than indicated by all the specs I could find on the web. 0.025" worked for me.

If anyone knows what the gap on the OEM Bosch plugs is, please post. If this hadn't worked, my next step was going to be to buy a set of the Bosch plugs.

Hope this helps someone out there.

Dave

Carson City, Nevada