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Thread: Bosch Pt +4 spark plugs: prongs broke off and caused misfire

  1. #1
    Join Date
    Sep 2005
    Location
    San Jose, CA
    Posts
    1,358
    My Cars
    97 M3 Coupe

    Bosch Pt +4 spark plugs: prongs broke off and caused misfire

    Found a serious issue with the Bosch platinum +4 spark plugs (FGR8KQE) I installed 5k miles ago. I'm now switching to NGK and starting with the standard copper plug model 2288 (BKR6EK). I've been using those on my S2000 without issue. Plus, some research indicates other E36 & E46 users having good experiences using NGK plugs.

    Problem statement: Engine would sometimes start misfiring after stopping back to idle.

    Symptoms:
    • Intermittent misfiring occurrences when coming back to idle, only about once per drive.
    • Engine was strong and powerful when the misfire wasn't occurring.
    • CEL code P0306: "Cylinder 6 misfire detected."

    Findings: All 4 of 4 prongs on the cylinder 6 spark plug were broken off. 2 of 4 prongs broken off on cylinder 5. The rest of the spark plugs looked poor after only 5k miles.


    Question for forum members: Should I worry about any damage, or perform further tests/maintenance? The engine is actually running fine now. I can't tell if there are any issues. I'm guessing the prong could have been pulled out of the exhaust valve on the vacuum part of the cycle. However, I'm also thinking that the prongs could have only broken fully off after I just removed the plug.


    Full story:

    Took the e36 to the track for three consecutive weekends, then it sat for ~4-5 weeks.

    My wife and I decided to take it out to dinner to keep the tires fresh, and the car randomly started misfiring at idle. I killed the engine quickly, and then it started back up perfectly. It ran the rest of the night just fine. So I chalked it up to a freak occurrence.

    But then I drove it to work and had more of the same. Car would start misfiring after coming to idle, I'd pull over, start it back up, and it would run smoothly. This happened twice, and this time it threw a CEL (check engine light). The code was P0306: "Cylinder 6 misfire detected."

    The strange thing is, I even drove the car to redline a couple times after this, and the engine pulled powerfully.

    So anyways, I started my diagnosis by testing the coil packs. Each one measured the correct 0.4-0.8 ohms (between left and right pins 1 & 15).

    But then I pulled the plugs and the issue was obvious. The plug on cylinder 6 was in terrible shape, and didn't retain any of the prongs.

    Pictures are worth 1,000 words:




  2. #2
    Join Date
    Dec 2006
    Location
    Orange Beach, AL
    Posts
    237
    My Cars
    '06 525i
    Man, I stopped using Bosch Platinums YEARS ago in my '71 VW Squareback when I had one's center electrode migrate out of the ceramic insulator and short out on the ground electrode.

    Stick with the conventional 4 prong Bosch plugs that the owner's manual calls for, Bosch FGR 8 KQC.

    Easiest test you can do is a compression and/or leakdown test on the affected cylinder. You can also purchase this inexpensive borescope off of Amazon and visually check for damage. https://www.amazon.com/USB-Borescope...40107488&psc=1

    Good luck!
    2008 Sequoia
    2001 E55
    1998 M3 (RIP thanks to Hurricane Sally)
    1982 320i (ditto)
    2006 525i

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