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I have a 2002 E39 520i. One of the cylinders has stopped working (or misfiring) and I am hoping it is because no spark is generated. I am very new to engines so bare with me please. :)

My question is: The root of this problem can be either a bad ignition coil or a bad spark plug, right? How do I know which one needs to be replaced, or do I replace both?

Thanks!

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    If these are individual coils (coil on), you can possibly swap out two coils. If the problem follows the coil, then it's the coil. If it doesn't follow the coil, it's the spark plug.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 2, 2016 at 21:07
  • Oh alright, that's what I thought as well. Yes it is a coil on configuration. What do you mean by swap out two coils? Why not just one?
    – Balázs Vincze
    Jul 2, 2016 at 21:09
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    You are changing it out with another which is on the engine. You don't have to buy anything at this point, just testing. So, if problem is seen at Cyl #1, then switch #1 and #2 (or something similar).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 2, 2016 at 21:10
  • Yes, but how would I know which one is bad just by replacing?
    – Balázs Vincze
    Jul 2, 2016 at 22:47
  • Read my first comment. Also, if you have a misfire and you don't know where it's at, get the codes read. If you don't have any codes, it probably isn't a misfire at all. OBDII systems (which your car has) will recognize a misfire and report it to you (initially) as a check engine light.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 3, 2016 at 10:16

2 Answers 2

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To test the coil, as you said there's only one cylinder that isn't working, just switch the coils of two cylinders.

For the spark plugs, again you'll just switch two if you want to test one against the other, however unless the spark plugs just got replaced, I would generally replace them anyways, as it's so infrequent that you get in there.

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  • Thanks, but unfortunately I do not know which cylinder is misfiring :/
    – Balázs Vincze
    Jul 2, 2016 at 22:48
  • Your original post said one cylinder stopped working or is misfiring. If you don't know which cylinder is misfiring, then you don't have a misfiring cylinder. An engine that "skips a beat" could be due to any side of the combustion triangle: air, fuel and spark.
    – tlhIngan
    Jul 3, 2016 at 6:44
  • @BalázsVincze there should be engine codes that tell you which cylinder in missing or misfiring
    – Cullub
    Jul 3, 2016 at 20:16
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The best way to test a spark plug in home conditions is to remove all plugs, put them in coils, put the spark plug (metal part, thread) on a engine metal part (earth), and turn a key, or better to ask someone to turn it. And see a beautiful blue spark in every plug. If there is any missing spark, put any other plug in that coil, so you can figure out if it is a spark plug or coil.. Sometimes spark plugs might give you a spark when removed, but won't give it in the engine. This is why the ideal way would be to test them under 12 bar pressure. But that would cost more than a new spark plug. I have a 100% standard car, and I love high revs, so when I notice any misfire I just replace all the spark plugs. If you don't do it the disease goes into a coil and lead. After that one cylinder stops working, your Lambda sensor reads rich Air/Fuel ratio, because 1 cylinder does nothing, corrects the fuel injection, gives lean mixture, the rest of cylinders have higher load because one is missing and slowing them down, and they receive a deadly dose of fuel and air mix, and if you are not in limp mode or just drive slowly, you melt your pistons. This is the worst scenario in 1 faulty spark plug..

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