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In the sequence of my previous question about the engine vibration (Engine is vibrating when idle and seems like missfiring), I have been trying to diagnose the problem on my own. What I have found out is that there is a possibility of one cylinder having a weak spark. When I unplug the cable on one spark plug, I see no difference happening on engine performance, while when I remove the cable from others engine starts to struggle. I have changed the wire, but the result is exactly the same (well the situation has improved slightly, but still not much better). My next guess is it might be the rotor and the cap which needs a replacement?

Is there anything I can do to diagnose the problem?

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What you found is not necessarily a weak spark. The test you described with pulling off the spark plug wires off one at a time to see how the engine behaves is a general misfire test. All you can really conclude is that the cylinder that makes very little change is misfiring.

When dealing with ignition components, they are generally replaced as a set. You don't just to the wires or the rotor by themselves. Replace the cap, rotor, plugs and wires all together. Once you have good ignition components and the misfire is still there you can look at other things. Verify the quality of the spark with a quality spark tester.

There is a trinity when looking for misfires; fuel, air and spark. Fuel is self explanatory, no fuel or a lack of fuel causes misfire. Spark is also self explanatory. Air is a little more complicated because it involves the whole induction system and cylinder compression.

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  • Why would you recommend replacing almost the whole ignition system altogether? Especially with an older system with a dissy etc., it's unneccessary to do that. While the spark plugs wear out rather quick with todays additive crammed fuels, the rotor and rotorcap can still have years of life left.
    – Bart
    Jan 12 '17 at 11:37
  • @Bart Most cars made in the last 15 to 20 years use either single or double platinum or iridium plugs. The lifespan of these is about 100k miles which happens to be about the lifespan of the cap, rotor and wires. Further, if just doing maintenance then throwing in a set of plugs only "could" be acceptable but when doing diagnostics there is nothing worse than chasing a problem because you assumed a component was good. Ignition components are cheap enough that a clean sleight is a good place to start.
    – vini_i
    Jan 12 '17 at 11:50
  • While I agree with your premise here, what you are suggesting is Remove & Replace wrenching, which IMHO, is the worst form of diagnostics you can perform. Replacing something on the sheer hope it helps the situation is not good diagnostics and usually costs far in excess of fixing the actual issue. Mind you, if the car is due for it, absolutely change out the parts as part of your regular maintenance plan if for no other reason than to eliminate them as a possibility. They were due for replacement anyway (maybe that's what you're saying and I'm not reading it right). Jan 12 '17 at 18:23
  • @Pᴀᴜʟsᴛᴇʀ2 I agree with you whole heartily. Diagnose the problem, not just throw parts at it. The two exceptions are; if the parts were due to be replaced or for a base line. This is only necessary when the ignition system is put together with unknown or piece wise components.
    – vini_i
    Jan 13 '17 at 2:15
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It seems you have found the bad cyilinder already. Try swapping its spark plug with one from another cylinder to see if the problem is now caused by that cylinder. You then know for sure if the spark plug is the troublemaker, or if it's somewhere in the path to it. The former option is the most likely. If it's indeed the spark plug, and if they have a decent amount of miles on them, i'd recommend to replace the set of plugs. They generally don't cost that much.

Make use of this opportunity to judge the plugs on their colour. Coffee brown is the best, but a slight amount of whitish deposits is not uncommon with today's additive crammed fuels. You can google spark plug colours for more information, there's plenty info on it.

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  • I have changed my spark plugs two days ago, since one of the old ones was bad. And spark wires were changed 45000kms before. I have swapped the spark wires and it does not seem that that particular wire is bad :/
    – Serhiy
    Jan 12 '17 at 11:52
  • @Serhiy If you changed the plugs two days ago, and swapping the plugs and wires doesn't change anything, it's unlikely that the ignition is the problem. You can check the rotor cap if there's corresponding terminal is badly corroded/eroded or something, but if it isn't it's probably not your ignition system. Do you have multipoint injection or carburettors? It could also be the compression but i very much doubt that.
    – Bart
    Jan 12 '17 at 18:59

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