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Two days ago my Opel Corsa D LPG's engine started to misfire, and by means of an OBD interface I found out it was cylinder 1.

First thing I thought about were the spark plugs, as I haven't changed them in the last 36k-40k kms. When I took them out, though, I found them quite clean and not corroded at all. Plus, I've measured the gap with a makeshift thickness meter and they seem ok (1.1 mm, as spec'd).

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Today I've checked the coil pack with a multimeter and it seems that cylinder 1 coil is broken (others show 15MOhm to common, this one seems an open circuit). So I'm going to change the coil pack, but I had it changed few months ago, after a similar failure (I didn't do it myself then, had the car repaired by a shop since I was far from home).

Now I'm thinking of blaming the shop for not completely fixing the fault, but I'm wondering: could the cause be the (relatively) wide gap of the spark plugs? Other recommended parts for my model have a 0,7 to 0,9 gap instead, and I suppose this limits the peak voltage of the discharge, thus (I suppose) stress less the coil (beside other effects).

Current installed spark plugs are NGK ZFR5F-11, some sites indicate Bosch 0 242 235 666 as a possible replacement. Couldn't find any recommendation by Opel itself.

  • A wider than spec'd gap is hard on a coil. Before you go blaming the plug, though, I'd get a good measurement on the gap. "Makeshift" does not sound very precise. – Pᴀᴜʟsᴛᴇʀ2 Jul 8 '18 at 12:53
  • @paulster2 indeed it's not very accurate but mind, the spark is marked as compatible with the model, in fact I think it's the original one – clabacchio Jul 8 '18 at 12:55
  • Spark plug gaps will open over time. I don't know if .2mm is enough to cause issues. 1.1mm is only .043", which is in the ballpark for "normal" to me. – Pᴀᴜʟsᴛᴇʀ2 Jul 8 '18 at 13:04
  • @paulster2 sorry that wasn't clear, 1.1mm is the spec'd value for that specific plug, but other models from other brands, also indicated as compatible, have a narrower gap – clabacchio Jul 8 '18 at 13:08
  • That's why it seems so "normal" to me :o) – Pᴀᴜʟsᴛᴇʀ2 Jul 8 '18 at 14:00
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Coil packs do go bad and become problematic and intermittant. They are constructed with very thin wire winding to keep them small, but the smaller size itself falls victim to heat more easily by going open. If you are operating a car or truck in hot climate, keep a spare and an OBD-II code reader in the glove box. Your choice of LPG is quite admirable. I saw a friend operate a truck on LPG for 25 years and he removed his spark plugs and checked the gap every 5 years and reused them because they were pristine clean.

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