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Thanks everyone, that is quite a spread of miles out of a set of coils and it looks like mine are typical.
When the dealer changed mine from original to Beru 10 years and 40k miles ago I did notice a difference, the engine ran smoother. There was no misfire or CEL.
When I have just changed them now, all six had at least one small crack, but the difference isn't quite as noticeable, it is running a lot better, but was after stripping out one side and cleaning the coils/plugs. I did test the coils with a meter, the values for resistance were within spec.
I did get a CEL a couple of months ago, but that was when the battery was quite weak, so in my experience to answer a couple of questions posed here:
- If you have a weak spark, you might not get a misfire or CEL, but you will get poor burning of the fuel and it will lead to rough/poor running.
- Cleaning all the crud off the coils and the plugs will always help.
- If the coil has cracks in it then it will start to break down, especially when wet and this will produce a weak spark/CEL.
- I got Beru coils in the UK for £31 each, at that price it isn't worth fitting anything else. I also got 6 Denso original fitment spark plugs for about £28, again not worth cleaning the old ones which looked fine to me.
So for about £210 (and a couple of afternoons) I have new coils, new plugs, the old plugs didn't show anything out of the ordinary so I am happy that my 986 at 93k miles is good for another few miles.
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