The improvement when I fitted the MSDs was vast, but that clearly came in part from very 'aged' COPs at 200k, and the fact that I changed all the injectors for new at the same time. I'm sure that when COPs get old, they fade off gradually, so you wouldn't notice. Only when 1 fails that you think about it.
I'm having to replace one of my MSDs too now, not through failure as such, but because the long spring inside the COP's pencil-boot which connects to the sparkplug was almost rusted away as a result of coolant leaking into the plug-well from an intake gasket leak at #8. Caused I am certain by a garage not re-torquing down when warm as required (and asked). The intake bolt behind #8 is difficult to get to for torquing down without burning your hands on the EGR gubbins when hot. An expensive failure all round, but you try proving it! Only when I started getting misfires did I remove the COP and saw what had been going on. Gooze were happy to oblige, at Euro 63 each for singletons. Ouch.
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