Hi Robin,
Have you gone right back to basics on this e.g.
- compression test on each cylinder
- spraying carb cleaner for air leaks as a lean burn would cause this
- each plug out in turn still connected to the HT lead but grounded and turned the engine over to make sure you're getting a good spark on all cylinders (fuel pump fuse removed).
- pulled the lead on each plug in turn with the engine running to check it bogs on all cylinders (not sure this is a good plan H&S wise with HT leads mind you)
- just pulled each plug to check whether any are wet with fuel, oily or damaged etc
- checked each injector is firing (screw driver on each with your ear against the end)
All my previous misfire issues on various cars have come down to a bad pencil coil. In all cases the car would idle fine and even pull away gently but as soon as you tried to accelerate the misfire would start. I've always had an OBD2 scanner to hand with makes life easier on modern engines as it will tell you which cylinder and often that it is actually a coil but the symptoms seem very familiar.
You won't necessarily get back fires or smell petrol either if just one cylinder is not firing 100%, just a serious shudder.
I wouldn't suspect the cam sensor as that would either prevent the car starting or see it revert to a batch injection mode which would just be less efficient (not sure if m20 was fully sequential though anyway). Most other sensors such as coolant, air and O2 should all have a similar effect and you'd notice a lack of power and high fuel consumption rather than complete breakdown.
my money's on either an air leak or something in the ignition chain between the coil and the plugs
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