Thanks for the reply, but I am afraid that I didn't explain it quite right - you see - if I remove the plug all together is does stop (14CUX says: engine not running = I'm not giving you any fuel) - this is sort of half-out when (I assume) some but not all spikes are getting through so the ECU is probably seeing: four and a miss, four and a miss, four and a miss, etc.
Interestingly the gas system also looks for flyback pulses (T'd off the same line) to see whether to supply gas or not - but I assume that it is rather more crude and as long as it sees something then that's ok.
One day I would love to see an entire functional spec for the 14CUX - I would love to know what it's thinking!
