EUREKA!! (bet you don't see that word often).
That link from Jwalker explains everything!!! MANY THANKS.
Now, back to basics: we must remember that a higher voltage (a better spark) is required to set fire to a compressed fuel/air mix than to get a spark in just air or exhaust gases. This is why you frequently get misfires only under heavy acceleration - a really strong spark is required.
And in a nutshell this is how the cam sensor and injector timing is done:
This engine does use the spare spark system.
In any spare spark system, the pair of spark plugs fire differently. One is positive at the tip and the other fires negative at the tip as they are sent opposite voltages from each end of the same coil. In this engine 1 and 3 are sent negative pulses, and 2 and 4 get positive pulses.
Feedback is generated by the slightly different times of the spark caused by one cylinder with compressed fuel/air and one without. The coil that ignites the compressed mix fires a few microseconds later than the wasted spark, as compressed fuel/air needs a higher voltage than the cylinder that does not have a mix in compression. It takes a few microseconds longer for the coil to reach that higher voltage needed.
Now, just thinking about cyls 1 and 4, the engine determines which happened first - feedback from the positive pulse (cyl 4) or feedback from the negative pulse (cyl 1). From this feedback, a signal is generated that "fakes" the cam sensor and importantly, it also tells the ECU which cylinder has done the work for the sequential injection procedure.
Here's an extract from that article:
"The engine always starts firing the 2/3 coil first upon cranking. Charging of the 2/3 coil always begins near the second crank notch. Charging of the 1/4 coil always begins near the fifth crank notch."
Speculation: I guess that on start-up the cyl 2 and 3 injectors are paired up temporarily so that whichever cylinder is on its compression stroke will fire and therefore can be found. Then, when the ECU knows which one was the "active" cylinder, it reverts to true sequential injection. But I repeat, pure speculation.
The article also says that as there is no fuel ignited on the overrun, there is no valid signal, so the firing sequence is generated on start-up and remembered.
So that explains why fault code 1514 was displayed and not a cam sensor fail as I would have expected - it doesn't have one. When the valves stopped doing their job, the fuel density was wrong, as measured by the coil feedback and compared to the throttle position, so it declared a fault with air control in the throttle body.
It also explains why we don't need to worry about whether 1 or 4 are in compression when we time the engine.
Mind now settled.
Thanks again for that link.
