Annoying misfire

[S7Paul]

My 450 failed the MoT last Saturday due to high HC emissions. The tester said it was misfiring, and that the HC readings were the result of unburnt fuel going into the exhaust. Since then, I've replaced plugs, plug leads, coil, distributor cap, rotor arm & (because it was broken) the stepper motor. I have also set the throttle pot & AFM to the prescribed voltages. End result of all this - just as bad as before! I've put a temperature sensor on each header pipe, and they are all about the same temperature, so it seems to be random, i.e. not affecting the same cylinder every time. I'd booked it in for a re-test tomorrow in the hope that all the new bits would fix it, but there's no point taking it back in as it is. What else could be causing it? Could it be the ignition module?

[blitzracing]

Yes or coil.

Mark

[S7Paul]

Well, it's a brand new Bosch coil, so that should be OK (especially as fitting it has not fixed the problem). Guess I'll have to try a new ignition module then.

[GreenV8S]

Is it definitely a misfire, or could it just be running rich? That could be caused by a wide variety of faults.

[S7Paul]

I suppose that's possible, after all, if it wasn't for the need to do something to get it through the MoT, I probably wouldn't be fiddling with it at all. Apart from slight lumpiness at tickover, it's running perfectly, and still goes as it should. What's the easiest way of finding out, i.e. is there an adjustment I can make to see if it smooths out (e.g. AFM screw)?

[S7Paul]

Just to close things off, I have now replaced the ECU temperature sensor and the ignition module, and it has finally passed the MoT. On the fast idle test, HC has come down from 279 to 57, and CO from 0.379 to 0.096. I'm still not really sure what made the difference, but now I have the MoT certificate in my hand and can drive the car again, I don't much care.