The APC doesn't make the car misfire, it just backs the boost off if it detects pinking. You would notice it as a slight waver on the boost gauge under hard acceleration.
Has your car got the Direct Ignition system? If so that could be a prime candidate to cause misfiring. I take it you've got the correct plugs fitted, with the correct gaps set?
What is the boost gauge reading when you get this misfire? It may be that what you're feeling is the 'overboost' switch cutting off the fuel. If that's the case, you may have a bad APC valve. What engine is it? 2.0 or 2.3? try to give us some more clues as to when it happens, ie: high revs? high/low speed? What's the boost gauge showing etc?
9000S 2.3 Turbo.
Thanks for your reply, a few more details for you...
2.0 Turbo with direct ignition, correct plugs and gaps, and the problem started as an intermittent one, and is now annoyingly permanent! As for it happens; under hard acceleration, between 2-4k revs and the turbo gauge will read roughly 2/3 (not in the red).
If it is the APC is there a way to test this?
If you want to test the APC solenoid valve, it is mounted at the R/H top of the radiator (looking into the engine bay). There are 3 pipes to it marked C,W, and R. The 3 stubs of the valve are also marked.
Internally, it comprises 2 small solenoids separated by a disc, which acts as a valve when either solenoid is activated. The centre stub is connected to the top stub when the bottom solenoid operates and vice versa.
At the 3 pin electrical connector, the centre pin is connected to 1 end of both the solenoid coils. The other ends of each coil are the other 2 pins. Therefore test for continuity between centre pin and each pin on either side. No continuity with either means a duff coil and new unit.
I doubt this would give a misfire. An inability to maintain a constant (higher) speed is more likley, with a slight change in throttle position giving an exagerated response. Pete`s suggestion re The DI system is the more likely culprit. If you have cat fitted, these don`t like misfires so go easy till its fixed.
Thanks for all the advice everyone, I removed the DI unit (it's a red one), took out the spark plugs and re-checked them (which seemed fine) then put it back together, and now it works perfectly!!!
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