Car has just been in for a service and developed a misfire.

Only thing I can think it could be is spark plugs, had NGK put back in but not sure they were gapped before going in.

Car is a 2010 Impreza Sti and totally stock and only seems to start to stutter on boost at about 5.5 - 6k onwards with no CEL light on.

Car doesn't pull like it did pre service i don't think but then it could be me and seems to stall more easily.

How do I tell which cylinder is misfiring and if it is spark plug related?

 

Take it back?
You paid for a service, get them to do it properly.

Check plug leads and coil packs, easily damaged.

 

James and Jenkins?
Llandaff North?!

 

True...it's in for MOT soon so I'll tell them then.

Not James and Jenkins...an independent. Are they that bad then?

 

Misfire at boost usually means spark is being blown out. The major issue with this is unburnt fuel getting in the exhaust. If it ignites it could damage the Cat's just like Anti-Lag would. Get it too them before the MOT as you will need a bit of time to clear the exhaust of any unburnt fuel. Otherwise your HC's might be high thus failing the MOT.
If I am wrong please correct me as I am always willing to learn more.

 

Brand new spark plug taken out of a Mini that had just been serviced by a main dealership...owner came in for tyres but asked us to check out the misfire....



So yeah, whip out the plugs, check gaps. If they are platinum tipped make sure the tip have not fell off (can happen).

If individual coil packs, these can be damaged internally if pulled at an angle or twisted during removal (they use ceramic insulation between the windings=delicate).

 

Car is in for MOT today hopefully it'll get sorted.

Garage didn't seem to think it would be spark plugs and more like coil pack.

​​​​​Something I did notice was it only misfires in sport # mode and not sport or intelligent mode.

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