As per my other thread about my new P38, when i took it for its MOT it seemed to be running very rich, reeked of petrol, had no power and was generally pretty rough.

In an effort to cure the thing running like a burst arse I've ordered some new spark plugs. They havent arrived yet, but i decided i would whip the old ones out and check them over to see if any of them gave any clues to it running poorly. Started on the left side of the engine, first three plugs were dark and sooty with a white tip, then i discovered HT lead number 7 sitting next to the plug, on top of the exhaust manifold... I think i've unplugged it when i was changing the crank sensor, and havent put it back on! Pulled number 7 spark plug and sure enough its wet with fuel as you'd expect. Fairly happy that i'd found something actually wrong, i started on the other side. No missing leads this time, but plug number 6 was also wet with fuel. Hmm!

At first i wondered if 6 and 7 shared a coil (being wasted spark i figured the disconnected lead might have disturbed the other cylinder) but no, 7 shares with 4, which was fine, and 6 shares with 1, which was also fine, so i'm unsure its a coil issue?

So 6 not firing is currently a bit of a mystery, but at least i'm getting somewhere with it. I'll maybe pull number 6 lead off completely and check it over for damage, they look like fairly new magnecore leads. I guess its also possible that number 6 injector is dribbling or otherwise not spraying properly?

Running on 6 would cause it to be a bit gutless. Number 7 plug not connected will cause that bank to run very rich. The lambda sensors measure Oxygen in the exhaust but not fuel, so with one cylinder not firing, it will see a huge quantity of Oxygen (from the cylinder that isn't firing) so assume it is running very lean and pump extra fuel in to try to burn all of the Oxygen rather than just some of it. With all 4 on that bank firing, it should clean things up although it may need a bit of running to let the fuel trims adjust and settle.

Number 6 not firing may be simply down to a dodgy plug or lead. NGK BPR6ES are the best for the GEMS engine, and HT leads that are more than a couple of years old should be regarded with suspicion too.

Its got some weird denso plugs in it just now. I've ordered some NGK plugs, The NGK thing suggested BPR6E, with 6ES as an alternative (i cant even figure out the difference, as "S" means standard, and no suffix is also means standard...). I actually ordered BKR6E though which is the same thing as BPR6E but with a smaller hex on it, which means you can use the smaller socket which fits easier in tight spots.

I'll check the receipt for the leads to see how old they are, i think they were fitted back in 2010, but its done less than 10k miles since then.

Got it running today with the new plugs. Sounds and smells much better, revs cleanly and exhaust smells mostly "normal".

6 and 8 i think were actually covered in oil i think, so i'll need to keep an eye on those after i've done some miles and see whats happening.

See what the MOT man says!