L322 2006 4.2SC 118k on clock. Got it around 6 months ago.

Had some starting issue which turned out to be alternator. Had plugs and oil changed at same time just cause I had just bought it etc.
Didn't use "in anger" for a while but it ran fine at low speeds around village.

One day I boot it and I'm hit with god awful misfires and reduced performance mode and engine management light.

I stick in in local garage and they eventually tell me that the only codes they have are for the misfire which is in bank 1 in cylinders 1,3,5,7. It never changes. I told them to replace all 4 O2 sensors which they did (shot in dark but it wasn't bad idea anyway to switch them out after so many miles)

I got myself a icarsoft reader and have been trying to see if I can trace what's causing the misfires. As if, but can't fault me for trying.

I've removed, cleaned and replaced MAF (proper cleaner)

Checked all turbo pipes using carb cleaner trick
Visually checked wiring etc

It starts and will run for 30 seconds and then gets chuggy and shakes a bit.

Yesterday I removed coil from cylinder 1 and 2 and switched them. Remember 1,3,5,7 on bank 1 always misfire.

1,3,5,7 misfired

4 misfired

2,6,8 fine.

Also had new P0134 o2 sensor circuit no activity detected bank 1 sensor 1! Hoping this isn't a new fault and is just knock on effect from the rest of issues

Did not expect bank 1 to not misfire but wtf why has 4 started now when I didn't touch it? I was very careful and methodical when switching them over too so I was heavy handed.

Now getting to stage I think I'll need to get breakdown to take it to main dealer to sort. I'm in Highlands so there isn't a ton of choice for me garage wise either lol.

Suggestions greatfully received at this stage. Sorry for ramble and if I've not been coherent. Atm I'm going to switch cylinder 1 and 2 coils back and see if everything returns to "normal" misfires.

Thanks for any help and for reading

OK I've put coil packs back on "original" cylinders but misfire in 4 is still there. So now I've got misfire in 1,3,4,5,7.

Is there any chance this is fuel related or injector related?

Given the number of miss-fires and originally all on one bank then it points to an issue that's common to all of them which will be unlikely to be individual coils or injectors (unless common wiring to them or you are VERY unlucky!).
I would be looking at the OBD data and deciding where to go from there.
My hunch would be a vacuum/manifold leak given it effects multiple cylinders. It's easy to miss leaks even with the brake cleaner trick, I didn't spot the last one I found on a relatives motor until I put the smoke machine on it and that was after the brake cleaner trick which found nothing. That leak turned out to be in a couple of places, a split in the main intake which only opened up when the engine moved on it's mounts from the torque so difficult to spot (easy with smoke) and a split blanking cap on a vacuum port which I found before smoking it.
I've had a similar issue on another one of mine with a split intake which only got worse when the engine torqued.
It's possible yours might be similar, give it a boot full, the engine torques and moves and any split hoses that would normally 'seal' when idle suddenly open up, gulp in loads of air and cause a miss-fire. Now the split is opened up it's doing all the time.

Thanks! I'll see if I can have someone do smoke test for me and see if it's that. My vacuum pipe(?) had popped of at back of engine and I've cleaned that and put back in along with some shrink tube to hold in place. Engine did not sound any different after doing this so maybe that indicates what you're saying is what's happening here.

Thanks for help

In case this is of use t o anyone in the future.

Main dealer checked and did their thing. All coils plugs etc all fine. Fuel delivery, injectors etc fine.

Turned out the driver side Cat was badly breaking up and making all readings by sensors further down the line, be totally off. New cat fitted (bye £1k) and all is good. Misfire cured.

Also got clean bill of health for everything else which was good

i was just going to say, look at the cat,
i have a similar problem with mine (2005 M62) and the BMW forums all mention this
i have yet to look at mine
how did they know it was us RR 2005 4.4 petrol
Any spelling mistakes are the fault of spellchecker
Previous MG J2, CITROEN light 15 x 2 gone unfortunately
Present MERC 180e, RANGE ROVER L322, JAGUAR XJS, MERC 280SE, MG F, JAGUAR S-Type 6v 2003, jaguar s-type 8v 2004, Ford Cougar

No idea how they worked it out but it looks like the sensors were basically restricting fuel and making bad mix due to cat upsetting readings and this causing misfire.
I had taken a live stream of data from my obdii but I'm not expert and so all I saw was fuel trim being almost opposites on misfiring Bank to the normal running bank ie Bank one had no fuel trim while bank two was OK with 18% etc

Did you fix this? Current stable:
2006 BMW M6
2007 4.2 V8 S/C FF
Gone:
2002 4.4 V8 FF

Hi, yeah was bad cat. Thing was totally bored and causing dodgy readings for the O2 sensors. No issues since

I too had a cat failure and now waiting for the other one. . . Range Rover 4.2 SC
2 Range Rover Classics - one from new but both rusty now.
Rover 3.5 Coupe had since new.
Subaru Legacy 3.0.
BMW K1200RS.

I see thanks Current stable:
2006 BMW M6
2007 4.2 V8 S/C FF
Gone:
2002 4.4 V8 FF

THIS IS EXACTLYWHATSHAPPENING TO ME!!!

Bought4.2 supercharged. Avert said drives well has cat faults wngine management light comes on now and then. Sell assured me I would be fine (I wanted to get home alive)

Got home but the fumes in the cabin were horrendous butno errors. Took for an MOT next day I told them cats are dead can they check etc fumes and co2 horrendously high

Told I needed remap and new fuel pump. Caris running rich. Get home the fumes areto much for me so take it back said do the spark plugs. Had those changed smell was gone but car still didnt feel right. Tookto another garage asked tp check cats and exhaust. They are fine but sparks are shot to bits (4 days old). Car is missfiring cylinders 1,3,5,7 I had OBD tested and got these errors along with multiple missfire and bank 1 sensor error

Pressure tested engine and sparks cleaned and put back. Smell gone white smoke gone. Told it was injectors 90% its them. Coil packs and sparks ok. Its missfiring

Went to quickfit and told no car is missfiring
Cats and exhaust good. So ithought ok it must be injectors. Went to put my footdown on way home and car went in to restricted mode max was 50 on a 70 not good

Reading this post is exactly the same as me. Sofar I have spent over £1200 and its still a lemon. If it is the cats which i am now 99% sure itis. How many and how much???