Short version - New coil pack needed.
Long version - car was running fine at Cadwell Park, left it on the trailer overnight then took it for a pre-Germany service. On starting it up, it was running rough again. Plugged in and it gave the following:
P0530 I though we can probably ignore as I’ve removed the AC system.
P0138 could be because of the decat and remap.
P0425 and P0426 I thought might point to a faulty or damaged pre-cat lambda so I ordered a new one.
P0440 I thought we can probably ignore as I’ve removed the emissions canister thing.
The new lambda sensor did nothing, so I’ve now got a spare. There’s a theme here to my fault finding…
I’d originally ruled out injectors having replaced them and also coil packs having swapped them over and not fixing the issue (more on this later). That left me scratching my head thinking it could only be a loom or connection problem.
I’ve been down to the car today and taken off the whole injector/coil loom. I’ve unpicked all the wrap, cleaned all the connectors, done a full visual inspection, and tested the continuity of every single wire using a multimeter. No issues. Thinking the worse, I’d bought a second hand one on eBay for £30 which arrived today. I’ve now got a spare (stop laughing at the back).
I re-wrapped it all back neatly in loom tape and it now looks neater than it did before.
While everything was apart, decided to reattach the plugs to the coils and ground them on the engine block and just crank the car and see if it was a spark or fuel problem. Strong sparks on Cylinders 1, 3 and 4. No spark on Cylinder 2. Now that gives us something to go on. It’s not the spark plug as they are new, it’s not the loom as I’ve just tested it all. It MUST be the coil pack, but I’ve ruled that out already, or have I?
Stupidly I think I remember swapping coil pack 4 and 3 and the issue remaining. I never went as far as 2, because it seemed to make no difference. Swapping coil 4 and 2 resulted in no spark at 4. Aha, light bulb moment. Put in a spare coil and voila, sparks on all four! Could that be it?
I buttoned everything back together, and boom, rough running gone. I feel like an idiot for not doing a proper diagnostic routine.
With the engine now appearing to run fine I decided carry on with the rest of the service, kindly being allowed to use the ramps. It makes such a difference having the car in the air!
Engine oil and filter changed, gearbox oil changed.