Last May I did the rocker covers and their gaskets. Changed all 8 spark plugs with NGK but only changed two ignition coils that were soaked in oil.
Fast forward to September, I began to experience cattle grid-like shudder then within a day or two got CEL blinking, narrowed it down to cyl. #5 misfire. Turns out it was a failed plug. Changed ALL ignition coils (NGK) but just the one faulty plug and all was fine until...
Yesterday went into sport mode and forced a brutal kick down to overtake for safety reasons in a situation potentially degrading into downright dangerous.
The car shook once or twice, thought gears were slipping for a second, but finally the CEL blinked briefly, indicating a misfire.
I drove the remaining 10km to work uneventfully even if I reved in manual, but on the way back home, I thought there was again a shudder or two so I went on driving very conservatively.
Worth noting as a clue, I had perhaps once, a misfire on a morning
startup with vibrating engine and CEL briefly flashing. A restart sorted it for good.
Consumption has been great so far since the coils were replaced, at about 15,5 l/100km on average compared to 19,8 when I bought the old lady!
I have booked her with my mechanic for Monday morning so we will be getting the codes read first. Fingers crossed it's not cyl . #5 again.
I am considering to change all plugs for good measure, this time. Will also do an oil change and air/cabin filters as well as both front drop links.
Any comments and ideas appreciated.

Thanks

By way of update

Went to the garage and tried to retrieve fault codes. None had been registered, although there was noticeable shudder throughout the 20 minutes drive there.
Did a test drive with my mechanic riding along and managed to get a CEL blink under heavy revving and acceleration. Pulled P0305 2 and #5 cyl misfire.
Moved #5 cyl plug to #6 and the misfire moved along, to #6.

Changed all plugs

Below are the plugs as pulled from the engine this morning.

The No6 plug was installed in September when I had the initial misfire problem. We had moved plug 6 to cyl 5 back then and installed the new plug on No 6.
We decided to wait for the spark plugs change until service was due, I.e. this month.

Here's some more info



Plug #6 installed September at 191500 km - has 4700 km on it and looks ok

All other Plugs were replaced in May at 183520 km - 12680 km on them and look somehow

Car presently has 196200 km on the clock.

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Is it a 4.2sc or the BMW engine?

I don't see on mobile app.

Plugs for 4.2 are different to those, that's all

It's the BMW 4.4

Updated pics post with some narrative.

The bunch of the "old plugs" looks really bad. The first one of them failed at 7980 km

The No6 looks nothing like the rest of them at 4700km.

I hadn't changed all the ignition coils when I did the rocker cover gaskets. Chances are they were soaked in oil which subsequently contaminated the plugs insulation???

And if so, why No5 again? I don't believe in coincidences although this could very well be one???

Wrapping this up for the collective benefit: Leaky cylinder #5 due to HGF or cracked head. First spark plug change lasted for 3 months as per above. Second change lasted for several months without misfires and all in all for a year (27k km) until the eventual spark plug failure.

I'm weighing my options between repairing and scrapping the car...

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And now this is getting interesting. I was there when my mechanic was undoing #5 cylinder spark plug. I swear it gave with zero effort and I therefore suspect it was loose.

My suspicion is now proven right by comparing the washer to one that had been properly seated. It didn't come loose. It was never torqued down properly I think.

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