I was quite happily cruising along in my 840ci, went to overtake a lesser car and instead of the usual kick in the back as the power came in, I got lots of misfires and the car shaking pretty badly. I let go of the throttle and the shaking stopped, but something was certainly amiss. When I slowed for the roundabout (a round intersection), the revs dropped pretty low, but the engine kept running, pulling away was very lumpy and it seemed I had lost at least 2 or 3 cylinders.

Diagnostics showed 'Misfire recognition KAT - damaged cylinder 8 OBDII Error!!' - and clearing the errors got all eight cylinders firing again and the engine sounded reasonably happy. The next day all seemed well - that is until I gave the loud pedal a good push and it all went pear-shaped again. This time, the errors were a bit more expansive:

That's just about all the cylinders on Bank 2 that have given up. There isn't much that can cause a complete bank to fizzle out with the other bank still working nicely. If it was Bank 1 that had all the problems I might have worried that the timing rails had failed and one camshaft had skipped timing - but there were no horrible noises from the engine and it was Bank 2 that was affected.

The most likely cause was a blocked catalytic converter. I have heard of this happening before, and the explanation of the sudden failure and the ability for it to 'get better' again was explained by the custom exhaust fitter. He told me that parts of the honeycomb detach, and are blown to the rear of the housing where they block the gas-flow. When the engine is stopped they fall back out of the exit and allow the engine run correctly again.

To allow the car to be driven to the exhaust shop I drilled a couple of large holes right at the front of the catalytic converters to allow the exhaust gasses to exit. This seemed to do the trick as long as I kept to reasonable power levels. I almost didn't make it there - but that was due to the camshaft sensor giving up, not a problem with the exhaust, still, it jollied the day up!

The evidence speaks for itself, one badly broken up and the other starting to melt:

I had both catalytic converters changed for 200 cell Sports cats including fitting for 499, the same job from BMW is 1700 (I have the bill from the previous owner who had this done a few years ago). The cause of the failure is probably due to deteriorated rubber bungs as described here.

It is worth saying that Sports Cats make a HUGE difference in top-end power, especially when cruising in 5th and giving the car some hefty throttle, it really takes off. I doubt if the cats will last as long as BMW ones, but I can afford for that cats to be changed two more times and still have 200 in my pocket!

All done, time for a cup of tea......