BMW 4.6 is poor acceleration and hesitation

[Aman]

Just dug up some info on catalysts:

Catalysts often last 10 years, but actual life depends on mileage and engine tune. 50,000 miles is a more reasonable benchmark, and you can generally expect to replace the catalytic converter once during the life of a vehicle.
If failure occurs early, it's important to find and fix the cause of the problem before fitting a new CAT otherwise the replacement can be expected to fail prematurely too.
There are three main ways they fail:

• Melt down - unburned fuel enters the catalyst and ignites on contact. The extreme heat literally melts the ceramic matrix. Poor ignition timing, a faulty oxygen sensor, worn or defective spark plugs, incorrect fuel mixture and other ignition/fuel injection related faults could lead to this failure.
• Carbon deposits - oil or antifreeze entering the combustion chamber/exhaust system can lead to a build up of carbon on the matrix, which increases back pressure leading to overheating and poor performance.
• Catalyst fracture - the ceramic matrix is fragile and can break-up as a result of excessive vibration or external impact. As the matrix breaks up back-pressure increases and overheating can result.

In my case there was no misfire from idle to approx 3500 rpm, but higher than that the car didnt rev freely / started to misfire
When I replaced coils and plugs, and $$$ later, the misfire persisted (only at high load / high revs)
When the damaged cat was removed, I looked at the inside from either end of the cat it looked undamaged. The fracture was actually in the middle of the cat and you could hear loose bits in the middle if you shook the cat.
Im still going to get diagnostics done etc to make sure engine/sensors are ok, but I think its important to realise that sometimes the damaged cat can be the cause of misfire (if the misfire is at high revs/high load) and it will generate lots of misfire codes and changing coils and plugs wont help.
It all depends on how the cat failed, which is not always due to mixture settings.
So, sometime misfires can cause a damaged cat, and sometimes the damaged cat can cause the reduction of airflow, and misfires

[upallnight]

Were both cats fracture or only one? Do you remember which side cat was fracture if only one cat was fracture?

[Aman]

Originally Posted by upallnight

Were both cats fracture or only one? Do you remember which side cat was fracture if only one cat was fracture?

The left side (kerb side in UK) was fractured

[bcredliner]

Sometimes cats fail long after the upstream cause has been fixed so when you check for codes there are none. Not saying that is the case here but it well could be.

Still a good precautionary measure to check for codes as you are doing.

[Aman]

Update:
Occasional misfire noticed
Engine is still able to rev freely to high rpms
Check engine light is now on again
Looks like you guys were right
Just as well I kept that appointment for tomorrow!

[upallnight]

It doesn't take too long for a cat to be ruined due to misfiring cylinders. I wouldn't be driving the X until the misfire is fixed. But hey that just me.

[Aman]

Update:
Started car this morning (going for diagnostics test)
No misfires
Engine light is off (eventhough it was staying on yesterday)
I thought when engine light stays on, you can only remove it using a computer?
Confused

[TerminatorX5]

engine light comes on for many ifferent reasons, for some reasons it stays on until remove by a computer, for others it will reset itself on next start up, for others yet, it will reset itself after so many miles after the problem is gone... the codes that gave you the light are still store in the car, so reading the car is a good idea...

[Aman]

Originally Posted by TerminatorX5

engine light comes on for many ifferent reasons, for some reasons it stays on until remove by a computer, for others it will reset itself on next start up, for others yet, it will reset itself after so many miles after the problem is gone... the codes that gave you the light are still store in the car, so reading the car is a good idea...

Thanks for clarifying that

[Aman]

After 1 hour live diagnostics including driving car, no misfire detected and oxygen sensors are fine
Only a fault code for electronic thermostat was found