BMW 4.6 is poor acceleration and hesitation

[Aman]

Originally Posted by upallnight

Great mechanics since they couldn't figure out that the cats were breaking up and clogging the exhaust. Their reply that it was "PROBABLY THE PROBLEM" does not leave me with a warm and fuzzy feeling inside. But hey that just me.

The cats were the problem because something made the cats the problem. Mixture is controlled by O2 sensors and if you have a bad O2 sensor eventually the mixture will also destroy your new cats.

If you read the postings by MrProject he noted that he didn't want to drive the car and risk destroying the new cats until he was sure his problem was solved.

If you don't fix the problem now, you will be posting later why your new cats are clogged again.

I will check for other issues, but its useful to note that cats can sometimes fail due to age
http://www.xoutpost.com/members/sethhatfield.html also advised me this is what happened to him when he wrote his experiences in this thread (page 6 I think)

[TerminatorX5]

no, a slight misfire, or retard of the spark will not set off check engine light... severe misfire, misfire under heavy load, several cylinder misfire will set off the light... sometimes it is not a misfire per se, it is just engine roughness, as monitored on the live feed from the engine that can be first sign of impending trouble... the things about the cats, since they are downstream from the main events, they can be a casualty of the problem upstream... of course, there is a loop back, where the info from the O2 sensors and the cats is fed back to the front, thus creating a vicious circle... as long as you have rectified the issued upstream and downstream, you should be good...


listen to your engine while it idles, without any loads, no AC, no seat heaters, no radio - just plain engine and only normal systems...

as you listen to the engine, the engine noise should be even, without any extra over- or undertones that fade in and out... if you hear them, that means that the software is making adjustments in spark, timing of the valves, blah, blah blah... some very light varioations are natural, due to varying fuel pressure (still, pressure should be constant), and the fuel quality - but if you use a 1st tier fuel supplier (do a search on this forum for 1st tier fuels), after couple of full tanks of gasoline, your engine should just purr... if not, then hook up live monitoring software and see what is going on there...

[upallnight]

You can get a Bluetooth OBD II device and an App call Torque. You can monitor the long term and short term fuel trim. This will tell you if the O2 sensor is adding gas or taking gas away from the mixture. If the O2 sensor is always taking gas away from the mixture you have a problem with a rich mixture, and a rich mixture will eventually destroy your new cats.

Torque will also list pending codes. Like TerminatorX5 stated after so many misfires the DME will throw up a check engine light, however, every time you shut down the engine, the computer will reset the number of misfire code so it will always start at 0 when you first start the engine. If it hasn't reach the number require for a check engine light by the time you shut the engine, it will restart the count the next time.

[upallnight]

http://www.xoutpost.com/950598-post13.html

Even though he think that his cats failed due to age and mileage, if you read his post he also posted about a misfire. Misfire is the most common reason why cats are destroyed.

[bcredliner]

Looking forward to hearing if something caused the cat to die or it was just old age. Complete diagnostics will at least give you peace of mind.

[upallnight]

@ Aman were both cats clogged and broken up or was it just one cat that was the problem. Also was the cat that was the problem was it on the side of the engine that had all the misfires cylinders?

If one cat was still good and the cat that was bad was the side with all the misfires cylinders it would be safe to say that the cat did not die from old age.

[sethhatfield]

Originally Posted by Aman

It was 110% the cataysts
The car now flies like mad
I didn't change the oxygen sensors as the exhaust place said the cats were probably the problem

Change the O2 sensors as a precaution. Its always a good idea. You can do it yourself easily/cheaply and the mileage usually always improves. I had already changed mine last year so they were fine.

They won't cause a SES light until they are completely inoperative even though they are feeding the computer inaccurate readings.

[Aman]

A few photos:

[Aman]

Originally Posted by sethhatfield

Change the O2 sensors as a precaution. Its always a good idea. You can do it yourself easily/cheaply and the mileage usually always improves. I had already changed mine last year so they were fine.

They won't cause a SES light until they are completely inoperative even though they are feeding the computer inaccurate readings.

Thanks for the advice
Here in the UK the cost of the 4 sensors is the equivalent of approx $400 for the 4, not including cost of fitting (not so cheap?)... I heard it is only worth fitting good quality sensors eg bosch or ngk... can you remember how much your sensors cost?
The cost of getting the new cats supplied and fitted (as in the photos) was the equivalent of $620.... this is the going rate here in the UK
Of course if you had the skill to TIG weld / pipe bend yourself you could get the cost down (I dont have this skill)
To fit the genuine BMW cats was costing the equivalent of $2500 including fitting
I have a diagnostics appointment booked for tomorrow and was going to get the sensors tested by a live test examining their voltages, as well as checking everything else
Although the car is no longer misfiring, it is always best to make sure, I guess

[Aman]

Originally Posted by TerminatorX5

no, a slight misfire, or retard of the spark will not set off check engine light... severe misfire, misfire under heavy load, several cylinder misfire will set off the light... sometimes it is not a misfire per se, it is just engine roughness, as monitored on the live feed from the engine that can be first sign of impending trouble... the things about the cats, since they are downstream from the main events, they can be a casualty of the problem upstream... of course, there is a loop back, where the info from the O2 sensors and the cats is fed back to the front, thus creating a vicious circle... as long as you have rectified the issued upstream and downstream, you should be good...


listen to your engine while it idles, without any loads, no AC, no seat heaters, no radio - just plain engine and only normal systems...

as you listen to the engine, the engine noise should be even, without any extra over- or undertones that fade in and out... if you hear them, that means that the software is making adjustments in spark, timing of the valves, blah, blah blah... some very light varioations are natural, due to varying fuel pressure (still, pressure should be constant), and the fuel quality - but if you use a 1st tier fuel supplier (do a search on this forum for 1st tier fuels), after couple of full tanks of gasoline, your engine should just purr... if not, then hook up live monitoring software and see what is going on there...

Thanks
Could not detect any misfire, but I am taking car for a live diagnostics check tomorrow to make sure