They are two different failure modes. If the substrate breaks in the converter, the converter rattles. If it overheats, it can get plugged.

Since Simon's don't rattle, they don't appear to be broken. If they were plugged, they don't flow, and then they would cause a continuous loss of power, not a hesitation.

It may be worth looking at the sensors, separate to the converters, but I would have expected codes in that case.

m5james , my 1997 740i is very sluggish from cold , does not want to upshift unless I press the gas a lot more so I shift manually through to 4th then shift to D .after a mile all is well .

idling in traffic for a few minutes I get an occasional burp/misfire with a slight vibration , this will repeat every 30 seconds or so .

I won`t be having any testing or diagnostics done for a few weeks but I wonder if you have any ideas as to where I could start looking .

you mention removing O2 sensor , will the same result happen by just removing wires .
thanks

I can't explain it, but mine was intermittent. I happened on and off throughout the day before it finally called it quits in the evening.

Point is, he already knows he need to replace the 02's anyways, so all I'm saying is that while he's replacing them, drive the car around and see if performance improves dramatically between having an extra 1" of flow or not. He can check in any order he wants really, but if he replaces both sensors (as he should), it's still sluggish, then leaving one 02 sensor out a time will help eliminate the possibility of a failed cat.

Cold vs warm engine idle changes could be a MAF or CPS sensor issue. Checking the MAF is quite easy...if you unplug it and drive it around, then suddenly everything is happy in the world, that means your MAF has failed to send good signals to the ECU. Unplugging the MAF makes the ECU default to a base map, but if you plug in a bad (failed/wornout) sensor, then it's feeding the ECU bad info and the computer acts off that bad info.

The 02 sensor thing would be for testing a plugged cat, which are different symptoms that what Simon and I experienced. In this case, if that cats are plugged, simply unplugging the 02's wouldn't work because we're trying to accomplish more exhaust flow and less backpressure, not bypass a sensor reading. I'd just unplug the MAF, followed up by seeing if you've got any codes, and posting your results from there.

I really appreciate all the replies from everyone =)

I couldn't have asked for a better group of people to assist me with this issue. I figure I'll go ahead and replace my 02 sensors when I get some free time and then hopefully all will be well. If not then I'll go ahead and get some mflow cats.... The only reason I am bothered by doing that is because I fear that it will be louder then the stock cats. Idk what the thing is with people that have loud ass cars and like it. I love my x as quite as it is now and I'd really hate to hear loud roars while I accelerate.

However after replacing the 02 sensors and if I still have this issue then I have no choice but to go aftermarket on the cats since the stock ones are out of my price range. But to be honest I just can't believe that it's the cats since they do not rattle and the issues aren't the same. But as james said it doesn't have to rattle to be clogged. I appreciate everything and I'll update the post asap.

thanks for all the help thus far everyone

I hate obnoxiously loud exhausts too, especially backfiring, popping and droning ones that some poeple do...with the exception of my bike :) I replaced the cats on my 7 long before I ever did the Magnaflow mufflers, and in my case it didn't make the car louder, I could just tell it was slightly deeper. For anyone who could care less about cars probably wouldn't even notice, but for someone like me, I could hear the slight difference and it was by no means something that anyone would consider unpleasant or "modified" in the terms of sound.

Simon, just because it's very easy, I say just remove each 02 one at a time for the hell of it before you order anything. Remove the electrical connector, remove the 02 from the pipe, reconnect the electrical connector and suspend the 02 (you'll see how when you're under there) and go drive the car around the block for a mile or two. Do it on each side, one at a time...just remember, it's only the precat sensors that you need to worry about. Maybe 30 minutes of your time and at least then you'll know you don't have plugged cats if nothing changes.

Completely unrelated videos since you're not doing mufflers...
YouTube - Bmw 740iL with Magnaflow exhaust Rev - this is how mine sounds with Magnaflow 14816 mufflers. Nice rumble at idle, no popping in the revs, no drone.
YouTube - BMW 740 with Eisenmann Quad 83mm Sport Exhaust - this is Eisenmann, easily 4-5x as much, but sounds the same
YouTube - BMW 740 Eisenmann Exhaust by Need 4 Speed Motorsports - ditto
YouTube - BMW 740i sport,magnaflows, resonator delete takeoff! - this is a guy w/ resontator delete, so I guarantee in person his is louder than mine.
YouTube - bmw e38 magnaflow custom exhaust - this farting and popping sound I hate, but it also doesn't help this guy has the 728iL. Mine sounded similar to this when I did the mufflers AND installed an x-pipe, which I have since removed and reinstalled the factory resonator....listening to it, you can see why. Live and learn.

Nobody suggested replacing the converters. James' suggestion was to check them by running with the O2 sensors removed, as that would change the back pressure if they were indeed plugged up.

The worst thing you can do is just randomly replace parts one after the other. That isn't fixing anything, it is just spinning the parts roulette wheel and hoping that you eventually get lucky.

As Weasel noted above, what you really need is a proper diagnosis.

I agree he needs a diagnosis, but for the rates that some shops charge for that kind of time, it's sometimes almost easier to throw parts, in the case it being 02 sensors. I've mentioned how to test for a bad MAF, so that purchase was probably unnecessary, but then again they age, so in the long run it didn't hurt anything more than his wallet and budget for the repairs he really needs.

For example, he could replace both 02 sensors himself (easy job for something that need to be replaced anyways simply due to mileage over 100k) for around the same price as an hour or two of dealership diagnostic time. He could also kill two birds with one stone with my backpressure idea.

Simon...quit writing, go into the garage and spin off those 02's bro! We could have them swapped out 2-3x by now :)

I am reminded of the very well phrased words of Weasel:

You think professional is expensive, just wait until you pay for amateur.

Those words are there for just that reason. :D