It's an 01 4.4 with 123k and I have changed my maf sensor and rebuilt transmission for other reasons as well as this one on my mind... I changed spark plugs as well. I never changed my 02 sensors though...

i had an intermittent prob with one o2 sensor in the past. it would cause the mpg gauge to go haywire like what you have. engine response was also incorherent and sometimes vibrating abnormally even though the X can still be driven. the fault code was also intermittent. it was pinpointed to the o2 sensor by monitoring the o2 sensor function after plugging in. hope this helps.

Another thing I have noticed when I accelerate fast and then brake at an intersection, besides my MPG going to 8 fast and then slowly stabilizing, at the same time my MPG is going haywire my RPMs sort of SINK under the line where it normally is when its stable and then returns to normal and then i feel like a jerk and it goes away.

Could this be caused by faulty O2 Senors? :dunno:

i remember my X behaved similarly. engine rpm will drop when car is at idle to the extent that it seemed about to stall but never did stall. suggest that you go get your X plugged in and monitor the o2 sensor function.

It very well may be a faulting O2 sensor giving bad readings to the DME to work with. If you have access to an OBD2 reader that reads live data you can monitor the readings, otherwise it is definitely worth a shot with the upstream sensors. (before cat)

I was thinking of it now, Is it possible that this can be caused due to my Catalytic Converter?

See, the story behind that goes like this. Around a year ago I went to the stealer just to give my X a full check up. according to them my Catalytic Converter was bad and it HAD to be replaced. They wanted $1200 for the job. I told them ill look into it. After going to 4 different WELL known indys in my area they all said at the time that it they both seem to be fine. There is no rattling and there is no SES light so just ignore what they said.

Now its been a year and I am thinking, could that be related somehow to this issue? Since the entire time I have had this issue? :/

It is possible I guess, if it is still cleaning the exhaust properly it wouldn't throw the cat converter efficiency faults... But if it is blocked up it usually causes some rich mixture faults and bogging down/sputtering etc. under load or at higher RPMs. The cat converters are expensive from BMW, but I've seen a couple guys use magnaflow universal cats without problems or faults. But I'd make really sure you actually need cats before you worry about that as the symptoms don't match up.

I had a clogged cat that caused my 7 to intermittently fall on it's face. The more power I gave it, the slower the car actually went. It got to the point finally where I couldn't get the car to even run. Mind you, this all happened within a day, not over months and months of time. I had the car towed to the nearest shop (my house was too far) and was gonna let them run their code reader...misfires on every cylinder, cat and 02 codes across the board. Not telling them what I did for a living, I left them car with them to charge the battery (slowly died from continual restarts/dying) and entertained them by letting them look over the car and they came back with a $4000 repair bill...wanting to do fuel pressure tests, remove the heads and check for carbon deposits causing the valves to stick, flow testing and cleaning the injectors, etc. Holding in my laughter, I got my keys back to find out they had broken my hood release. I went from laugher to being pissed and they claimed that my hood was jammed...odd, NEVER had an issue before. Their "free inspection" turned into a "we charge for the extensive looking over we gave your car", so told them we'll call it even for breaking my hood release, and they agreed.

So after running through in my head the codes I had, knowing I've already replaced one cat due to rattling, my last suspect was the remaining cat. I didn't suspect it since it was a factory replacement under BMW before it hit the 100k mark because they we're known to fail prematurely on the M62, which is the same motor in your X. While still in the indy's parking lot, I limped the car onto driveup ramps and removed the pre-cat 02 sensor. Car fired right up and purred like a kitten, mind you a loud ass kitten, but still purred. I removed the 02 sensor from the harness and drove to the nearest exhaust shop with a 1" hole in my exhaust...you'd be surprised how loud that can be! $200 (incl. installation) and I had my second Magnaflow flow cat installed and not a problem since.

Moral of the story...climb under your car, beat on the cats a few times to check for rattles, and go ahead and drive the car with the pre cat removed. Try doing one at a time, and if it gets better on either side, there is your culprit. I've also told you once before, after 100k you should probably replace both pre-cat 02's regardless due to them being wear items just like spark plugs. If in fact the car runs better with the 02's removed, I'd honestly replace the cats in a pair as well The piping is 2.25 and I think the model number was 94005. As a plus, you'll get a deeper exhaust note and regain some lost power if it's in fact clogged cats.

A failed catalytic converter is essentially creating exhaust restriction, so that is why it would lose power, lots of codes, etc. That isn't the symptom here, and the OP already said that there was no rattle from the catalytic converters.

I would look elsewhere for the cause of a momentary hesitation.

My 1st cat rattled like no other, forcing it's replacement. The 2nd cat did not rattle at all and was probably replaced within a few months after the 1st. It was simply clogged up so badly that nothing would flow, so removing the 02 sensor gave it that little bit of flow I needed to be able to get the car driveable enough to a shop for replacement.

I was under the belief that a failed cat would always rattle, but that simply wasn't the case for me. Removing the precat 02's, one a time and driving around, then seeing if the power returns will just be another way to see if either of Simon's cats are clogged and failed in the same fashion that mine were.