Experts - Please Help - Im Fed up with this issue already!
 

In my other posts I mentioned that I have this annoying issue where whenever i hit the brakes my MPG jump to 8 and then when i come to a stop it goes back to normal at 40. Now this is the most annoying thing ever because whenever I feel this it has some sort of down shift reaction.

1. Transmission - Recently rebuilt fully - all new sensors and completely was rebuilt
2. MAF - Alot of people on the previous posts stated that it was the Mass Airflow Sensor, I went ahead purchased that for no reason since I just replaced it and still a no go. I feel my gears changing a LITTLE bit smoother but thats it, the MPG issue is still effected.


What else could be the reason? Please anybody! 02 Sensors? or something else? Please this is really annoying the crap out of me!

Thanks!

Are you talking about the little MPG gauge needle? If so then that has no affect on driving/shifting and the gauge dropping down just before the vehicle stops is normal behavior. If the vehicle is downshifting rough at that moment it isn't caused by the gauge but something else.

So what is the actual issue you're having? Rough downshift?

Well when i hit the brakes it comes to a little rough downshift and at the same time right after when i accelerate there is a bit of a delay.... where it takes a second or two until the acceleration kicks in

Does the engine rev when it hesitates to accelerate?

Not that I can tell, and overall since i changed the MAF ive noticed that there is a delay in general now, whenever the X is at a stop and i accelerate its just a 2 second delay from the time i hit the gas pedal until the X actually moves....

Does it matter if you're in sport vs manual? I take it the engine still revs normally in neutral/park... Also, did you get OEM airflow sensor or aftermarket?

Sport or manual it does the same thing. and yes i got the oem bosch maf.

Could the hesitation be the hydraulic delay on the transmission?

Any answers for this issue? Its not as bad but still its a very annoying thing to have while driving the X....

With issues like this one needs to feel it/experience it to make any proper diagnoside. Just thought of something, what year is your X?

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.

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

I agree....good think I'm not an amateur! I spoke with Simon this evening, it's not hard at all to remove the 02's and see what happens, so there is really nothing that anyone can screw up here.