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  1. #1
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    Member No: 138107 zm850i is an unknown quantity at this point zm850i's Avatar
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    Car runs without O2 sensors. What???

    I was bragging about how smooth my 8 runs to a BMW mechanic, he told me that some BMWs run better with the O2 sensors disconnected. He said that most e34s have those disconnected, and run better without.

    So I went home, crawled under the car and disconnected the O2 sensor plugs, to test this.
    Car started without problem. Took it for a drive and there was absolutely no difference in performance or fuel economy. Very surprised to note there was no rough idle.

    This has lead me to believe that my O2 sensors are dead. O2 sensors tell the computer how much unburned O2 comes out of the engine so the computer can readjust the air/fuel mixture as needed. This could explain why my car feels much faster then other 850s (well i only driven 1 other 8) and also why my Consume button always goes back to 13mpg regardless of how I drive. The best Ive had it was at 17mpg on the highway but if I do not reset it for few days goes back to 13 again(combination of city and highway).
    My assumption is that computer thinks there is no O2 coming out of the engine so it tries to ram more air/fuel mixture, and therefore car runs faster also uses more fuel.

    Anyone cares to comment on this?


  2. #2
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    well yes and no. up to a point it can alter the

    ignition and air/fuel mixture upon data from the O2 sensors but I hardly believe that you are actually only getting 13mpg regardless of driving style. Also why is it always 13mpg ? I think it is an another problem.
    As far as O2 sensor being dead or alive a resistance test can show the difference. O2 sensors are not terribly expensive. Go to Autozone and buy a Bosch unit. A unversal is oke, as long as you solder the cables not just tie them. The resistance goes up and can do magic problems if the connection is not heat-sealed. Use a good O2 sensors as many cheap ones on the market and some are really bad quality.
    I assume your car is a pre 96 as if you disconnect the O2 sensor on an OBDII car the check engine light will come on. On the OBD-I cars the O2 sensor is only in front of the cat so the function is half-done. There are no comparisation between before and after the cat so yes the car can run fine w/o the O2 sensor. It will run very fine if you remove your cat, but you did not hear it from me.
    You might even gain a few extra HP and some nice rumble by this. I would say it is worth doing it if you will not spend the money on a cat. leaving a dead cat in place is worse then not having a cat at all. A cat will take away a few ponies but hardly noticeable. The 635CSI will however work perfectly without the cat and you will gain somehting like 20HP instantly. Older the car the worse it is with the cat.


    1997 840CiA
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  3. #3
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    I wouldn't do it. Engine designed of O2 sensor

    13 MPG is definitely a failed Digital Motronic Electronics (DME).

    So, I'll call the O2 sensor and raise you a component!!

    The engine will run without the Mass Air Flow (MAF) sensors also.
    I wouldn't do that either.

    The O2 sensor is the component that helps the engine run at the best Air Fuel Ratio (A/F)perfromance point for idle and part throttle conditions. Also the Cats and the O2 sensors perform together to minimize exhaust emissions. Depending on the Motronic version the O2 sensor will set injector fuel trim to maintain an appropriate A/F for throttle position just beyond part throttle.
    Engine components such as the Mass Air Pressure sensor and engine RPM sensor to provide the correct A/F for medium throttle and Wide Open Throttle (WOT).

    Disconnecting the O2 sensor will make the engine reference the DME (Motronic - MAP data) as the default A/F since it can detect a failed O2 sensor.
    Also it is possible this A/R will fry the Cats if not corrected long term.

    I assume your 8 is outside North America or you would have a "Check Engine" light on the instrument cluster telling you your O2 sensor had failed. Without this internal DME diagnostics it requires external diagnostics equipment to determine the contition of the Digital Motronic Electronics and the sensors.

    It is also possible the MAF sensors have failed!!
    Rod...
    BMWCCA GGC

  4. #4
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    Yes | No

    Re: well yes and no. up to a point it can alter the

    So this is the reason why im getting like 8 mpg in my 850 eh? my mechainc told me they were bad i just never repalced them yet.

  5. #5
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    Re: HMMMM bad DME?

    Your post made me think... I've only been getting 12mpg for three years now. A few mods, Dinan chips, straight exhaust through Magnaflows, no cats or resonators, 3.91lsd, and 18" M Parallels.

    (I also just completed a cosmetic overhaul including a CSI kit, Brembos, and total repainting. It really looks great!)

    I've done the IMs, banjo bolts, DK cleaning and Synching, and now the DME question arises. How would I know if I have this problem and more importantly, what can I do about it!

    Thanks,

    Mike in Denver

  6. #6
    zm850i
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    Yes | No

    Re: well yes and no. up to a point it can alter the

    My consume2 button never gets reset and it sits on 13.2mpg
    the instant fuel consumption (code 2) sits between 20 and 25L/100km (i believe this converts at 12 to 14mpg) around the city, and the best I get on open road is 10 to 12L/100km at 90km/h, which is great..

    The cats can be a reason for high consumption of fuel.
    Had a look at them some time back, and they look original, 16 years old, the car is euro 1990 850i with 150,000km (around 96,000ml), so it seamed as perfect time to replace. Well my choice was no cats actually, but with the strong regulations here in Australia no decent shop would touch this job, they say they will get fined if i get busted with them off. I am i little worried about the aftermarket parts going on the 8 and backyard job was not an option. Maybe Ill buy some exhaust tools latter on and do it my self. lol

    I am glad that you mentioned "better no cats then shot cats" because i wasn't aware of this and decided to keep the "dead" cats, simply because the car is faster, and rumbles nice and deep, also my garage door resonates like crazy from the deep rumble so I am willing to put up with the increased fuel consumption.

    Why do you think "leaving a dead cat in place is worse then not having a cat at all"? other then the fuel consumption being high but not getting ALL the power benefits of no cats.

  7. #7
    zm850i
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    Re: HMMMM bad DME?

    With all your mods Mike, i doubted you have a faulty DME.
    Biggest factor to your 12mpg are the no cats. Actually, that can be said its economical for a no cats V12.

    Christ, and no resonators? That thing must be fairly loud? I bet you feel like a king sitting on and launching from the lights. 8)

  8. #8
    zm850i
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    Re: I wouldn't do it. Engine designed of O2 sensor

    Definitely a failed DME? How? Those other work or they dont.

    Going by your theory about MAP data, and I totally agree with you there, after I replace the O2 sensors will make the engine reference the DME (Motronic - MAP data) as the default A/F again. Wouldnt it?

    Yes, my 8 is euro spec and I live in Australia, have no Check Engine light on.

  9. #9
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    Re: HMMMM bad DME?

    ZM850i,

    Actually, I just did the exhaust a month ago, the milage improved slightly. It is kind of loud on accelleration right before each auto shift but has a great idle rumble and sounds very nice and mellow at highway speed!

  10. #10
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    a bad cat ? maybe there isn't any cat at all !

    well the bad cat can cause lots of problem. Once the cat is broken inside it will clog up and actually choke the car. at one point the gas mileage will increase and will slow the car down so much that it will feel like a 3 cylinger geo metro with handbrakes on up to the colorado rocky mountain.
    if your car indeed feels faster then you might have an "empty cat" which means some point of the cars life an owner or the shop removed the guts of the cat. Very common when cost and rules are major players. If this is the case then it explaines the rumbling noise and louder sound, and also the extra omph. In this case the cat is gone so don't mess with it.
    The O2 sensor is still worth replacing, under 100usd and can make a difference in fuel consumption. the fix 13mpg is too much for the car ! It should be at least 14-16 in city and even better in highway with normal driving style (speed limit)


    1997 840CiA
    1990 Z1
    2002 Astro Van
    2002 Lexus GS300
    1998 Yamaha Blaster/Breeze
    1975 Fiat X 1/9

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