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Thread: Help! Trying to undo conflicting information, Z3 1997 Oxygen sensor

  1. #1
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    Help! Trying to undo conflicting information, Z3 1997 Oxygen sensor

    Everyone, I am hoping someone could un-conflict this nuttiness. This is a BMW Z3 2.8L. 85000 miles

    I have the infamous 0402. I did scan tool this with an iCarSoft i910 (I love this thing, by the way.)

    The cat on this 1997 is BMW part number 223 7521 003. A second code is 1-433-691

    I see clearly there are two oxygen sensors: One pre-cat; the next is post-cat.

    I cannot see any oxygen sensor at the exhaust manifold. The i910 reports 4 oxygen sensors; but two seem to be "standing by".

    Question 1: Does the 1997 BMW Z3 6-cyl have only two O2 Sensors? Documentation in this forum suggest "post facelift" Z3's have a different arrangement from "pre-facelift".

    Question 2: Where are the respective locations of Sensor 1, Sensor 2, Sensor 3, and Sensor 4?


    Any insight you might be able to share is very much appreciated!!

    Thank you all in advance!

    Ken

    Red Bank, NJ

  2. #2
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    Typically, this era of BMW engine of this displacement has four O2 sensors.
    Two forward pre-catalytic converter, two post converter.


    Sent from my iPhone using Tapatalk

  3. #3
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    In the situation where there are four oxygen sensors; wouldn't there be two Cat converters?? In this case, it appears the exhaust manifold has two pipes in a "Y" configuration to a single catalytic converter.

    It seems the 1997 2.8L has a few weird configurations to the M52.

    I am about to lift my other BMWs and compare...

  4. #4
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    Depending on model, year, engine, and original market, the car can have either one or two cats. There will be one O2 in front of each cat, and one behind.. However, P0402 is an EGR code, which no Z3 has.


    /.randy

  5. #5
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    OEM site shows 1997 2.8 01/99 to 12/97 has 1 pre cat sensor and one post cat sensor.
    http://www.realoem.com/bmw/enUS/show...diagId=18_0252
    Put your last 7 digits of your Vin number in the search page to check what your car came with from the factory
    The fact that your code reader shows 4 sensors may be misleading, the system is looking but they are not there so you only see the two that are there.
    http://www.realoem.com/bmw/enUS/select


    Last edited by colb; 04-14-2019 at 04:10 AM.

  6. #6
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    Colb: Thank you VERY much! You NAILED it.

    VIN Number 4USCJ3328VLC01253 suggests: DOM: 1996. ..all details are exactly as you suggest.

    Two Oxygen Sensors for the older model car.

    I assume O2S #1 is forward, O2S #3 is aft. (downstream.)

    Now I can address this - one way or the next. This is a "fun" car - I am not too keen to toss an OEM $1200 into a Catalytic converter.. I do happen to be an EE. ..And I do know a thing or two about RC circuits to smooth out the O2S response. ...

  7. #7
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    Yes #1 is pre cat #2 post cat normally.
    If the Cat is still good then the sensors are probably needing replacement due to age, they don't last forever and given the age of our cars and mileage it would be a good idea to change them. If you have diagnostic software that can monitor live data you could check the function of the sensors. Pre Cat sensor when viewed should show a switching voltage as it ranges up and down, softwhere usually shows this as a smooth wave form rising and falling. The post Cat sensor when viewed ranges far less and can look almost static but that is dependant on what it is seeing in the exhaust. Best to replace with oem sensors, either Bosch or Seimens brands that the factory would have used. They are not cheap but far better than aftermarket sensors, buy cheap pay twice is my experience. The sensors are important and affect the fueling of the car and of course affect the emmissions which for all of us these days needs to be effective in order to pass our annual tests. If things don't improve with new sensors then start looking at the Cat, if you have to replace that your new sensors will of course fit onto the replacement.
    Last edited by colb; 04-14-2019 at 12:08 PM. Reason: correction

  8. #8
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    Help! Trying to undo conflicting information, Z3 1997 Oxygen sensor

    I believe the OE sensors are Bosch, if so NTK produces the Bosch sensors, and overall they have been working fine for me.


    Sent from my iPhone using Tapatalk
    Last edited by bluptgm3; 04-15-2019 at 09:26 AM.

  9. #9
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    Hi Gents. Thank you all VERY much for the input. I plan to follow your instructions...

    OEM sensors are indeed Robert Bosch. Older model was packaged differently.. I have one spare; the upstream sensor (short lead).

    As I have the upstream; I'll change it until I get the second downstream model. I already removed the downstream O2 Sensor -- coked a little - cleaned it carefully and blew it out with clean air. (no solvents.)

    Sensor #1 (front) varies from 0.6V - 2.5V; Oscillating. downstream oxygen sensor is 4.xxV and steady for the interval I examined.

    Hypothesis: is upstream O2 Sensor is worn; causing rich mixture. Coking up the Cat and downstream sensor.

    My plan, as follows:

    1. Install the new upstream;
    2. use the cleaned downstream
    3. Drive the car 150 miles (the CEL trips after 100-125 miles)
    4. Decide whether the downstream sensor is the problem...
    5. Or if the Cat is the problem.

    Again, everyone == thank you. When I make the determination; I will send an update. ..With the hope I'll help the next guy out there with a similar problem.

  10. #10
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    Sounds like a good plan upstream sounds like its a bit lazy, they do get like that with age. Post cat when viewed in diagnostics always seems to be quite static when I have checked mine. Another check would be to put a gas tester on the tailpipe and see what CO it is putting out, not sure what reading you should see but obviously a high reading would indicate rich running. Must have a look on the web to see if the CO% is listed anywhere. I have a Gunsons gas tester I used on previous carb equipped cars and it was preety accurate having compared readings with garage testers at annual test time. easy on old cars but all this electronic trickery is progress they say.

  11. #11
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    You can't tell much about a sensor, especially a titania sensor (passive), by quoting two voltage points. If you really want to test it that way, you need a lab scope. No one does that. Also, the rear sensor is only turned on while the DME is running the cat efficiency test. It's not on all of the time. When it's off, it will show a voltage in the 4.xx range.

    There is an easy test. Look at your long term fuel trims. Or trim in your case. If it's near zero, less than +/-5%, then the odds are you have a bad cat. If the trim is out of range, then you either have a bad sensor (not likely) or you have a mixture control problem, aka a vacuum leak.

    O2 sensors in these cars are actually quite stout. Short of the ceramic heater being broken from impact shock damage, they really don't die. I replace a fair number of sensors due to heater circuit issues. The number I've replaced due to faulty/skewed operation can be counted on one finger. Just like the goofy-lubes pushing 3K mile oil changes, those pushing a mileage replacement interval are those selling O2 sensors.


    /.randy

  12. #12
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    Randy, that is wildly valuable information. I did not know recognize the sensor is biased; then sampled only over intervals. Hence my reading of a 4.25V quiescent value is nothing more than a bias voltage from the ECU. I do indeed have an oscilloscope (it's what I do for a living..) ..However, I would need to build a "break-in" harness. Moreover, it is unclear whether the sensor digitizing loop is single-ended or differential (I guess I could reference the frame as ground?)

    Please explain this: What is the long-term fuel trim? Is this a parameter one could read from the i910 gadget? Could you hint what parameter to stare at? This i910 has a million different devices on the CAN bus, many which do not exist on a Z3. Further, it is unclear what some of the parameters actually are. I get the mass airflow, RPM, etc. Some of the other sensors are not too clear to me, nor what would be considered an "acceptable range" for a given parameter.

    Colb, I'll be replacing that upstream sensor saturday morning..

    With two other BMWs (and two MB's); I am trying to learn a bit more about these ODB diagnostic computers. I'll be ordering the i910 for MB this weekend..

  13. #13
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    Ah.

    The far more common zirconia sensors are active. They generate a voltage nominally between 0 and +1V, with a realist operating range of 0.2 - 0.8V. The MS41.x DME used by the M52 and S52 uses titania based sensors. These are passive, changing resistance rather than generating current. As I remember, the DME biases to +5V, and the sensor sinks to signal ground. Signal ground is not the same as chassis ground. I've never really mapped out the exact voltage vs mixture curve. I prefer to use an OBD2 data stream where the DME spits out a simulated zirconia voltage that I am used to working with.

    As an aside, even computers using zirconia sensors will bias the signal line towards 0.45V as a neutral reference to cover for a mal-adjusted air gap (aka unplugged) condition.


    /.randy

  14. #14
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    Alright.. Some feedback -- and hopefully some information for the next poor soul in this situation.

    Confirmed the voltages to-the-letter, Randy. You nailed this. Voltages on the downstream Oxygen sensor seem reasonable most all of the time. There are momentary aberrations. Replaced the oxygen sensor (downstream) -- and have been driving the car.

    As of this morning -- I drove the car to NY State from NJ -- 192 miles with two pauses in-between -- no CEL. In the past, 2-3 drive cycles for a total of 100-140 miles would trip the CEL with the 0420. It is unclear whether a drive cycle must bring the car down to "cold start"; as today the engine was restarted at temperature -- I did not bring the i910 with me to read ahead.

    Will report back what might be the end-game.

    I did buy a 1" "spacer" -- just in case. (I want to avoid installing it, however.)

  15. #15
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    Follow-up: 240 miles on Z3 with new Oxygen sensor. No CEL.

    Summary: Change upstream Oxygen Sensor; de-coke (thick with coke) downstream sensor. P0420 reset -- Apparently solved.

    Thank you all again for your assistance.

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