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      08-19-2014, 07:12 AM   #1
torchmedia
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z4m engine stutter

Hi guys,

My car is having a frequent, yet unpredictable stutter coming through the engine/exhaust that has really got me stumped. I am hoping you can provide some feedback before I start replacing parts.

Here's a video clip of the issue


Here's what I know:

1. It happens with the engine both cold and hot
2. It happens in dry weather and wet
3. It seems to mostly happen within 2500-3300RPM
4. It does not happen during heavy or WOT acceleration
5. I've inspected the engine for any loose hoses but haven't found any.

The engine has thrown a code, 2198 which is O2 stuck on rich (http://www.obd-codes.com/p2198)

However it has also thrown code 2096 which is too lean! (http://www.obd-codes.com/p2096)

The only modification to the car is an RPi exhaust which I recently installed. And yes, these issues did seem to start around the same time that I switched exhaust. (I also ran the car for about 5 days with no mufflers at all before RPi was installed).


Could it be that I just need so replace O2 sensors?
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      08-19-2014, 07:50 AM   #2
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does light say service engine soon??
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      08-19-2014, 07:52 AM   #3
torchmedia
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Yeah it does. It's come and gone a few times. I have a generic OBDII reader which is how I read the codes listed.
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      08-19-2014, 11:57 AM   #4
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Are the codes for the same bank, sensor 1? Replace the O2 sensor.

I'm not sure if the Z4M is like most cars in this regard, but rumor has it that it is: under WOT, the ECU goes into an open loop mode, effectively ignoring the O2 sensor readings. If the car is fine under WOT, this may be your issue.

Does it happen in the first 15-20 seconds after startup? O2 sensors require 30-60 sec to warm up and start providing accurate readings, so sensor feedback is ignored during that time.
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      08-19-2014, 08:50 PM   #5
torchmedia
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pokeybritches, I can't say for sure about the first 15-20 seconds: I normally don't touch the throttle at all during that period of time and just allow the engine to idle. If it was doing it, it may not have enough effect for me to discern it.

I plugged the car into the OBDII reader again tonight, here are the screens.

Notice both lean and rich codes! And notice that it shows sensor #2 on both banks at 99.2%. I'm not sure what to think of that.



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      08-20-2014, 12:08 AM   #6
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That O2 Bank 2 Sensor 2 voltage looks too low.
I just checked mine and they're both around 770 mV.
I think low = lean.
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      08-20-2014, 08:26 AM   #7
torchmedia
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StickMon, you're right.. after reading up on this they should both be up in that range. However, I thought it was the first sensor that controlled the fuel mixture, and the 2nd sensors only served to confirm that the cats are working?
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      08-20-2014, 02:04 PM   #8
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I think so.
Maybe Bank 2 Sensor 1 is the actual failed one, and the Sensor 2 reading is just showing that Bank 2 really is running lean.
I take it you can't get Sensor 1 data with that scanner?
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      08-20-2014, 08:22 PM   #9
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I'm going to try getting a read on sensor 1 data again tomorrow. I like your theory. Fingers crossed!
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      08-21-2014, 12:04 AM   #10
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IIRC the sensor 1 (wideband) voltages should be 2.00 V and if you can read the current, the nominal is 0.
On these, I believe lower voltage and/or negative current is rich.
Higher voltage and/or positive current is lean.
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      08-21-2014, 04:19 PM   #11
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I had a similar stutter when one of my throttle position sensors was beginning to go out, just throwing that out there.
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      08-25-2014, 09:02 PM   #12
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Thanks z4msteve, will keep it in mind.

I replaced Bank 2 Sensor 2 today and it made absolutely no difference I have another o2 sensor on order now for Bank 2 sensor 1, but won't get it til later this week. If that doesn't do the trick I will go crazy. lol.
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      08-25-2014, 11:53 PM   #13
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Ok. Lots of possibilities. ECT sensor, TPS sensor, fuel pressure regulator, etc.

Primary o2 is only one of many possibilities.

http://www.freeasestudyguides.com/rich-fuel-trim.html
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      08-28-2014, 09:10 PM   #14
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Success!

I installed the new o2 sensor in position 1 of Bank 2 today, and the engine is now running completely smooth again. What a relief.

Thank you to all who have helped with your input.
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      08-29-2014, 12:20 AM   #15
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It's good when a plan comes together.
Any chance you can rerun the test you have in post #5 just for a sanity check?
I'd be interested to see if the Bank 2 Sensor 2 voltage came back up in the 700 odd mV range.
It's only a matter of time before I get into the same fix, but with the mileage on my car, when one goes, they all go.
I'll just drop a grand and replace all 4 plus the EGT sensor.
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      08-29-2014, 07:01 AM   #16
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I did re-run the scanner yesterday. What I observed was the bank 1 and bank 2 voltages ran in the mid-600 to low-700 range most of the time. When they did change, the changed at the same time and by the roughly the same amounts.

Before, Bank 2 voltage was fluttering all over the place.

Looking back I probably should have tried changing the first o2 sensor first and I would have saved the extra $140. Live and learn!
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      08-30-2014, 03:41 PM   #17
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Are sure it's nothing to do with Vanos?
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