I have an E36 318i that is running rough and stumbling. It got me to thinking about the oxygen sensor. If I just disconnect it, other than a check engine light, what will it do the way the car runs? With no voltage signal it'll think that it is really lean? Or really rich? I can't picture this. Is there any chance that this is a way to test the O2 sensor and that the car may clear up if I do this or is it a complete waste of time?
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Aug. 1, 2011 10:02 p.m. glueguy Reader
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Aug. 1, 2011 10:10 p.m. oldtin Dork
Has it thrown a code yet? obdI or II? Check vacuum lines and intake hoses for cracks and leaks. They're sensitive to leaks.
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Aug. 1, 2011 10:21 p.m. Keith SuperDork
It'll think it's really lean, and dump in a bunch of fuel to compensate. Like oldtin, I'd want to know if it was OBD-II. If so, plug in and look at the sensor readings.
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Aug. 1, 2011 10:26 p.m. glueguy Reader
92, OBD-I, no codes. Could be a number of suspects, this just got me more thinking about how the system would react if one were to disconnect the sensor.
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Aug. 1, 2011 10:47 p.m. oldtin Dork
Any stored codes? Turn ignition to the on position, pump gas pedal to wot and release 5 times in quick succession - count the number of times the cel light flashes - will give one long flash followed by blinking. Count the blinks. how to check obdI codes
My e28 would run really rough when it rained hard - shorted out the 02 wires. If you pulled the battery cable for a few minutes it would reset and run fine until the next time it got soaked.
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Aug. 2, 2011 11:20 a.m. Jerry From LA HalfDork
glueguy wrote:
92, OBD-I, no codes. Could be a number of suspects, this just got me more thinking about how the system would react if one were to disconnect the sensor.
It'll go full rich into "limp home" mode. Vacuum leaks, etc.
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Aug. 2, 2011 11:23 a.m. 92CelicaHalfTrac SuperDork
I don't actually run an o2 sensor on the MX6. From what i understand, it could technically be "pre OBD-1," but it does throw a code. Also throws it into safe mode, which means it dumps ALL the fuel, and ALL the timing.
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Aug. 2, 2011 12:36 p.m. itsarebuild Reader
Jerry From LA wrote:
glueguy wrote:
92, OBD-I, no codes. Could be a number of suspects, this just got me more thinking about how the system would react if one were to disconnect the sensor.
It'll go full rich into "limp home" mode. Vacuum leaks, etc.
i dont know if they changed anything on the '92 models, but my 87 325 has a disconnected o2 sensor and it runs fine.. a little rich, but not so much that it wont pass the smog test. it passed this morning as a matter of fact....
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Aug. 2, 2011 1:41 p.m. Zomby woof SuperDork
OBD1 has no limp home mode. Once it sets the code, and doesn't correct itself, it will go into a default setting.
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Aug. 3, 2011 6:20 a.m. foxtrapper SuperDork
On a number of older cars I've disconnected the O2 sensor, and haven't seen diddly in the way of change in how the engine ran.
That may be a reflection of the state of the O2 sensor in each of those cases. Perhaps the sensor was so old/bad and slow to react it was already being ignored. But I rather don't think so. Too many cars that all ran fine, with and without the O2 sensor connected.
All in all, at least with the older cars, the O2 sensor seems a trim adjustment to the position on the map. With the general guide being air flow through the meter. Vacuum or load in some fashion, RPM, and temperature being secondary. The O2 sensor being a distant third.
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Aug. 3, 2011 9:58 p.m. Streetwiseguy Dork
European, post 89, it will probably sit in the middle of its range, but you will have to clear the memory to make it go there quickly. Thats actually how I test O2 sensors of that vintage, because it seems they will sometimes continue to swing voltage high and low, but they are out of range. Disconnect the batt, if it starts and runs ok, but gradually goes rich or lean, I disconnect the battery (again) and the O2 sensor and see if it then stays ok.
Edit: I mention post 89, because prior to that unleaded fuel was not universally available over there, so the Euros mostly ignored the O2 sensor. A totally screwed sensor would drop mileage maybe 5 mpg. Post 89, 25 mpg.
