Faulty O2 Sensor
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  1. #1
    Tadpole Pawley's Avatar
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    Default Faulty O2 Sensor

    Can anybody fill me in on typical car behaviour which has a dodgey oxygen sensor??

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    I've got a 205 Si that is running very rich, and hesitating violently only when the thermostat passes half way.

  2. #2
    1000+ Posts N5GTi6's Avatar
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    Quote Originally Posted by Pawley
    Can anybody fill me in on typical car behaviour which has a dodgey oxygen sensor??

    I've got a 205 Si that is running very rich, and hesitating violently only when the thermostat passes half way.
    From my experience when the sensors get a little old, they get a little clogged up with carbon and crap, and they get a little lazy - ie: they don't register the correct readings any longer. Your's sounds like it's in that area. When they don't register correctly any more, the ECU pumps in more juice to get a proper reading - hence the rich running.

    Not sure whether the SI uses a single wire sensor or multi wire, but they're cleanable to a certain extent. You'll have to have access to either an ultasonic cleaner (my local fuel injection place has an ultrasonic chamber) or a blowtorch to heat it up and get the crap out of the air flow 'vanes'.

    Acknowledging that this thing 'plugs' a hole in your exhaust and the car will be undrivable when you remove it, you'll have to climb under the car (hopefully whilst it is on stands or ramps - not a hydraulic jack!) detach the electrical leads from it, and use about a 22mm spanner (sorry - can't find my spare sensors in the garage to size them for you...yes, I have 'spares'!) to undo it. A socket probably won't go over the head of it.

    Remember to use some copper grease or some sort of heat resistant grease when putting it back in, otherwise the heat from the exhaust will make sure you never remove it again!!

    Cheers

    Mind you, you could just have a clogged air filter!!!
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  3. #3
    1000+ Posts alan moore's Avatar
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    I would look more toward the engine temp sensor for the injection than the O2 sensor. The temp sensor will affect the mixture much more than the O2 sensor, that generally can only alter the mixture by 20%. There are hopefully specs for the resistances you should see on the temp sensor in your workshop manual.
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  4. #4
    Too many posts! JohnW's Avatar
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    Quote Originally Posted by alan moore
    I would look more toward the engine temp sensor for the injection than the O2 sensor. The temp sensor will affect the mixture much more than the O2 sensor, that generally can only alter the mixture by 20%. There are hopefully specs for the resistances you should see on the temp sensor in your workshop manual.
    When the oxygen sensor on our 1995 306 XT failed, all that we really noticed was slightly rough running and poor acceleration in parts of the rev range. It didn't run rich that I noticed. After a while, we thought "not running properly" but there was never a major problem.

    Hope this helps.

    Cheers

    JohnW

  5. #5
    Tadpole Pawley's Avatar
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    Quote Originally Posted by alan moore
    I would look more toward the engine temp sensor for the injection than the O2 sensor. The temp sensor will affect the mixture much more than the O2 sensor, that generally can only alter the mixture by 20%. There are hopefully specs for the resistances you should see on the temp sensor in your workshop manual.
    where does the engine temp sensor sit?

    story goes as such:
    i did a driveway service on the car, replaced oil and filter, plugs and leads, injector cleaner and fuel filter.
    problem presented as described above
    replaced old leads.... no fix
    replaced old plugs.... no fix
    got compression test... all good
    put new longer nosed plugs... no fix

    took the car for a quik drive this afternoon. its fine at start and until the engine warms up a bit, then the missing and chugging starts.
    i have a haines manual, printed in the UK, so doesnt have too much on the Si in it...... but thats out in the shed at the moment

  6. #6
    Moderator Alan S's Avatar
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    First basic rule when troubleshooting anything mechanical; if it was operating correctly prior to you doing the work; that's where you start looking.
    Forget the O2 sensor, engine temp sensor & everything else until you retrace your steps.
    Working on the assumption that you installed the fuel filter correct way around, the first place to look is the plug leads. If they are not properly connected, (by that I mean fully seated onto the plug top) they will cause a problem.
    Have you got them in the correct order? Haven't mixed up in particular nos 2 & 3?
    Have a look in the dark & see if there are any sparks tracking around the coil when the engine is running rough. Also, be sure that the LT wires are clean inside the socket & plug as it is possible for the LT to track across to the HT side and cause misses.

    Hope that is some help. If that all checks out, then start looking for other causes.

    Alan S
    If it ain't broke, use a 12" shifter.....that usually does the trick!!

  7. #7
    1000+ Posts N5GTi6's Avatar
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    Ahhh, now the truth comes out - the problems have manifested since a service and other fiddling - back to square one then before pulling out exhaust sensors.
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  8. #8
    Tadpole Pawley's Avatar
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    Quote Originally Posted by N5GTi6
    Ahhh, now the truth comes out - the problems have manifested since a service and other fiddling - back to square one then before pulling out exhaust sensors.
    well............. yeah i guess
    but as stated, as far as i can everything has been returned back to its original state (incl plugs, leads, and fuel filter).
    i've had the car to a mechanic, and he himself replaced the old plugs and leads with new ones.
    i queried with him about the order of the plug leads, but am still not sure.
    looking at the engine from above:

    O O O O
    1 2 3 4

    and then the coil pack from the passenger side:

    O O
    2 3
    O O
    1 4


    Correct?

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