Faulty O2 Sensor

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Pawley

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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.
 
Pawley said:
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.
Click to expand...

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!!!
 
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.
 
alan moore said:
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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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
 
alan moore said:
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.
Click to expand...

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 :clown:
 
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
 
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.
 
N5GTi6 said:
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.
Click to expand...

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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