O2 Sensor or Vacuum Leak?

[tssfka]

Hi all,

I'm trying to diagnose a problem with my 98 V70XC.

I did some work to replace the PCV system as the vacuum lines had cracked through. This included removing the inlet manifold and replacing with a new gasket. Shortly afterward the car was running a bit rough and then threw a check engine light. The trouble code was for the front O2 sensor circuit.

I cleared the code and thinking I may have introduced the problem, I removed the parts again that I had fiddled with to get to the PCV. On the way I checked as many vacuum connections as I could and reinstalled the inlet manifold with a new gasket again. I'm pretty sure I got the inlet manifold back on correctly and gave the bolts a little extra torque than spec just to be sure.

After all that, the car seems to run a bit smoother but the O2 sensor real time readings are still looking problematic.

At idle the front O2 sensor is varying wildly from 0.3V to 0.6+V and the stft is also varying.

The rear O2 sensor is rock steady at 0.485V.

Any opinions as to whether this is more likely to be a vacuum leak that I haven't found or a faulty O2 sensor?

Would a bad O2 sensor cause the rough running?

TYIA and pardon my ignorance.

[Edit] With further reading, perhaps the front O2 sensor's behaviour is normal? Anyone know how it should behave? At idle should it be a slowly fluctuating value or rapidly changing?

[tssfka]

Found some interesting data in http://en.wikipedia.org/wiki/Oxygen_sensor

Seems that normal O2 sensor range is 0.2 - 0.8 V with 0.45 V the neutral point. Sounds like my sensors are operating okay but if the response of the front sensor is slowing then likely my CEL will come back.



Please feel free to comment on this and the first post if anyone has had experience.