OriginalSterm

I have reason to believe my car has a bad/intermittent O2 sensor, but what are some of the signs? The car is running awfully rich at idle, both smell and narrow band O2 monitor show this. The last time the O2 bought the farm, the narrow band monitor read off the scale lean but the exhaust smelled rich (as expected). Last year on the track, the wires for the O2 got melted against the exhaust and shorted out. I cut them apart, spliced them back together, and added thermal sleeve to the wires. Is this reason enough to get a new sensor installed?

WBO2 on the dyno showed slightly rich at idle and incredibly rich at the upper RPMs (~9.5-10.0). Dialing fuel out with the PowerPerfect didn't help much.

testarossa_td

Just buy a new sensor...cheap/safe bet.

Chris White

A worn O2 sensor is much slower to respond. An ‘open’ circuit will make the car run richer when the sensor is supposed to be in the loop. On full throttle the O2 sensor is disregarded (in a stock DME based system).

Chris White

Bri Bro

How did you splice the wire together? The outside air for the O2 sensor comes down the wires. Block off the ouside air and the sensor will not function.

OriginalSterm

New O2 sensor it is then...

SD Porsche Fan

A new 02 sensor and a set of plugs really smoothed things out for me. Gas mileage has also improved. The car just seems a little more eager now. No stumbling off of idle, more responsive.

KuHL 951

This thread brings up a related question I have. My son GF just bought a 97 328i. The car was just smogged last December and the PO shop had to put on a new cat. He did not change the O2 sensor and now the car doesn't even come close to passing smog. It's running very rich (HC) at both the 15 and 25 mph pulls but fine otherwise.

My question is "can a new cat be damaged in just 6 months/5K miles if the bad O2 was feeding it a rich brew?" I've heard of cats going bad quickly if run rich and never allowed to get really hot.

Bri Bro

That should do the trick.

OriginalSterm

although i don't really understand the whole "O2 coming down the wires bit", but someone in another thread claiming to be an engineer for Bosch mentioned the same thing.

Bri Bro

When the sensor first came out, they had a hole in the shell of the sensor that supplied the outside air reference. The problem with this approach was that road grim plugged up the hole and the sensor stopped working. Some clever engineer noticed that the amount of air needed was extremely small and the space between the strained wires was more then adequate to supply the outside air reference to the sensor. So, the air is allowed into the strainded wire at the connector far away from the road grim and the problem was solved.

http://www.forparts.com/Bos02update2.htm

OriginalSterm

makes sense to me!

wi54545

You could just disconnect the sensor. It works for me. My car would run extremely lean at idle and low throttle settings and extremely rich is higher settings. Disconnecting it made it idle properly and run normal AFRs on boost. I used my piggyback (mafterburner) to smooth it all out and hopefully will pass emissions this November. I get 29 MPG on the highway and average around 25 overall. Works great, lasts long time.