TheSamba.com :: View topic - Second time is a charm- but why??

Fri Mar 28, 2008 3:19 pm

Ok,

I went to the DEQ for the first time with Ruby and it failed the HC emissions in a bad way- off the charts at 474 ppm (supposed to be under 220) and CO at .165). I couldn't get her in for a tune up so I fiddled around a bit myself.

I figured it may either be a faulty CAT or O2 sensor. Everything else seems to be running fine- I am getting great gas mileage and plenty of power with minimal hesitation. Purs like a sewing machine.

After hunting around here and checking out the Bently, I decided to unplug the O2 sensor and figured it wouldnt hurt to get it re-tested. I passed with flying colors (HC at 110 ppm, CO at .017).

What is going on? What may be going on with my O2 sensor? It seems to be running a lot cleaner without it. Does this mean it is faulty and needs to be replaced?? What else could this be signalling? Is there any harm running without it? I'd rather have everything be running as it should, so...

Or did I just get a bad draw with the first test and the O2 sensor is unrelated?

thanks for any advice,
rob g

Sun Mar 30, 2008 2:45 am

I would like to bump this, because my van just failed its idle only test and the HC is running in the same range, and I have a NEW o2 sensor

Sun Mar 30, 2008 5:38 am

With the O2 sensor unplugged, the engine runs in "closed loop" mode, meaning the ECU uses a fixed program, and ignores input from the O2, temp-II sensor.

Check fuel pressure delivery, vacuum lines, the O2 and it's wiring, and the temp II sensor.

Sun Mar 30, 2008 1:37 pm

It was an idle only test so doesn't it ignore the O2 sensor on idle?

Sun Mar 30, 2008 2:40 pm

Mon Mar 31, 2008 9:09 am

bucko wrote: With the O2 sensor unplugged, the engine runs in "closed loop" mode, meaning the ECU uses a fixed program, and ignores input from the O2, temp-II sensor.

Check fuel pressure delivery, vacuum lines, the O2 and it's wiring, and the temp II sensor.

You got it mostly right, bucko me boy, except that with O2 disconnected, the system is open loop.

The loop being referred to is the lambda feedback loop, where the O2 signal voltage varies in relation to exhaust oxygen content compared to ambient air, and the ECU responds to that by varying injector duration, and the O2 signal responds to the change in mixture, ECU responds again, always trying to narrow mixture around lambda, or 14.7:1 air-to-fuel ratio, ad infinitum.

So when you disconnect the sensor, you've opened the loop.

Also, temp2 info isn't ignored when open loop, only the O2 signal because it isn't being supplied to the ECU. As I understand it, Temp2 is not used to determine mixture once engine temp has passed the warmup threshhold, but is still used by the ECU to modify ignition timing.