Seconday oxygen sensors

[Stefan]

I have a 1999 Boxster (wait, don't kick me out of this forum yet) with a 1999 3.4L 996 engine.

I am having trouble tracking down a rough running problem (detailed here http://www.renntech....n-engine-warm).

My secondary O2 sensors are quite old (145K miles). They aren't giving off any faults. Nor were the primary O2 sensors but when I replaced them, my MPG went up significantly.

Is it possible for poorly functioning secondary O2 sensors to affect the way the car runs? Or are they simply to test the efficiency of the cats?

[txhokie4life]

I have a 1999 Boxster (wait, don't kick me out of this forum yet) with a 1999 3.4L 996 engine.

I am having trouble tracking down a rough running problem (detailed here http://www.renntech....n-engine-warm).

My secondary O2 sensors are quite old (145K miles). They aren't giving off any faults. Nor were the primary O2 sensors but when I replaced them, my MPG went up significantly.

Is it possible for poorly functioning secondary O2 sensors to affect the way the car runs? Or are they simply to test the efficiency of the cats?

SWAGGING HERE:

It could be that they are still within tolerance but the information that they send to the DME causes it to change

how it is controlling your fuel mixture. Maybe making it run richer which I think means more gas. You might have been

sending some unburnt gas out the tailpipe -- wasting fuel.

somebody more knowledgeable should be able to really tell you what is what.

A google search gives me this (and seems to support my SWAG -- jeez I just might have to trade my pocket protector for some coveralls :-)

O2 sensors and fuel mixture

unfortunately the discussion from the previous month does not seem to exist.

Mike

[Stefan]

Since one of my other suspects is leaking injectors that article is helpful.

But regarding this question, which is about the secondary O2 sensors, I don't see anything there. The mixture adjustments are done based on input from the primary O2 sensors. As far as I understand, the secondary O2 sensors don't contribute anything. I am looking for confirmation on that fact so that I can eliminate this as a possibility.

[wvicary]

On a conventional O2 sensor the primary sensors are used for short term and long term fuel trim but some ECU's use the secondary O2 sensor to also contribute to the long term fuel trim. Their main use is to monitor efficiency of the convertors and they would only effect fuel consumption if the were biased at their upper or lower limits but then they would also trigger a fault code.

[RFM]

Secondary O2 sensors have to stay approx. stable at 0.6-0.7 V (depending the used type of sensors) into a good functional system, the best and fastest way to analyse, and to solve combustion problems, is to use the results of a 4 gas tester (HC-CO-CO2-O2) at idle and at 3K RPM in combination with the PIWIS. Diagnostic tools alone, are not always good enough to solve real combustion problems, the use of both tools and knowledge can provide the solution. This obviously applies to non modified engines.

[Stefan]

I guess I'm surprised that the secondary O2 sensors' output is important for anything other than generating a CEL since my understanding was that the European cars don't even have them. Since these sensors on my car are well beyond their expected lifespan and the primary ones were definitely bad it is a fair assumption that theses are too.

[number9ine]

I guess I'm surprised that the secondary O2 sensors' output is important for anything other than generating a CEL since my understanding was that the European cars don't even have them. Since these sensors on my car are well beyond their expected lifespan and the primary ones were definitely bad it is a fair assumption that theses are too.

Hmm. I'm not familiar with the problem domain, but what about your catalytic converters? Excess backpressure could certainly lead to a rough idle.

Mark

[Stefan]

The cats are actually much newer than the rest of the car. I'd estimate they have about 40k miles on them. If they weren't functioning efficiently I would think the secondary O2 sensors would detect it and a CEL would be triggered.

[RFM]

Euro cars use secondary sensors since 03. Euro3 use only a pre cat. sensor, Euro4 use a pre and a second sensor and 2 cats on each cylinder bank, Euro5 use a pre and a second sensor, 2 cats on each cylinder bank and DFI engines. A little complicated but feasible.