O2 sensor disconnected

[Biff]

Last Sunday I decided to change my spark plugs and discovered that my O2 sensor was not connected at the male/female plug location left side engine bay. I haven't noticed poor running conditions and my idle was rock solid. I reconnected the plug an perceive no change in running condition. Is my O2 sensor shot?

[don gilbert]

what kind of car is it? on an SC's, I think the service interval on the 02 is 30K, and like 60k on the 3.2 and later cars.

[Biff]

Sorry....its an stock '86 Carrera.

[hcoles]

I've unplugged mine, either way couldn't tell much difference. Someone here (e.g. Steve Wong) knows what happens when the CDU doesn't get a signal there and what map is used.

[don gilbert]

you should be able to tell a diff. on your fuel mileage.. the 02 only works (closed loop) at idle and part throttle. When you go to wot, system goes into open loop (not working) so it has no effect on performance, just fuel mileage and emissions. When ecm is not getting a signal, it goes to a fixed rate, usually on the rich side.

[Biff]

So basically it effects in a negative way the emissions and gas mileage?

[Gogar]

yes. Unhooked = crappy mileage, stinky tailpipe. That's mostly it, though.

[mca]

Mine is disconnected (82SC) and I get 25mpg highway which is pretty good.

[tcar]

Originally Posted by mca

Mine is disconnected (82SC) and I get 25mpg highway which is pretty good.

As the above posts point out, the O2 sensor has NO effect on the highway.

Only has an effect that affects mileage and emissions at idle and low throttle.

[mca]

Are you saying that the sensor is out of play at constant acceleration?

Never heard that before.

[mca]

Sorry if this is getting off topic, but I still believe that the sensor is in closed loop during highway driving. That IS part throttle.

Therefore highway mpg WOULD be affected by the O2 sensor.

I disconnected mine because I did not like how the sensor made the car run at part throttle / constant acceleration. I felt a slight "trailor hitch" or surging which was most noticeable in 3rd gear around 2500-3000 rpm. Unplugging the sensor lead to smoother running - so the sensor wad certainly in closed loop.

[don gilbert]

Tcar, thats not quite correct. The sensor on a motronic car works until the wot switch is met. The slight surging MCA feels on the highway are the gentle fluctuations of the ecm keeping the mix at stoiche, (easy to feel on k-jet, not so much on motronic) which is a good thing, for mileage, emissions, AND making your engine last longer. The inception of the 02 is what made cars have a life span of up to 300k instead of 100k Leaving the sensor disconnected on a motronic car or k-jet will keep it on the rich side, which over time will wash away your ring seal and dilute your oil.

[stlrj]

Originally Posted by Gogar

yes. Unhooked = crappy mileage, stinky tailpipe. That's mostly it, though.

If the base mixture is set correct per spec, there should be no noticeable difference in performance, mileage or smell.

All base mixture settings are done with the 02 sensor disconnected and set rather lean at <1%. With the 02 sensor connected, only minor adjustments are made by the DME to keep the mixture stoichiometric.


Cheers,

Joe

[don gilbert]

those minor adjustments add up to gallons over a period of time.

[ischmitz]

Without the O2 sensor the 3.2 DME will not react to mixture changes and keep its trim value centered in the adjustable range. In addition to that the open connection can pick up electric noise and cause the trim value to "walk" all over the place. The lead acts like an antenna to ignition noise. So it is not a good idea to leave the O2 sensor not connected.

The trim range is approximately ±17%. This is quite significant and can cause a lot of change in engine behavior. The base mixture adjustment only applies during idle and has little effect once the throttle is opened.

The O2 mixture trim is only used during part-throttle (highway cruising, moderate city driving, etc). During WOT the control loop is disabled.

Later DME versions for the 3.6 will throw up fault codes and use a default value for the mixture but the 3.2 unit isn't that sophisticated. It simply has an integrator stage that either drives the mixture trim up or down with a slew rate of about 1 - 3 seconds (depends on RPM). If you run a catalytic converter a correct mixture is crucial for maintaining a proper catalytic converter temperature.

And yes, emissions suck when the O2 sensor is disconnected - so do yourself a favor and buy a new sensor and connect it. You might as well use this feature.

Ingo

[tomphot]

Originally Posted by don gilbert

The inception of the 02 is what made cars have a life span of up to 300k instead of 100k Leaving the sensor disconnected on a motronic car or k-jet will keep it on the rich side, which over time will wash away your ring seal and dilute your oil.

I have an ROW car that was designed to run without the O2.
The car ran like crap when I got it - I removed all of the feds stuff and got rid of the wires that were spiced into the box. The car runs like a champ now.

So I'm wondering - why did Porsche design a car that had a short engine life for the rest of the world?

[don gilbert]

{So I'm wondering - why did Porsche design a car that had a short engine life for the rest of the world]---Toms quote


Thats just the way it is Tom. Thanks? to our government, we now have cars that get great mileage, cleaner air coming out of the tailpipe than goes in, engines that last longer. The 02 keeps the mix so correct at idle and part throttle that there is no waste. (fuel running past your rings or blowing out you tailpipe.) I don't think you will find any ROW Porsche's or any cars now-days that don't have an 02. Most have 4 now. The 02 system has had a bad rap since they come out, just because most don't understand what it does, they think it is killing performance, when actually its the best thing to happen to a combustion engine. it only works at idle and part throttle to keep your engine clean, and mileage up. Whats wrong with that? And as far as Porches making a car that has a short engine life, I never said short, I said 02 street engines will last longer. Just a fact.

[Jim Williams]

Re the discussion on the purpose of the Lambda System -

The prime reason for the Lambda System was to reduce emissions.

However, in addition to fuel economy, one of the reasons presented by Bosch in their tech book on K-Jetronic for the Lambda System utilizing the O2 sensor was to present a stoichiometric mixture to the 3-way cat to reduce emissions. Not only is this the best mixture for complete fuel combustion, but the TWC operates most efficiently in this range for reducing noxious emissions. This is also covered in the Charles Probst book on Fuel Injection. This same logic applies to Motronic.

[rswamy]

84 911 ROW, I can't find O2 sensor harness. Read following post.

Oxygen sensor harness Help

[rswamy]

84 911 ROW, I can't find O2 sensor harness. Read following post.

Oxygen sensor harness Help