Went to look at my O2 sensor, and found this...

[Hotwatermusic]

Background: I have a 74 911 with an 83 SC engine, the lambda box is under the passenger seat and connected (to what now, I don't know). I have no cat convter. And now I see I have no O2 sensor either. Well, I have one, but the wire has been cut and connects to nothing. Can anyone tell me what exactly I am looking at here, why someone would have done this, and what I am missing or gaining by having no O2? I had always thought the O2 was needed for a smooth running car. But maybe I don't need it, I don't know. The car runs alright, but the idle is spotty and the cold weather is torture on the CIS. I've only had the car for about 6 months, so I am slowly starting to figure out all the weird little things I hadn't noticed before. Thanks for any info and advice!

[Vin-barrett]

With the O2 not connected, the lambda box is buzzing the frequency valve at a fixed value.
An advantage would be, being able to set your mixture richer than factory settings (more power).
Disadvantage would be fuel consumption and emissions.

I unplugged mine long ago and like it like that way for reasons above, however, you will probably get mixed answers about not using the O2 sensor.

[psalt]

An advantage would be, being able to set your mixture richer than factory settings (more power).


This is not true. Output (power) is measured at WOT. CIS lambda ignores the O2 sensor input past 35% throttle. Either way, WOT fuel delivery is determined by the default FV pulse and the fixed air sensor height adjustment.

There are more benefits in running the O2 sensor than disadvantages.

[McLovin]

Originally Posted by Hotwatermusic

why someone would have done this

my guess is they did it to mask some kind of running problem they weren't smart enough to fix properly.

[Forza]

Originally Posted by psalt

An advantage would be, being able to set your mixture richer than factory settings (more power).


This is not true. Output (power) is measured at WOT. CIS lambda ignores the O2 sensor input past 35% throttle. Either way, WOT fuel delivery is determined by the default FV pulse and the fixed air sensor height adjustment.

There are more benefits in running the O2 sensor than disadvantages.

Agreed; you're fooling yourself if you think you can so easily outthink Porsche. That stuff is meant to be there. Drive it like it is at present if you like, but don't think for a moment that it's better than the original design.

[Hotwatermusic]

How much of a headache am I looking at trying to get a new O2 sensor up and running on the car? How would I even begin to diagnose what has been done beyond just cutting the wire? Would getting a Fuel/Air gauge like the LM-1 talked about lately in another thread give me an idea of what the engine and lambda are doing without the sensor? I don't have any yearly emissions testing to go through with the car, so is my biggest issue with having no O2 poor gas mileage?

[psalt]

I don't have any yearly emissions testing to go through with the car, so is my biggest issue with having no O2 poor gas mileage?



No, the big picture answer is, proper fueling from the O2 sensor feedback loop is the main reason for the large increase in service life of SI auto engines in the last 30 years.

My advice is to get a generic, one wire Bosch sensor for a few dollars, hook it up and see what happens. You need to be careful about the shielding of the wire, but it can be successfully spliced. Then hook up an analog dwell meter to the test port and the system will speak to you. Not quite OBDII, but you can diagnose the entire system and make accurate mixture adjustments. You may find that the system was hacked to mask the typical CIS vacuum leaks, but they can found and corrected.

[Forza]

Actually, I pretty much dealt with this same issue when I bought my '83 SC. The O2 sensor was all coked up, and the O2 sensor relay and acceleration enrichment relay (the small box attached to the larger lambda control unit) were fried. Once I installed new ones, the frequency valve emitted the proper signals, but these changes revealed a shody WUR, which I tried refurbing but ended up replacing with a freshly rebuilt unit.

So, from your pic you definitely need a new O2 sensor! You may also need some combination of the OXS relay, AE relay, lambda box, and/or WUR. Check the frequency valve, too. The sensor and relays are inexpensive and new; the lambda box and WUR are reasonable if used/rebuilt. CIS testing equipment and a multimeter will lead the way....

Oh, and I strongly agree with psalt: a functioning OXS system ensures 14:1 under varying circumstances. Earlier today I drove from about 5,500 to 9,000 feet for the fun of it, and the car's temp stayed steady and ran smooth the entire time. Can't say that for my twin-carbed 2002ti!!

[RWebb]

"twin-carbed 2002ti"

but still a great car!

and worthy of a thread with pics in OT...

[psalt]

a functioning OXS system ensures 14:1 under any circumstance.


This is not true for CIS lambda, nor for any other engine management system with closed loop control. Under acceleration or WOT, the mixture is much closer to 13:1, not 14:1, for several reasons. Under part throttle cruise, the system dithers the mixture back and forth around 14.7:1, usually with an accuracy of 0.1 AFR average. You could more accurately say the system almost never "ensures 14:1 AFR". There is some altitude correction with CIS lambda below 35% throttle, but it cannot correct for air density like a system with a mass flow air sensor, outside closed loop.

[Forza]

Originally Posted by psalt

a functioning OXS system ensures 14:1 under any circumstance.

Yeah, you're right: I Glenn Becked that one. Total BS. Pretty much as soon as I wrote that I was like, ???? but I couldn't get back to my PC to retract it.

[psalt]

I'm jealous, my 2002's only have single carbs...... But one has a pukka, NOS, 2002 turbo steering box.

[Forza]

Originally Posted by psalt

I'm jealous, my 2002's only have single carbs...... But one has a pukka, NOS, 2002 turbo steering box.

No need for twin-carb jealousy; I sold my Zero-Two prior to getting my SC!

[steely]

For what it's worth - and I don't have a clue yet as to understanding some of this stuff or if this even applies - but my PO and his mech disconnected the O2 sensor (at the connector) and swore it was better this way. Something about the cat delete and the SW chip. But they also admitted that the car had cold start and idle problem which they couldn't figure out.

As soon as I reconnected it (for grins), the cold start and idle problems disappeared - wah lah!
I know mine is a different vintage than yours - but I've left mine on ever since.

[don gilbert]

as Paul and others said, unplugging the 02 has nothing to do with performance, as once you go past the 35 degree throttle switch, it is not working, (open loop). It only works at idle and part throttle. Having an operable 02 will get you close to 25 mpg on the hwy, make your throttle response great, lower emissions, and make you engine last longer. You can still bump the mix up on an 02 car, and still have the great mileage and driveability. people who unhook it thinking it will increase performance are wrong. You can get a Bosche one wire at Oriellys for under 20 bucks. And the relay under the seat is around 20 from Pelican. (you must use the correct relay, the uni 5 pins from the parts store will not work) The o2 relay runs pretty warm, and have a high fail rate, most i have looked at are bad. The ecm is one tuff box on the 80-83 sc, it is prob. ok, i pulled one out of a flood car once, dried it up, and the thing worked.