why disconnect the O2 sensor?

[KFC911]

One more try:

[Tim Walsh]

Keith,
Try using only alphanumeric filenames. spaces and "(" screw it up.

That is the O2 sensor on my engine but I have no clue what it's from (SC or 3.2). It's also no longer on the engine since MFI has no use for the O2 sensor.

[ruf-porsche]

Originally posted by stlrj

The fast and the stupid .......

Sound like a nice movie title for next year car flick for the summer.

[JackT]

So does disconnecting make the car faster?

[Superman]

No.

[1fastredsc]

No power increase, it's more of an issue of longevity to the engine. When running that lean with an o2 sensor hooked up, you can shorten the life of an air cooled motor. I don't speak for all of them, but i believe the sc originally wasn't designed for emission stuff, just had it slapped on. This is why we get such a large power gain from getting rid of the emission stuff and backdating the exhaust. As far as polluting, we are kind of a select crowd, there aren't many air cooled enthusiasts on the road. Now for a water cooled motor that has better control of it's head temps to not run a cat is plain ignorance. Besides, having your car tuned properly and having constant attention is a bigger factor in passing the smog nazi test than anything else.

[VaSteve]

Originally posted by 1fastredsc
No power increase, it's more of an issue of longevity to the engine. When running that lean with an o2 sensor hooked up, you can shorten the life of an air cooled motor. I don't speak for all of them, but i believe the sc originally wasn't designed for emission stuff, just had it slapped on. This is why we get such a large power gain from getting rid of the emission stuff and backdating the exhaust. As far as polluting, we are kind of a select crowd, there aren't many air cooled enthusiasts on the road. Now for a water cooled motor that has better control of it's head temps to not run a cat is plain ignorance. Besides, having your car tuned properly and having constant attention is a bigger factor in passing the smog nazi test than anything else.

By the SC series, ho wcould it *not* be designed for emissions stuff? It had been in place for a while at that point. Wasn't it the 74-77's that suffered all the pulled head studs and what not as a result of hotter temps due to emissions?

[stlrj]

Yeh... and all the bulletproof air cooled 911's run 02 sensors with the 78/79 SCs as the only exceptions...how do you explain that?


Joe

[911nut]

At an air-fuel ratio of 14.7/1 you get maximum efficiency (MPG) and lowest emmisions. At an air/fuel ratio of 12.5/1 you get maximum torque. If anyone doesn't believe me it's all in Posbt's book on CIS. Look it up.
Disconnecting your O2 sensor and resetting the idle mixture to 3-3.5% CO gives the engine a real kick in the pants. My car really came to life after I did this. Also, part throttle, low speed operation is improved as you get rid of the surging caused by the frequency valve playing with the fuel pressure in the fuel distributor. Also, the car will idle smoother for the same reason.
Call me stupid if you like but I prefer a powerful, good running car instead of a EPA-castrated transportation device.

[stlrj]

resetting the idle mixture to 3-3.5% CO gives the engine a real kick

Paul, were your readings before or after the cat?

What were your CO readings when you where having all the surging with the 02 sensor connected? Was it set to spec, less than 0.8% CO taken ahead of the cat? If not, that could have been your problem.

Joe

[slakjaw]

when you unplug the o2 sensor it goes to .440, you can measure the voltage at the plug it will be around .440. this is not rich not lean, kinda in the middle.

also, the o2 sensor is absolutely useless at idle, they are designed to work at 2000 rpm or more. unpluging it should make no diffrence in idle quality unless there are other problems with the fuel or iginition system.

also, once you pass 80% throttle the computer will give the motor a set value of 12:1 or even 10:1 air fuel ratio. at that point it dosent even care what the o2 is seeing. so i dont under stand how unpluging it could give much more power.

i have the flu and just worked a 12 hour day so maybe i am just out of my mind right now.

Kyle

[911nut]

Originally posted by stlrj
Paul, were your readings before or after the cat?

What were your CO readings when you where having all the surging with the 02 sensor connected? Was it set to spec, less than 0.8% CO taken ahead of the cat? If not, that could have been your problem.

Joe

I have no cat, just a test pipe.
The CO reading was at .8% prior to my tweeking it.
When I say surge, I mean a slight surge, actually a "lope". It happens at low speed (25-30 MPH) and it's the result of the frequency valve doing it's job (when the O2 sensor is hooked up).

[911nut]

Originally posted by slakjaw

also, the o2 sensor is absolutely useless at idle, they are designed to work at 2000 rpm or more. unpluging it should make no diffrence in idle quality unless there are other problems with the fuel or iginition system.

also, once you pass 80% throttle the computer will give the motor a set value of 12:1 or even 10:1 air fuel ratio. at that point it dosent even care what the o2 is seeing. so i dont under stand how unpluging it could give much more power.

Kyle

The K-lambda CIS is designed to yield 14.7/1 A-F ratio at all throttle settings other than WOT to keep the cat from being overwhelmed. Unpluging the O2 sensor and turning the set screw changes the relationship between the air sensor plate and the piston inthe fuel distributor. Exposing more of the metering slits on the piston for a given air flow will enrich the mixture. At all throttle positions other than WOT there will be more torque.
The k-lambda computer will vary mixture even at idle. It's the constant overshooting of the control system that causes the uneven idle.

[Lorenfb]

911nut provided the most accurate comments about the O2 sensor. Once
disconnected on either a K-Lambda or Motronic, the mixture setting should
go the neutral mode (about .50 volts input). Here the CO setting can be made
before the CAT. Ideally the setting is about 1.5% up to maybe 2.5% to 3.0%.
This will give the best overall performance. A much higher value can possibly
damage the CAT.

Once the O2 sensor is connected, the CO should come down to less than .5%.
Under hard accel, e.g. full throttle, both systems ignore the O2 sensor. The
K-Lambda system utilizes an enrichment relay for additional fuel. Check out
this website (www.systemsc.com) on the Technical & Graphs pages for some
additional info.

Have Fun
Loren
'88 3.2

[sammyg2]

My SC passed smog just fine two years ago with the O2 sensor disconnected, and that was in So Cal on a roller dyno test.
That means that disconnecting the O2 sensor does not necessarily result in more pollution.

I have the O2 sensor hooked up now and it does cause a very slight modulation in power at cruising speed.

CIS is just not accurate enough to control the air/fuel mixture at exactly 14.7 to 1 throughout the entire range of operation. The lambda system tries to make it more accurate but this can cause some swinging from overcompensation. My O2 sensor reads either rich or lean, nothing inbetween. It is this switching back and forth which causes some modulation. A very small change in the lambda reading makes a significant change in HC.
The early SCs did not have an O2 sensor. The lambda system was an add on to the existing system in an attempt to meet ever tightening regulations under certain operating temperatures and conditions.

By disconnecting it some folks can set it where they want it and get rid of the very slight surging that many KE-Jet cars seem to have at steady cruise.
I could keep making adjustments until I nail the mixture just right at steady cruising speed, but chances are I would have to compromise on the ideal mixture setting at some other operating condition.
Does that make me ignorant? I don't think so. Does it mean my CIS system may not be operating quite as well as when it rolled off the showroom floor? Of course it does. It's 24 years old.
I may eventually get it perfect if I throw enough money at it, or I may just leave it where it runs well and I'm happy with it and just go drive it.

[1fastredsc]

Originally posted by stlrj
Yeh... and all the bulletproof air cooled 911's run 02 sensors with the 78/79 SCs as the only exceptions...how do you explain that?


Joe

That's not totally fair, especially since before 78 the 911 motors had mag cases which are the main reason as the why the studs can pull. Well that and from what i hear, the thermal reactors.
911 nut is also right, 12.5 is where peak torque is acheived. With the o2 sensor hooked up, 14.7 will always be the goal fuel mixture.
As far as the design, the optimal design for an NA exhaust system is to have equal length headers which with the sc and carrera aren't possible because of the wonderful heat radiating cat by the left bank of cylinders.
And as said before, taking care of your engine is much more important to "pollution" than having a cat strapped to the exhaust. I had a 944 hollowed out cat pass emission in NJ with no problems, but a nissan pickup flunk with a brand new factory cat. Turned out that giving it a good tune up and running some heavy duty injector cleaner through it is what passed it, not the new cat.

[wanabe911 owner]

Do I connect or leave alone, I have only owned the car for a month. Idle hunts until it warms up, sometimes will shut down when getting off the gas and on the brake like pulling to a stop. I have fixed a few obvious vacume leaks.

[Thack]

The lambda control unit is designed to cycle the freq. valve at a certain duty cycle when the O2 sensor is unhooked. So you can unhook the O2 sensor and the ECU will not adjust the mixture. That is why you can adjust your mixture at idle while warm and be close for most throttle openings. Also the O2 cannot tell the ECU what to do untill the warm up switch reaches about 350* C. The O2 sensor can adjust your mixture for altitude. It is really a good system when adjusted properly. The ECU is really an add-on for the 1979 CA models and the 1980. It actually came from the 924 Turbo and was not really designed for the 911. Maybe that is why it is not perfect.

Mike

[Thack]

Correction, the O2 sensor must reach 350* C. The temp switch warms up and then lets the ECU adjust mixture at a certain temp value. I assume close to the same time when the O2 sensor reaches 350* C.

[wanabe911 owner]

So I connected the 02 sensor last night, no change at first, then when the car warmed up, two things happened. 1 the engine quit stumbling when I get off the gas to slow to a stop. 2 My hesitation problem started again! A little background about a month ago I took my car to a local service center, Vertex Miami (just purchased) to sort out a few issues, one of which was that the car would no smoothly accelerate from a standing start, once past 3000-3500 RPM's or at WOT it would take off. So he had it for a month, changed injectors, checked AFM, compression, and fixed quite a few vac leaks. Said he could not get rid of the hesitation. So I told him I would pick it up. I really wanted the car back, so I drove the car, they had it warmed up, and the hesitation was gone, but it never shut off on me. So I took it home, on the way home every single time, I would slow to a stop (off the gas on the brake) it either stumbled, or shut off completely. Sorry about the ramble. Any ideas on what I should do next.