Oxygen Sensor - ok to unhook it?

[Sonic dB]

I was wonding if one of you experts could advise me on this...

At the PPI, the mechanic noted that my car "hesitated" a little on accellaration and said that it probably needed an 02 sensor...but wasnt a big deal and could be done whenever time and budget permitted.

I did notice this hesitation too...especially when the car is cold...sometimes it even "cut out and then restarted".

* Last week, I had the clutch done. The shop dropped the engine and did the work but forgot to hook the 02 sensor back up.

I got the car back and really noticed that it was running sweet... very smooth accelaration and powerful.

Had to take it back to the shop, to get main seal leak fixed and the mechanic noticed that the 02 sensor was unhooked.... he hooked it back up again and said "if you get the hesitations or problems you can just unhook it or replace it. Some guys just unhook it".

Questions:

1) A new 02 sensor is rather expensive to replace. Will it hurt my engine in any way, if I just unhook it and leave it that way?

2) Assuming that the answer is NO, and that the sensor is just used for emmissions.... am I contributing to screwing up the environment by unhooking it (an ethical question)

The car recently passed CA smog test with 02 sensor in place...so this is not an issue.

Thanks very much.

[Sonic dB]

Also...one other newbie question:

Assuming that I just unhook the oxygen sensor... are there any special considerations that I should adhere to, for performance, environment, and saftey to the engine (fuel types or other mods etc.)?

[speeder]

I have heard of people around here unplugging their lambda sensor on SCs, but was under the impression that motronic, (3.2s), will not run w/o it(?)

[Sonic dB]

forgive my ignorance speeder...is lambda the same thing as the 02 sensor...or are you referring to the forbidden dance of the Latino culture?

[Mike the mechanic]

Sonic, the lambda is the same as O2 sensor. I have found that 3.2's run better with the 02 disconnected. I noticed smoother idle and better part throttle response with no loss in fuel economy.
Mike

[ruf-porsche]

The O2 sensor controls whether the engine is burning lean or rich while it is idleing. The only long term damage I can see if you disconnect the O2 sensor is that if the engine is burning rich, it can damqage the catalytic converter. If you live in an area that has emission testing, you may not be able to pass the emission test with a damage converter.

You can afford a clutch job, but can't afford a new O2 sensor?

[thrown_hammer]

I pulled all the steel wool out of my cat. So if I unhook the O2 sensor it will run better? Sound like a plan.

[BGCarrera32]

The O2 sensor controls whether the engine is burning lean or rich while it is idleing

I have to disagree with that. As I understand it, the O2 sensor is only out of the loop (when plugged in) at initial cold startup while the engine warms, and maybe at WOT (not positive on this though). Another reason is that there have been several complaints of flat spots in the rpm range from owners of DME equipped cars (you see this especially at the 3400-4000 mark) when the sensor goes south. Obviously, the sensor was put there for a reason; to measure air/fuel mixture across the range, and the sooner you find the real problem the happier I think you'll be. If the car looses its ability to meter fuel, you will end up with an overly lean/overly rich condition.

Just my 2 cents...hopefully Chris Bennet will chime in...

-BG

[ChrisBennet]

(Sound of chimes)
With the O2 sensor disconnected, the system reverts to using the base fuel maps which probably ends up being a little richer than when the O2 sensor is correcting things. I don't think it goes to "full rich".
With the O2 sensor connected, the system will keep correcting the mixture i.e. a little richer, a little leaner, a little richer, etc.
When O2 sensors get old they get "lazy" and don't respond as fast.
This is just conjecture but I wonder if a lazy O2 sensor could confuse the engine management into using "last week's recipe" for the fuel mixture because the O2 sensor was still indicating a "too rich/lean" condition even after the mixture was corrected, causing it to over correct?
Personally, I'd prefer to have a good working O2 sensor just for the enviromental benefits. It's the least I can do after I gutted my cat.
-Chris

[JavaBrewer]

I took my 86 in for a full service about 8 months ago. Before that it was running fine...owned it for a couple of months before that so I have nothing to compare to. Anyway, the shop found that the 02 sensor was unplugged so they plugged it back in and told me that I should see an increase in performance and fuel economy. The story as told to me is that when the 02 is either dead or not wired the computer goes to a richer "default" setting. Running in this mode might cause harm to your cat and will probably speed the melting of the polar ice caps but net you a smoother running 3.2.

The postive effects were that my car did accelerate with a bit more vigor in the 4K+ range and I noticed a gain in MPG, but I'm not sure if that was also the result of the full service and valve adjustment. The negative effects were that the car developed a slight stumble at idle and now has 2 flat spots in between 3K and 4K RPMs.

The shop indicated that its normal for a Motronic 3.2 to exhibit these symptoms and that my 02 was fine. A friend of mine owned a 79 SC and a 85 Carrera and said that his SC was a smoother running engine over his 3.2. I'm still not convinced that a 3.2 should run this way but have not started throwing money into new parts - 02 sensor, cyl. head sensor, etc... until I get some more advice. My next full service (15K) is coming up soon so I might just wait for the shop (Black Forest of SD) to report back on their findings.

[addictionMS]

I run my 3.2 in my race car with out the O2 sensor, I can assure you there are no flat spots. The motor also has a chip, headers, and AMF from autothority.

I do sometimes stall at idle, never looked into it, but I should.

Jim

[targa80]

This theory of operation is from the Bosch CIS fuel injection management book. This applies to my car since it is a Kjetronics CIS with Lamba. The pricipal of operation should be the same for your car even with newer equipment.

Because lambda (O2) sensor operation depends on high temperature, the system operates open-loop before the sensor warms up. This can cause driveabilty probems and increased exhaust emissions due to imprecise mixture control. To correct this, many cars have a lambda thermoswitch, located in the coolent (on the cylinder head of air cooled Porsches) to give the control unit an additional input and insure proper fuel metering during engine warm-up.

When the engine is cold, the thermoswitch is closed. The control unit sends a fixed, slightly-rich duty cycle signal of 60% to the lambda control valve (frequency valve). When the thermoswitch warms enough to open, the control unit sends a fixed middle signal, 50% duty cycle. When the lambda sensor (O2 sensor)reaches operating temperature so its signals are valid, the control unit switches to closed loop operation. Remeber in closed loop the duty cycle signals are constantly changing, cycling back and forth between about 45% and 55%, so that the air-fuel ratio is maintained near stoichiometric: Lambda = one.

If you are only having drivability problems when the engine is cold but runs great when hot then you might have a problem with the thermoswitch.

[ruf-porsche]

Originally posted by dmoolenaar
Running in this mode might cause harm to your cat and will probably speed the melting of the polar ice caps but net you a smoother running 3.2.

A friend of mine owned a 79 SC and a 85 Carrera and said that his SC was a smoother running engine over his 3.2. I'm still not convinced that a 3.2 should run this way but have not started throwing money into new parts - 02 sensor, cyl. head sensor, etc... until I get some more advice. My next full service (15K) is coming up soon so I might just wait for the shop (Black Forest of SD) to report back on their findings.

I don't think your friend 79SC has a 02 sensor. I believe the 02 sensor was first used in 1980 and later SC with CIS. The 79SC had an air pump to reduce the emission ( that is not unless someone already removed it).

[Wayne 962]

The 1979SCs do not use an o2 sensor. On the Motronic cars, the O2 sensor is a vital sensor for proper operation of the car. Hands down, you should not run without it, and you should not unplug it. Any 'performance' that you think you might obtain with it unplugged is really probably in your head, or when the car is not using the sensor values (like warm-up, etc.). The values from the o2 sensor are used to help the computer lookup the fuel injector values on the timing maps in the chip, and provide closed-loop feedback. In my opinion, running with a bad o2 sensor will rob you of performance and gas mileage.

-Wayne