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Unplugging the O2 Sensor? Bad Idea?

Fella's, 1987 930

I was having some problems with my car running rough, Idle fluctuations and this surging feel while driving. I decided to unplug my 02 sensor and readjust my AFR just to see what happened. I unplugged the 02 sensor and the car billowed black fuel smoke and started running very badly. I adjusted the AFR by turning the set screw about 3/4 a turn counterclockwise until it smoothed and started to die. I then turned clockwise to richen and the car smoothed out and idled great. I adjusted the screw like I used to with my webers carbs. The car runs better than ever with low end torque, smooth transistion from gear to gear and more power...

I plug the 02 sensor back in and the idle dipped and began to loose the smooth running feel.

OK now what...Should I leave the O2 sensor unplugged or is there another issue with the frequency valve or something? ..help. I need to take the car to a local smog shop and set my AFR at 2.5 per ther CO readout.

Has anyone had this problem?...I cant believe how well my car runs now..

Bob
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Old 07-25-2007, 10:45 AM
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Like you, I have years of tuning carbs myself, but with such an expensive engine, you really need to be making AFR adjustments with a wide-band sensor connected - Innovate LM-1 for instance. Tuning by feel is not advisable. Especially when you need to know what the engine is doing at full boost, WOT.

Most people that I have spoken too, have disconnected the the factory O2 sensor (plugging the port) and choose to set the mixture and leave it....not relying on the Lambda circuit to stabalize AFRs. I think at idle, my AFRs are set to 13.3:1.
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Old 07-25-2007, 11:08 AM
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However, I have lots of mods. Fully adjustable Leask WUR, Leask RPM solenoid, IA fuel head, twin plug ignition, headers, fully de-smogged including the AAV. So I'm not sure if there's more to it than just removing the O2 sensor and plugging the port.
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Old 07-25-2007, 11:12 AM
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Setting idle mixture can be done with a Gunston. Buy or borrow one and set to 3.0 - 3.5%.
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Old 07-25-2007, 11:20 AM
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The O2 sensor only affects the A/F ratio at idle. No danger in throwing it away.
Once you use a Gunsen or LM-1 and know where 2.5% is you will be able to tune it by ear. My guess is you're close now.
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Old 07-25-2007, 12:08 PM
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Quote:
Originally Posted by RarlyL8 View Post
The O2 sensor only affects the A/F ratio at idle. No danger in throwing it away.
Once you use a Gunsen or LM-1 and know where 2.5% is you will be able to tune it by ear. My guess is you're close now.

The O2 sensor feeds a signal to the lambda computer which, in turn, varies the fuel pressure in the lower chamber of the fuel distributor via the frequency valve, affecting enrichment. The lambda computer will keep the AFR at 14.7-1 so the cat will do it's job.
Unplugging the O2 sensor will run the system in open loop mode, allowing 12.5-1 AFR (by setting CO at idle to 3-3.5%) on CIS. A CO set at this value improves torque.
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Old 07-25-2007, 12:35 PM
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I was running 9% CO

I drove the car down to a local smog shop and the worker liked my car and decided to check it for free. At about 220 degrees oil temp it was running at 9% CO per his digital display. I pulled out the 3 MM allen and adjusted it to ~ 2.5-3.0 per the display. The car ran nicely before and is very crisp now. I would like to test drive it in the morning when it is cool. BTW, today the outside temp was 97...

It pops more now out of the fabspeed exhaust on deceleration...

Would this high temperature have any issues on the performance and AFR? What a dynamic question??? It seems my 930 likes cooler weather and runs stronger than a day like today.

should I plug the O2 sensor back in or just leave it unplugged?

Whatta ya fellas think?

Bob
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Old 07-25-2007, 06:45 PM
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Paul explains nicely how the O2 sensor works. You don't need it. Turbos run best at less than 14.7:1 A/F at idle as you have found yourself.
Higher temps do affect the A/F ratio. Generally it will run more rich as the heat rises. Less dense air = lower number in the "air" part of the A/F ratio.
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Old 07-25-2007, 07:39 PM
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Brian,

Adjusting it yesterday at 3% at 97 degrees, will I have a problem driving the car at 60-80 degrees. Will I be lean? Is that gunson gastester worth it. Is it good enough to periodically check and adjust at different times of the year at different temperatures. Winter adjust/Summer adjust type of thing?

Thanks

Bob
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Old 07-26-2007, 08:19 AM
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I don't see your location listed.
You shouldn't have any problems when it gets cold but it never hurts to check. Mine has a range of about 1% between cold dry weather and hot humid weather.
There is a relationship between the idle mixture and the cold and warm fuel pressure as well. I found that when my cold pressure was nearly out of spec my mixture would have less adjustability on the rich side once the engine is up to operating temps. On hot days the idle would oscillate so I tweaked the mixture just enough to calm the idle. All that went away when I re-adjusted my cold pressure. The A/F range between cold dry and hot humid stayed at 1% but the threshold for oscillation was no longer within that same range.
How much of that is due to old worn parts? I have no idea.
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Old 07-26-2007, 09:23 AM
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Brian,

Rancho cucamonga, Ca. Do you recommend the gunson?

Bob
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Old 07-26-2007, 09:35 AM
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Quote:
Originally Posted by robert_snyder View Post
...Is that gunson gastester worth it...

My $0.02, I thik you'll get more utility out of one of the Innovate wide-band based products. I use my Innovate LM-1 with a tail pipe clamp in my zork. It places the sensor in a very good location, near the turbo exhaust outlet, being that the zork is such a short pipe. YMMV in using a tailpipe clamp with a more elaborate (and longer) muffler/exhaust. Many claim that the clamp is not an ideal method of locating the sensor. The LM-1 is nice because you can data log and analyze and there are other peripherals that can be added.

While I have no hands on experience with one, I've heard the Gunson, can be sort of fussy. Of course this could all be based on user error.
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Old 07-26-2007, 09:58 AM
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I have a bias when it comes to the Gunson because I used to build and repair Gas Chromatographs in a former life. The Gunson is no more than a single point thermol conductivity detector. Knowing how they work is a huge advantage when it comes to setting up and troubleshooting the Gunson.

All that aside, if I was going to spend $100 on a Gunson I would hold off until I could add $200 to it and buy an LM-1 instead. VERY useful device.
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Old 07-26-2007, 12:28 PM
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Sand-man,

I'm thinking that way too. Maybe install the O2 for the LM-1 in my current O2 sensor location. I would like to keep an eye on my AFR. Thats probably what I'm going to do.

Also- I had a local mechanic tell me today that 3% CO is good but under load the
engine likes fuel and could get more toque at 6%CO at 1 bar...whatta ya think?

Is this true?

Thanks

Bob
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Old 07-26-2007, 12:32 PM
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Quote:
Originally Posted by robert_snyder View Post
Sand-man,

I'm thinking that way too. Maybe install the O2 for the LM-1 in my current O2 sensor location. I would like to keep an eye on my AFR. Thats probably what I'm going to do.

Also- I had a local mechanic tell me today that 3% CO is good but under load the
engine likes fuel and could get more toque at 6%CO at 1 bar...whatta ya think?

Is this true?

Thanks

Bob
To be honest, I've heard that locating a wide-band sensor BEFORE the turbo is not the way to go. Most of the setups I've seen locate the sensor AFTER the turbo, approximately 6" downstream of the turbo exhaust.
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back in the saddle: '95 993 - just another black C2
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Last edited by sand_man; 07-26-2007 at 12:41 PM..
Old 07-26-2007, 12:38 PM
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Yep, don't place the sensor before the turbo. The stock sensor is not a wideband sensor like the LM-1 uses.

Yes, on boost higher than 3%CO is needed. But do not confuse this with the CO at idle. Still set your idle CO at 3%.

Oh, I have an Innovate XD-16 kit to install...
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Old 07-26-2007, 01:40 PM
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