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Registered User
Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
CIS oxygen sensor question

My car is an '87 930 with CIS.
Is the oxygen sensor in the loop doing it's thing at all times accept at full throttle?
The way I understand it the full throttle position switch on the throttle body turns off or bypasses the oxygen sensor lambda system at full throttle so you get a richer mixture on boost, but it is working at all other throttle positions monitoring the CO in the exhaust and changing the pulse frequency of the frequency valve on the fuel head to keep the fuel mixture about as lean as possible to lower emissions at all other throttle positions.

My car has a K27 7200, 1 bar spring, B&B headers with heat and single out muffler.
It also has a Garretson half bay intercooler with stock recirculating valve with new seals in it, and it has powerhaus high lift cams.
All this stuff was on the car when i bought it and the previous owner didn't remember which powerhaus cams he had installed when I talked to him once.
The boost comes on around 4500rpm so I think they are timed pretty late.

Anyway, it has a little miss at idle and mild lean surging on slow accelerations around town.
If I disconnect the oxygen sensor it idles absolutely perfectly smooth with no miss at all.
Only thing is it is too rich at times during warm up from a cold start and it oscillates up and down at idle because it is a little too rich and if you blip the throttle in nuetral it will drop below normal idle speed and then come back up to normal idle speed. That is how a slightly rich mixture with fuel injection behaves at idle speed.

I am thinking the best thing to do is just leave the oxygen sensor disconnected and then lean out the CO adjustment on the fuel distributer till it idles smooth without the rich mixture symptoms.
Or is there a way to keep the lambda system operating as it should without idle and low speed running being a little too lean?

Any thoughts?
...thanks in advance,
Old 09-27-2007, 11:25 PM
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Noah930's Avatar
Join Date: May 2005
Location: Los Angeles
Posts: 13,855
Interested in any responses, as well. My car is stock (as far as I know), and the O2 sensor is unplugged, also. It runs perfectly smoothly, other than some roughness at cold start-up. However, when I plug the O2 sensor back together, it surges a tiny bit. I posted a similar question to yours last month, and got this response: O2 sensor & surging I haven't looked into the problem any further, since then. The car's been doing fine for the 10K miles I've had it, though.
1987 Venetian Blue (really looks like grey) 930 Coupe
Old 09-28-2007, 05:45 AM
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GO DAWG GO's Avatar
Join Date: Jan 2007
Location: Inland Empire of the left coast
Posts: 1,266
Set your CO


I had the same problem. Your Air /fuel is running fat (rich). What you can do for a few bucks is go to a local shop smog shop and unplug your O2 sensor set you CO at 2.5 and hook your O2 sensor back up or leave it unplugged. I set my CO and left it unplugged and the car runs perfectly. The smog shop liked my car and didn't charge me but I flipped him a $20 for the courtesy. It took a whopping 3 minutes after the car was at operating temp.

Most of the authorities on this site will agree. I'm relatively new member too and have jumped in this stuff head first...

Have a nice weekend.

1 bad 930
Old 09-28-2007, 05:46 AM
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mark '87 930's Avatar
Join Date: Feb 2006
Location: Toronto, Ontario
Posts: 727
This is just something I'm putting out there, but on mine I have it unplugged as well as it runs poorly under boost with the O2 sensor connected. My thought/theory here is that the O2 sensor bung position on the new headers (I run GHL's with heat) is that it's way to close to the Turbo (only a few inches before on mine) thus giving false conditions. When I ran the stock headers, the O2 sensor did it's job and really ran well. In fact, it had much better low RPM throttle response with the 02 sensor connected (I'm thinking it was because the car wasn't running as rich as before).

I ran an lm-1 and adjusted it accordingly without the O2 sensor and now have never looked back. There are no misses and the idle is very smooth.
1987 930
1956 Chevy 3100
2009 Subaru Forester
2003 KX250 X2(I like my toys!!)
Old 09-28-2007, 06:50 AM
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