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Chasing the wrong problem
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 20 Mar 2001 13:21:41 Share Post by Email
In Reply to: HELP! How do I correct lean mixture at idle on '88 9Kturbo?, Chris, Mon, 19 Mar 2001 13:39:10
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The voltage when the O2 sensor is disconnected from the computer is meaningless. The idea is that the O2 sensor acts as part of a closed loop system to control the mixture. With the sensor disconnected, the fuel injection computer detects this and assumes a value. It just turns out that value gives you 0 volts at the sensor.

Disconnecting the sensor and then worrying because the system isn't perfect is like saying a bicycle is poorly made because it won't balance by itself.

It sounds like your system is working properly, or close to it. But there are some things to try.

First off, the 380 ohm reading on the AMM is just a STARTING point. Saab recommends setting the AMM to 380 ohms, because that will put most cars into the range where the voltage on the O2 sensor is swinging around 0.5V. You ain't done yet. What you do now is adjust the screw in the AMM while monitoring the voltage from the O2 sensor (STILL CONNECTED TO THE COMPUTER). Crossing 0.5 isn't the important part - you want it to spend 50% of the time above 0.5V, and 50% below. If the voltage spends more time above or below, then you'll be running rich or lean.

I have personally seen John Moss, who heads Saabs technician training school, adjust the AMM. He started at 380, but it took some serious adjusting before he got the 50% duty cycle. Just for kicks we measured the AMM, and it was up over 700 ohms.

It's important for the mixture to swing above and below 0.5 volts, and to have a 50% duty cycle. That slightly lean to slightly rich swing makes the 3-way catalytic converter work.

Lastly, I'm suspecting you Don't have an overly lean mixture. Lean mixtures usually mean higher combustion temperatures and less fuel, which results in higher NOx readings. An elevated HC implies a richer mixture. Hydrocarbons are usually the result of incomplete ignition, which can come with a rich mixture.

Adjust the AMM to get a 50% duty cycle. Make sure the timing is dead nuts on. If your NOx readings are very low, I'd consider actually adjusting the car to a slightly leaner mixture (O2 sensor output spending maybe 60-70% of the time below 0.5V).

Good luck!
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