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Re: Air/Fuel ratio adjustment "old school"
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Posted by Dana [Email] (#2822) [Profile/Gallery] (more from Dana) on Tue, 19 Nov 2002 23:38:08 Share Post by Email
In Reply to: Air/Fuel ratio adjustment "old school", MS, Tue, 19 Nov 2002 21:54:31
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The problem is....the leaner adjustments you are making with the fuel enrichment screw will normally be countered acted by the pulse duration valve which will be adding enrichment. It is only when the screw adjustment travel is so extreme as you lean it out that the O2 sensor/Pulse duration control is out of control range that it can no longer add additional fuel to give the right mixture. This is generally about 75/25 to 95/05 pulse duration that one starts to sense that the car is starting to starve for fuel. But at this point the pulse duration valve has no additional enrichment to give when it is needed the next day for cold starting. In other words the fuel enrichment screw does not normally control the amount of enrichment directly....it is used to set the enrichment control system (O2 sensor/pulse valve) at the mid point of its control (55/45, 55% open duration of the pulse valve, 45% closed duration of each of the 70 valve cycles per second). Although SAAB indicates that the enrichment screw would never need adjustment, in real life airs leaks, aging electronics, etc. does indeed tend to shift the mid point of the control from the warm idle 55/45 pulse duration timing. Most of the time this is in the form of enrichment leaning since there would likely be air leaks with time.

I like straight forward procedures like the next person but one does need a meter which can read either "duty factor" or "pulse duration" which many digital multimeters can do now. My procedure to check if the control system is working and to make adjustments to place the control mid point on spec is...

1. Start the car and allow to warm to normal operating temperatures. Durating the first approximately two minutes, the fuel enrichment pulse duration should lock at 60/40 valve open to close time duratin ratio. At the end of this time the 60/40 locked enrichment ratio is released for control by the O2 sensor. Then the control system will add/substract fuel to maintain about a 50/50 pulse duration ratio for warm idle conditions.
If not you can adjust fuel enrichment screw to give you this 50/50 ratio which is the mid point of the control system.
2. Then disconnect the sensing wire from the O2 sensor while you are running. Short out the sensing wire to the car body. The control system fuel enrichment should add enrichment by increasing the pulse ratio above 75/25. It corresponds to an simulated output voltage from the O2 sensor of 0 volts (the car is running very very lean)
3. Then disconnect the sensing wire from the car body and just hold it. The mixture ratio should first become leaner by going below a 50/50 ratio and then slowly increase to a fixed 60/40 ratio.
4. Reconnect the wire to the O2 sensor and the control system should go back and regulate around a 50/50 pulse ratio.
5. If you depress the full open throttle valve switch the ratio should be 45/55 for non-tubo, 80/20 ratio for turbos.

Sorry this was so long but the important fact is....the enrichment screw should be used to set the mid point of the enrichment control system but does not adjust the actual enrichment unless the screw adjustment is so extreme that the control system is out of control range to provide the proper amount of enrichment for acceleration, idle, and/or cold starts.
2.

posted by 172.136.11...
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