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Thank you for your advice. I agree, the O2 sensor does seem most likely culprit. Against that is that the readings I'm getting from the O2 sensor are exactly what I would expect - looking at the state of the plugs - from an engine that is running lean due to a fault elsewhere in the fuel mixture feedback loops.
Can I just bat some thoughts out for comment to help me clear my thinking? My guess, and I welcome any advice on where to get more detailed information, is that after warm-up the AMM still has a role in giving advanced warning of changes in air conditions, e.g. when opening and closing the throttle. It may also have a role in stabilising the fuel system, e.g. if exhaust conditions go 'off scale' for the O2 sensor during wild transient conditions like during rapid gear changes. I suspect that the ECU algorithm keeps the AMM on side but makes it much less influential than the O2 sensor after warm up.
Assuming this, my reasoning goes as follows. 1. Faults of the O2 system are likely to result in RICH mixture because most faults result in less voltage being returned to the ECU and a lower average voltage would tell the ECU to richen the mix. The lowest voltage from the O2 sensor (around 2.5 volts) is a bit odd. It should have gone down to < 0.1 volt
It might be due to me using a digital multimneter because it only samples a few times a second. 2. My main thought is that it looks like something forcing an offset onto the feedback loop. 3. One possibility is that the algorithm in the ECU has gone bad. I'd sort of expect the ECU to be either pretty stable or really broken, and it seems to work fine for all other functions. 4. Another possibility and the one suggested by the local Saab specialist is that the AMM has drifted. It was certainly oiled up, but cleaning its insides made no difference to how th eengine runs. A peculiar thing is that in setting the AMM it makes little difference to O2 voltage reading at any setting once the O2 sensor has started cycling. It changes the O2 settings for a few seconds and then the O2 voltage settles back to its usual range. There is a limit at around 750 ohms where the engine knocks coming off idle. 5. I guess ECU might do a small amount of compensation for AMM offset in setting up the warmed up mixture. If so, AMM hot wire errors wouldn't contribute to a consistent offset in mixture weakness. 5. Boost cutback is almost always accompannied by subtle or not so subtle knock for a brief (1/2 sec or so) period. This suggests weakness also occurs for transients and takes more suspicion off the AMM. 6. A large error in intake air temperature signal could be the sort of signal that might be the basis of a continuous bias in the mixture. Again, it might be expected the ECU would do some stabilising compensation but whether they did that in 1987 I don't know. The fact that the IAT sensor has already been shown to be a problem causing consistent mixture weakness also puts the oily finger of suspicion on it.
The car is relatively low mileage for an 1987 auto at 121,000ml (194,000 km). It was serviced by the main Saab dealer for the first half and has a Mercedes dealer service at 160,000km . I bought it 2000km ago. The O2 sensor looks like it has been replaced not long ago because its outside and wiring is cleaner than the bits around it. Though it might have been just removed and cleaned as its wiring is all twisted.
I welcome your thoughts,
Terry
posted by 203.14.16...
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