A long time ago, I posted regarding a fuel economy issue: Air Mass Meter and Oxygen Sensor test results - do they need replacement? 200 1986 .

In summary, there are some suggestions in that thread, that since I had achieved closed loop operation with the idle mixture adjust on the AMM, that the AMM was fine (even though the voltage test with ignition on and motor not running was low, indicating a lean condition). Art wasn't necessarily ruling out an AMM issue however. I have since replaced tha O2 sensor, and got the odometer working. Mileage is about 20 mph (US gallons).


The other problem I have is with cold starts, which I also posted back then: starting problems in extreme cold only 200

Since then, I cleaned the throttle body, and afterwards, I had to screw the idle adjust screw all the way in to keep the idle speed down to an appropriate level. This would suggest a vacuum leak, but I can't find one using the "propane leak" method. The advice in the old thread was to try starting with the AMM disconnected. I did so this morning (-25C), and it fired up and ran smoothly. I shut it down immediately, reconnected the AMM, and when it started it ran rough for a while, like before. This quite definitively points to a lean condition.


Art's thoughts in the original post were "I would not spend money for an AMM especially for your poor mileage symptom if you think it is running lean." It appears then that these two symptoms conflict with each other; the disconnect AMM "fix" points to a lean condition, but the poor mileage results point to a rich condition. Is it possible for an AMM to be out of spec, such that it can be adjusted for proper closed loop operation, and deliver poor mileage and poor cold start performance?


For reference:
- new O2 sensor
- idle mixture adjust does achieve 50% duty cycle, but reads low on pin 3 with ignition on, engine not running
- ECT has been verified accurate via voltage measurements at ECU connector as per FAQ
- have used propane torch gas flow to check for vacuum leaks; couldn't identify any
- grounding test point does close IAC
- compression is even; using one gauge yields 155 psi; another shows 180
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near