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Art:
Before I forget, thanks for your excellent web page on the blower motor replacement. With your detailed description and pictures, the job was simple. Without knowing the tricks, it would have been a lot more painful.
You suggest not spending money on an AMM if I think it’s running lean, but I’m not positive that this is an accurate conclusion. It was based on three things:
1) in an old post, you had suggested that a static AMM output voltage lower than 1.5 volts would cause a lean operating condition (mine was 1.26). I guess your point is that if the AMM is putting out a lower than expected voltage, the ECU thinks that the amount of air passing through it is less than the actual amount, and hence injects less fuel than is required for the actual amount of air. If never adjusted, this makes sense. I'm thinking though that the system design could be based on knowing that a set amount of air will be passing through the AMM at the specified idle speed, and the trim pot can therefore calibrate the AMM/ECU combination to this known amount (as determined at the idle mixture test point) and make the two work together correctly. If this is true, then the static AMM output voltage is not such an issue, providing that the trim pot can bring the idle mixture test point output within range.
2) because the disconnected oxygen sensor's output voltage is supposed to drop to 0.2 when a vacuum leak was induced, it infers that it should be above 0.2 before the leak. Since mine was already below 0.2 before the leak, I assumed it was lean. The spec with the leak says 0.2 or below, so I suppose my 0.025 volts is within spec. Ideally I suppose the spec with all vacuum hoses connected should be in the middle of the range, about 0.5, but as you pointed out, it's hard to achieve without feedback.
3) the idle adjust test point was sitting at 13 volts constantly, which I understand indicates a constant lean condition
Before all these tests, I had assumed it must be running rich in order to use too much gas. I figured that if it was running lean, I would get decent gas mileage but just no power. You point out that if the oxygen sensor is slow to register the exhaust has gone from lean to rich, the injectors are injecting too much fuel for a period of time, hence degraded fuel economy. I suppose an oxygen sensor whose static output always indicates that the exhaust is leaner than it actually is, would consistently call for more fuel than required, and make the engine run too rich. If this is the case with mine, it would mean that all my readings suggesting lean are erroneous, and my adjustment on the AMM actually pushed the mixture into a false "normal" mixture, when in fact it may be constantly rich now.
I do have an oscilloscope. How does one use a scope to interpret whether the sensor output is slow to react or not?
As for my actual mileage, I have to confess that my assessment is subjective. My odometer doesn't work! I should pursue the odometer gear repair (assuming that's the issue) so I can accurately measure my fuel economy.
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