I've been getting poor gas mileage, so I did the following tests, as suggested by the FAQ, Art Benstein and others. The Idle Air Control valve by the way seems to be operating fine.

The first set of test results were with the idle mixture trim pot set to 350 ohms (likely the factory setting), as measured between green/yellow (pin 2) and yellow (pin 6) on my LH2.2 AMM:

Air Mass Meter (Art's test):
- ignition on, but car not running, voltage on white/red wire (pin 3) was 1.26 volts (my understanding is that this would make the engine run lean). Art suggested replacing the unit if it was less than 1.5 volts. Fanning air into the unit, I couldn't get it above 2.0 volts
- With engine running, it was 2.2 volts (Art suggested it should be high 2's or 3).

Oxygen Sensor measurements With the sensor output disconnected from the ECU (FAQ - Bosch test):
- 2500 RPM, all vacuum hoses connected: 0.035 volts (indicates lean)
- 2500 RPM, vacuum line to the fuel pressure regulator pulled and plugged: 0.82 volts (indicates rich)
- 2500 RPM, vacuum leak created by pulling the hose to the flame trap: 0.025 volts
- 750 RPM, all vacuum hoses connected (sensor still disconnected): 0.05 volts

Idle Mixture Test Point (FAQ - "Setting Base Idle"):
- Voltage at pink wire test point near battery stayed at 13V constantly.


Because I wasn't seeing the expected oscillation between high and low voltages at the idle mixture test point ("Setting Base Idle" test), I decided to adjust the AMM idle mixture screw, until the voltage was high (13V) half the time, and low (I believe about 0 volts) half the time. After this adjustment, the trim pot measured 440 ohms.

The new Oxygen Sensor measurements were now:
- 2500 RPM, all vacuum hoses connected: 0.045 volts
- 2500 RPM, vacuum line to the fuel pressure regulator pulled and plugged: 0.8 volts
- 2500 RPM, vacuum leak created by pulling the hose to the flame trap: 0.030 volts
- 750 RPM, all vacuum hoses conneced (sensor still disconnected): 0.8 volts


Both before and after the AMM adjustment, if the oxygen sensor is hooked up, the voltage at its output oscillates between 0.2 and 0.7 volts. I understand that the ECU's job is to make compensations to acheive this result, so it tends to indicate that the ECU is fine, but doesn't prove that either the oxygen sensor or AMM are within spec.

The only big difference between the two tests, is the oxygen sensor output (when disconnected) at idle. Originally it indicated very lean; with the new setting it indicates very rich. Is the 0.8 reading the expected result in a perfectly operating system?

I realize that my adjustment to the AMM may simply be compensating for an out-of-range oxygen sensor. My big question is, based on this data, can I definitively conclude that either or both the oxygen sensor and air mass meter need replacing?


Sorry for the long post, and thanks in advance for any input.

David