The air-fuel mixture of a 1988 BMW 320i is controlled by the oxygen sensor, which is located in the exhaust manifold. Because most 1988 320i models are pre-OBD I systems, the oxygen sensor does not have direct adjustment. There are some things that can be done if the sensor is faulty or if the mixture adjustment is extremely out of whack.
Oxygen Sensor
- Verify the oxygen sensor is fully functional. A multimeter can be used to test the sensor's resistance. Specifications are given in the vehicle's repair manual.
- Replace the sensor if it is not performing as expected.
- Note that the oxygen sensor uses a four-wire connector. Two wires are power and ground, and two wires are signal. When heated to the correct operating temperature, the oxygen sensor output voltage should swing from 0-1 volts rapidly.
Idle Mixture Adjustment (Pre-OBDI)
- Locate the warm-up regulator (or idle control valve). It is attached to the throttle body and has two fuel lines and a small vacuum line connected to it.
- There is a large flat-head screw on the device that can be turned in and out to adjust the mixture.
- Start the engine and let it warm up to temperature.
- Turn the idle mixture screw in or out until the engine idles smoothly. Turn the screw very small increments at a time and allow it to adjust before turning it again.
