1989 Geo Metro

Engine Performance problem
1989 Geo Metro Front Wheel Drive Manual 149225 miles

This car I have been working on seems to run rich and foul out the spark plugs. I was told by a guy that either the temp. Sensor or the MAP sensor could be the problem. I want to find out more on this issue
March 6, 2009.

Manifold Absolute Pressure (MAP) Sensor (Metro, Prizm & Tracker)
MAP sensor is connected to ECM by a 3-wire harness and to engine by a manifold vacuum hose. ECM applies a 5-volt reference signal from terminal of ECM to MAP sensor. MAP sensor has an internal mechanical resistor which varies resistance based upon changes in engine load (manifold vacuum). As internal resistance varies, return voltage signal varies at terminal of ECM.
ECM interprets this voltage change as changes in engine load and uses this signal to help determine control of fuel injector, shift-up indicator, idle speed control solenoid and EGR solenoid.
With key on and engine off, MAP sensor signal is interpreted by the ECM as a Barometric (BARO) pressure signal. BARO signal is used by ECM to determine control of timing and air/fuel mixture.
Manifold Air Temperature Sensor (Metro & Prizm)
Air temperature sensor is located in the side of air cleaner case. Its thermistor changes resistance with respect to temperature. High temperature causes low resistance. Low temperature causes high resistance. A reference voltage (supplied and monitored by ECM) is modified by sensor resistance. ECM uses this information to determine control (output) signals to injectors.

Dave H
Apr 6, 2009.