High Altitude Emission Control System

The high altitude emission control system is ECM controlled via input from the MAP sensor and ECM control of the MAP vacuum switching valve (VSV). Refer to COMPUTERS AND CONTROL SYSTEMS. Use the following test to verify VSV switching function.

1. With ignition "ON" and engine "OFF," blow into MAP sensor port of VSV. Air should pass through the VSV filter to atmosphere and not out the intake manifold port.
2. Attach a vacuum gauge to MAP sensor port of VSV.
3. Start engine and vacuum gauge should read intake manifold vacuum.
4. Suspect a defective VSV if it fails these tests. However, be aware that ECM and/or MAP sensor problems can affect the results.
5. Clear trouble codes when testing is complete. Refer to COMPUTERS AND CONTROL SYSTEMS.