Idle Speed & Mixture: Notes

NOTE: Mixture control adjustment screw opening is plugged to prevent tampering. Adjustment is not a normal maintenance procedure and should not be performed unless mixture control unit is replaced or vehicle fails emissions testing.

Remove mixture control unit and punch out mixture plug (steel ball) with a punch. Reinstall mixture control unit. Disconnect oxygen sensor electrical connector.
Remove plugs from header pipes (1 for each cylinder bank) and connect CO probe (5151) to each pipe. See Fig 1. Connect tachometer and start engine. Run engine until normal operating temperature is reached and adjust idle speed to specification.

Fig 1: CO Meter Installation
Courtesy of VOLVO CARS CORPORATION

NOTE: Idle speed cannot be adjusted on California vehicles with constant idle speed system. Idle adjustment is factory-sealed and no adjustment is required.
Turn dual probe adapter to center position. In this position exhaust gases are admitted from both cylinder banks for total CO level reading. Read CO level.
NOTE: When checking CO level, mixture adjustment hole must be covered to prevent a lean mixture.
With CO meter and tachometer still installed, install air cleaner and connect hoses. To adjust CO level, insert adjusting wrench (5102) into adjustment hole and adjust CO to specifications. See Fig 2.
NOTE: After each adjustment, adjusting wrench must be removed and adjustment hole covered to prevent a lean mixture while checking CO level.

Fig 2: Idle Mixture Adjustment
Courtesy of VOLVO CARS CORPORATION
To check CO balance between left and right cylinder banks, proceed as follows: Turn dual probe adapter toward left cylinder bank and check CO reading. If left bank CO level is not within specifications, correct by removing balance screw plug and adjusting balance screw "2". See Fig 3.

Fig 3: Idle and Mixture Adjusting Screws
Courtesy of VOLVO CARS CORPORATION

NOTE: Left balance screw goes to right side manifold and right balance screw goes to left side manifold.
Turn dual probe adapter toward right cylinder bank and check CO reading. If right bank CO level is not within specifications, correct by removing balance screw plug and adjusting balance screw "1". See Fig 3.
NOTE: CO reading should be equal for both banks and correct for the total system.
Recheck CO level with dual probe adapter in center position. If necessary, repeat adjustment procedure. Reconnect oxygen sensor electrical connector.
Recheck idle speed and adjust as necessary on vehicles without constant idle speed system. Remove test equipment, insert exhaust pipe plugs and seal mixture adjustment hole.