Toyota Corolla 1970-1987 Repair Guide

Mixture Control (MC) System


The mixture control valve, used on certain engines, aids in combustion of unburned fuel during periods of deceleration. The mixture control valve is operated by the vacuum switching valve during periods of deceleration to admit additional fresh air into the intake manifold. The extra air allows more complete combustion of the fuel, thus reducing hydrocarbon emissions.


  1. Unfasten the vacuum switching valve line from the mixture control valve.
  3. Remove the intake manifold hose from the valve.
  5. Remove the valve from its engine mounting. Installation is performed in the reverse order of removal.


  1. Start the engine and allow it idle (warmed up).
  3. Place your hand over the air intake at the bottom of the valve.

Keep your fingers clear of the engine fan!

  1. Increase the engine speed and then release the throttle.
  3. Suction should be felt at the air intake only while the engine is decelerating. Once the engine has returned to idle, no suction should be felt. If the above test indicates a malfunction, proceed with the next step; if not, the mixture control valve is functioning properly and requires no further adjustment.
  5. Disconnect the vacuum line from the mixture control valve. If suction can be felt underneath the valve with the engine at idle, the valve seat is defective and must be replaced.
  7. Reconnect the vacuum line to the valve. Disconnect the other end of the line from the vacuum switching valve and place it in your mouth.
  9. With the engine idling, suck on the end of the vacuum line to duplicate the action of the vacuum switching valve.
  11. Suction at the valve air intake should only be felt for an instant. If air cannot be drawn into the valve at all, or if it is continually drawn in, replace the mixture control valve. If the mixture control valve is functioning properly, and all of the hose and connections are in good working order, the vacuum switching valve is probably at fault.