On engines with the Carter YFA or Rochester E2SE carburetors, a Mixture Control (MC) solenoid is used to regulate the air/fuel mixture. During open loop operation, the MC solenoid supplies a preprogrammed amount of air to the carburetor idle circuit and main metering circuit where it mixes with the fuel. During closed loop operation, the MCU operates the MC solenoid to provide additional or less air to the fuel mixture, depending on the engine operating conditions as monitored by the various engine sensors.
See Figure 1
If the mixture control solenoid is suspected of either sticking, binding or leaking, test it using the following procedure:
- Connect one end of a jumper wire to either terminal of the solenoid wire connector and the other end to the positive (+) terminal of a 12 volt battery.
- Connect one end of another jumper wire to the other terminal of the solenoid wire connector and the other end to the negative (-) terminal of the battery.
- With the rubber seal, retainer and spacer removed from the end of the solenoid stem, attach a hose from a hand vacuum pump.
- With the solenoid fully energized (lean position), apply at least 25 in. Hg (172 kPa) of vacuum and time the leak-down rate from 20-to-15 in. Hg (137-103 kPa). The leak-down rate should not exceed 5 in. Hg (34 kPa) in 5 seconds. If the leak-down rate exceeds that amount, replace the solenoid.
- To test the solenoid for sticking in the down (de-energized) position, remove the jumper wire to the 12 volt battery and observe the hand vacuum pump gauge. It should move to zero in less than one second.