Deceleration Fuel Cut System: Operation
On 1977 3K-C engine, when engine speed is below 2100 RPM and carburetor vacuum is below 13 in. Hg, the computer is activated by a signal from ignition system and/or vacuum switch. See Figure. Current flows to fuel cut solenoid and valve is pulled back. Valve is now in open position, opening slow fuel cut circuit in carburetor.
Under continuous deceleration, engine speed above 2100 RPM, and vacuum above 13 in. Hg, the computer cuts off current to solenoid switch. Spring tension pushes solenoid and closes slow circuit in carburetor. A delay of 1 1/2 seconds is built into the computer to hold off operation of solenoid valve for a short period of time.
On 1978-79 models, when engine speed is below 2400 RPM (2000 RPM on 2T-C and 4M engines) and carburetor vacuum is low, the computer is activated and sends current to the fuel cut solenoid. See Fig 1. The valve cuts fuel flow rate in carburetor slow circuit. When engine speed and vacuum are high, the solenoid valve will not affect fuel flow.