Carburetor Feedback System
The purpose of carburetor feedback system is to maintain air fuel ratio at desired 14.7:1 ratio during normal operation (except warm-up and acceleration).
The carburetor on the feedback control system is designed to operate somewhat richer to set up rich limit of system operation. When a more lean operation is desired, the ECM commands air to be bled into carburetor's main metering system and into carburetor's primary bore. A lean operating condition is therefore easily obtained. The ECM receives inputs from the following sensors:
- Oxygen sensor
- Vacuum switch
- Throttle position switch
- Distributor
The ECM's output consists of a signal to an electric air bleed control valve (EBCV). When energized, the EBCV bleeds air into main air bleed circuit and into slow air bleed port of carburetor. This additional air leans air fuel ratio. When not energized, air fuel ratio moves to rich limit.
With engine running and coolant temperature below 45°F (7°C), thermostatic vacuum switching valve (TVSV) applies atmospheric pressure to vacuum switch B by connecting ports J to L of TVSV. With these conditions, vacuum switch B is off, electric bleed control valve (EBCV) is off and both air bleeds are off. The carburetor is operating toward its rich limits. With engine relatively cold, this is desirable. The computer will not be controlling or influencing air fuel ratios while engine is cold.
At temperatures above 63°F (17°C), with engine running between 1500 and 4200 RPM, the TVSV applies vacuum to vacuum switch by connecting ports K to L of the TVSV. Vacuum switch closes, signaling ECM. Throttle position switch is closed, also signaling ECM. With these 2 switches closed, if oxygen sensor senses a rich condition in exhaust (high voltage - 1 volt), ECM commands electric bleed control valve to be energized, thereby bleeding air into both main metering system of carburetor and intake manifold. This results in air fuel ratio becoming lean.
When air fuel ratio is detected as lean by oxygen sensor (low voltage - 1 volt), ECM will de-energize EBCV and close both bleed ports which results in air fuel ratio moving to its rich extreme. This is closed loop mode operation. To energize EBCV, the ECM completes its electrical circuit on its ground side. See Fig 1 and CARBURETOR FEEDBACK SYSTEM OPERATION chart below.