System Operation

The mixture control unit is located between the air cleaner and the throttle valve. It consists of the air sensor and the fuel distributor.
The air sensor measures the amount of air entering the engine and controls the amount of fuel injected by moving the control plunger in the fuel distributor.
The flow of intake air through the air cone will lift the sensor plate causing the sensor lever to lift the control plunger in the fuel distributor.
As the control plunger rises, it allows more fuel to be delivered to the injectors.
Since the air sensor is constantly measuring the intake air and controlling the amount of fuel by raising or lowering the control plunger, the basic air/fuel mixture is always correct.

The pictures below show the relationship between air flow and fuel flow:











The operation of the air sensor is based on the "floating body principle" which says that a floating body suspended in a cone moves in a straight line according to the rate of air flow through the cone.
The CIS electronic "floating body" is the circular sensor plate bolted to the sensor lever which moves freely around a pivot point. The movement of the sensor plate, lever and control plunger are controlled and dampened by a balance weight at the end of the lever and system pressure acting on the top of the control plunger.
At (1), the slight air flow at idle lifts the sensor plate enough to raise the control plunger slightly. The metering slit in the fuel distributor is opened to allow enough fuel for idle.
As air flow increases, more of the metering slit is exposed (2) until at full throttle (3), the fuel flow is at maximum.
The graph represents an engine's basic air-fuel requirement.
Notice that with CIS electronic fuel Injection the fuel flow is automatically corrected for any volume of air flow from idle throughout the full load range.





The sensor plate position has been changed because of the new fuel distributor. The control plunger now seats on an "0" ring. There is now a small gap between the sensor lever and the control plunger when the sensor lever is at rest position. This is necessary to insure that the plunger seals completely against the "0" ring when the engine is shut off. This results in the sensor plate resting below the edge of the sensor housing. When the engine is not running, the sensor plate and lever rest on a spring loaded stop.
The control plunger sits low enough to uncover the top edge of the metering slits. This allows residual system pressure to work directly on the injectors to eliminate fuel vaporization.





Should a backfire occur in the intake manifold, the sensor plate will be forced down against the spring loaded stop. This creates a larger opening around the sensor plate to reduce pressure and to prevent damage to the air sensor assembly.









FUEL INJECTORS
CIS injectors open at a pre-set pressure and spray atomized fuel continuously into the intake port near each intake valve.
When the engine is stopped, the fuel pressure is reduced and the pin and spring assembly at the end of the injector closes. This traps fuel under pressure in the system to prevent vapor lock and to insure quick hot starting.
All engines are now equipped with air shrouded injectors. Air flows through a passage in the cylinder head, through the injector insert and exits near the tip of the injector. The fuel leaving the injector is better atomized, reducing fuel condensation in the intake manifold.
There is now a second generation of air shrouded injector. The changes consist of: A circlip added to the injector to hold the sealing "0" ring in position; a new brass injector insert, a separate plastic injector shroud; and a fluted air directional shield attached to the end of the injector to improve air flow around the injector nozzle. These changes enhance the fuel atomization and provide even smoother engine performance.
The new style 2 piece insert assembly must only be used together with the new injectors that have the directional shield attached to the nozzle. This complete assembly may be installed in earlier vehicles with air shrouded injectors.





FUEL DISTRIBUTOR
The fuel distributor has many changes to adapt it to CIS electronic. It is now made of aluminum and is slightly smaller than previous versions. Some of the changes are:
- Deletion of pressure relief valve.
- The pressure regulating springs are now in the lower chambers and the differential pressure regulator (electro-hydraulic actuator) is mounted directly to the fuel distributor.
- The volume of the upper chamber is smaller, resulting in quicker response and good hot starting.
- The control plunger now seats on an "0" ring with the engine shut-off. There is now a few millimeters play between the sensor plate arm and the control plunger with the engine shut-off.
System pressure is always present in the upper chamber of the fuel distributor. The metering slit in the control barrel regulates the amount of fuel delivered to the upper chamber based on air sensor position and control plunger position. The amount of fuel delivered to the injectors, and consequently the fuel mixture, is adjusted by the differential pressure regulator.