Continuous injection systems are used almost exclusively on import vehicles. The basic technology for CIS was introduces in the early 1970s and has been continuously updated and refined. During the past twenty years, continuous injection systems have gained an excellent reputation for efficiency and reliability.

The major difference between CIS and electronically controlled throttle body and port injection systems is the way in which the amount of fuel injected is controlled. In a CIS-equipped engine, the amount of fuel delivered to the cylinders is not varied by pulsing the injectors on and off. Instead CIS injectors spray fuel continuously. What does vary is the amount of fuel contained in the spray. CIS systems do this by maintaining a constant relative fuel system pressure and metering the amount of fuel to the injectors.

Basic Operation

Metering is done through a mixture control unit. This unit consist of an airflow sensor and a special fuel distributor with fuel lines running to all injectors. A control plunger in the fuel distributor is mechanically linked to the airflow sensor plate by means of a lever. As the airflow sensor measures the volume of engine intake air, its plate moves. The lever transfers this motion to the control plunger in the fuel distributor. The plunger moves up or down changing the size of the fuel metering openings in the fuel lines. This increases or decreases the volume of fuel flowing to the injectors.

Air Delivery System Air enters the system through the air filter and is measured by the airflow sensor. The amount of airflow is controlled by the throttle. The sensor plate is located in an air venturi or funnel shaped passage in the mixture control unit. Because of the shape of this venturi or funnel, the airflow sensor moves more when more air flows into the engine. Any air that enters the intake without passing the sensor plate interferes with  proper air/fuel mixture, causing the engine to run lean. The same holds true for all other types of injection systems. Proper operation of all other types of injection systems. Proper operation of all systems depends on having no vacuum leaks.

Fuel Delivery System The main components of the CIS fuel delivery system are the fuel tank, electronic fuel pump, prefeed pump (some systems only), fuel accumulator, fuel filter, fuel distributor, and fuel injectors.

Fuel is drawn from the tank by an electric fuel pump. It passes through the fuel accumulator and filter before reaching the fuel distributor in the metering control unit. Some models use a prefeed pump to supply the main pump. This prefeed pump helps prevent vapor lock in hot driving conditions.

The fuel accumulator is needed to prevent a sudden rapid rise in fuel pressure inside the fuel distributor when the vehicle is being started. Besides stabilizing the pressure, the accumulator also maintains a rest pressure within the fuel system when the engine is off. This helps eliminate vapor lock in the fuel lines.

The fuel distributor consists of a fuel control unit, pressure regulating valves for each cylinder, and a system pressure regulator. The fuel control unit consists of a slotted metering cylinder. This cylinder contains the fuel control plunger. Part of the control plunger protrudes past the fuel distributor and rests on the airflow sensor lever.

Fuel flows through the slots in the fuel metering cylinder. There is one metering slot for each engine cylinder. Control plunger movement within the metering cylinder determines the amount of fuel released to the fuel injectors. Each cylinder has its own pressure regulating valve. These valves maintain a constant pressure differential of approximately 1.5 psi on either side of the fuel metering slot. This pressure differential remains the same, regardless of the size of the slot opening. Without pressure regulating valves, the amount of fuel injected would not remain proportional to the size of the metering slot opening.

The fuel distributor also contains a pressure relief valve that regulates system pressure. Like the fuel pressure regulators used on EFI systems, this regulator maintains a constant system pressure by allowing excess fuel to return to the fuel tank via a return line.

Control Pressure Regulator A control pressure regulator is also used to provide correct fuel pressure on top of the fuel control plunger. This helps regulate the engine air/fuel needs. A dampening restriction over the fuel control plunger also eliminates any fluctuations that may occur in the airflow sensor lever.

FUEL INJECTORS CIS fuel injectors open at a set fuel pressure. Once the engine is started, each injector continuously sprays finely atomized fuel into the intake port of the cylinder.

A vibrator pin or needle inside each injector helps break up and atomize the fuel. This vibrating action also helps keep the injectors from clogging. Clogging is much common on TBI and PFI systems that on CIS-equipped engines.

When the engine is stopped, the pin and spring assembly seal off the injector to retain fuel pressure in the lines. This helps ensure quick starting.

COLD START INJECTORS AND AUXILIARY AIR VALVES CIS systems are normally equipped with a cold start injector and auxiliary air valve system to control cold starting and engine idling. These systems operate similarly to the EFI systems discussed earlier.

Oxygen Control Feedback System

Continuous injection systems can be fitted with an oxygen sensor (sometimes called lambda sensor) for feedback control. The sensor is mounted in the exhaust manifold so it heats up rapidly when the engine is started.

Signals from the oxygen sensor are sent to the oxygen control unit. The ECU modifies the fuel flow in the mixture control unit so the engine operates on the proper ratio. The changing exhaust gas affects the oxygen sensor and it sends a signal in a closed loop through the mixture control unit to the engine.

The oxygen control valve (sometimes called a timing or frequency valve) operates on signals from the oxygen control unit. It opens and closes to allow more or less fuel to return to the tank through the fuel return. This is called dwell time. By reducing the pressure in the lower part of the differential pressure valve, fuel flow to the injector can be increased, enriching the mixture. Shortening the time that the oxygen control valve is open increases the pressure beneath the differential pressure valve diaphragm. This lessens the amount of fuel injected, leaning the mixture.

Based on a series of signals from the sensor from oxygen-rich to oxygen-lean, the control valve continually cycles from being open about 40 percent of the time to being open about 60 percent of the time averaging about 50 percent.

The oxygen control unit switches to open loop during conditions when the oxygen sensor is cold or when the engine is cold. This open loop operation holds the oxygen control valve open for a fixed amount of time. When testing the operation of the oxygen control unit and its control valve, it is possible to hear the change in sound caused by the change in open time. A better way to monitor the activity  of the unit is to connect a dwellmeter across it and watch the change in cycles.

Adjusting the carbon monoxide (CO) output level is accomplished by turning a mixture screw. If the mixture adjustment is covered with a temper proof plug, this plug must be drilled and removed from the mixture control unit. The oxygen (lambda) sensor wire can be disconnected and the exhaust sample taken at the pipe provided on the exhaust manifold. As an alternate procedure, the mixture is adjusted in closed loop, with oxygen sensor connected, using a dwellmeter. Additional details are provided on the underhood decal.

CIS-E Components

As mentioned earlier, CISs can also be equipped with certain electronic controls. They combine the benefits of a basic mechanically controlled fuel injection system with simple electronic controls for enrichment, cutoff, and closed loop feedback. Economy is improved through the use of minimum enrichment during warm-up and fuel cutoff during coast.

CIS-E uses an electrohydraulic actuator in the fuel distributor, which is controlled by an ECU. The ECU receives signals from the coolant temperature sensor, throttle switch, airflow sensor plate, and the oxygen or lambda sensor.

CIS-E differs from a basic CIS in three ways. First, the airflow sensor mechanism includes a potentiometer, which signals the position and movement of the plate for acceleration enrichment. Secondly, the system pressure regulator maintains a constant pressure. It also relieves electric fuel pump pressure and maintains pressure in the system for easy restarts by controlling return fuel flow from the fuel distributor. Finally, the electrohydraulic actuator is an electromagnetic differential pressure regulator on the fuel distributor. It operates a plate valve.

System Operation The system pressure regulator maintains a constant system of primary pressure. Constant system pressure is applied to the control plunger to counter the force of the airflow sensor mechanism. There is no control pressure and no control pressure regulator.

The electrohydraulic actuator provides enrichment reducing the pressure below the diaphragm of each differential pressure valve. This has the effect of increasing the pressure at each metering slit. In turn, this increases the amount of fuel delivered. For fuel cutoff during coasting or for rpm limitation, the electrohydraulic action is reversed. A decrease in the pressure at the metering slits cuts off delivery of fuel.