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
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.
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
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.
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.
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 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.
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.
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
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.
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.
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.
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.
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.
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.
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
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
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