CTS 2003-2005

Description & Operation


A number of components make up the fuel injection system. Some meter fuel, others meter air. All must work together for proper engine operation. The major components are discussed here.

Fuel Metering Modes Of Operation

The basic function of the air/fuel metering system is to control air/fuel delivery to the engine. The best air/fuel mixture to minimize exhaust emissions is 14.7 to 1, which allows the catalytic converter to operate most efficiently. Fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each intake valve.

The main sensor control is the Heated Oxygen Sensor (HO2S) located in the exhaust manifold. The HO2S tells the Powertrain Control Module (PCM) how much oxygen is in the exhaust gas and the PCM changes the air/fuel ratio to the engine by controlling the fuel injector on time. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a Closed Loop System.

The PCM monitors voltages from several sensors to determine the engine's fuel needs. Fuel is delivered under one of several conditions called modes. All modes are controlled by the PCM and are described here.

Acceleration Mode

When the accelerator is depressed, air flow into the cylinders increases rapidly and the PCM increases the pulse width to the injectors to increase flow. The PCM determines the amount of fuel required based on throttle position, coolant temperature, manifold air pressure, mass air flow and engine speed. The system goes Open Loop.

Battery Voltage Correction Mode

When battery voltage is low, the PCM will compensate for the weak spark by:

Increasing the amount of fuel delivered.
Increasing the idle RPM.
Increasing the ignition dwell time.

Catalytic Converter Protection Mode

The PCM constantly monitors engine operation and estimates conditions that could result in high catalytic converter temperatures. If the PCM determines the catalytic converter may overheat, it causes the system to return to Open Loop operation and enriches the fuel mixture.

Clear Flood Mode

If the engine floods, clear it by pushing the accelerator pedal down all the way and then crank the engine. The PCM then de-energizes the fuel injectors. The PCM holds the fuel injectors de-energized as long as the throttle remains above 80% and the engine speed is below 600-800 RPM. If the throttle position becomes less than 80%, the PCM again begins to pulse the fuel injectors ON and OFF, allowing fuel into the cylinders.

Deceleration Mode

Deceleration mode indicates that the PCM has detected conditions appropriate to operate in Deceleration Fuel Mode. The PCM will command deceleration fuel mode when a sudden decrease in throttle position has been detected while the vehicle is traveling over 25 mph. While in Deceleration Fuel Mode, the PCM will decrease the amount of fuel delivered by entering Open Loop and decreasing the injector pulse width. When deceleration is very past, the PCM may cut off fuel completely for short periods.

Fuel Cut Off Mode

The PCM has the ability to shut OFF the fuel injectors completely or selectively when certain conditions are met. The fuel shut off mode allows the powertrain to protect itself from damage and also improve its driveability. The PCM will disable the injectors under the following conditions:

Ignition OFF (prevents engine run-on).
Ignition ON but no ignition reference signal (prevents flooding or backfiring).
High engine speed (above red line).
High vehicle speed (above rated tire speed).
VSS above 106 mph (tire dependent, may vary).

Run Mode

The run mode has two conditions called Open Loop and Closed Loop. When the engine is first started and engine speed is above a predetermined RPM, the system begins in Open Loop operation. In Open Loop, the PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system remains in Open Loop until the following conditions are met:

Both HO2S sensors have a varying voltage output, showing that they are hot enough to operate properly (this depends on temperature).
The ECT is above a specified temperature.
A specific amount of time has elapsed since starting the engine.

The specific values for the above conditions vary with different engines, and are stored in the Electrically Erasable Programmable Read Only Memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio based on the signal from various sensors, but mainly the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1 mixture.

Starting Mode

When the ignition is first turned ON the PCM energizes the fuel pump relay for two seconds, allowing the fuel pump to build up pressure. The PCM then checks the Engine Coolant Temperature (ECT) sensor and the Throttle Position (TP) sensor. During cranking, the PCM checks the crankshaft and camshaft position signals to determine the proper injector synchronization. The PCM controls the amount of fuel delivered in the starting mode by changing how long the fuel injectors are energized. This is done by pulsing the fuel injectors for very short times.

Fuel Supply Components

Fuel Injectors

The Sequential Multiport Fuel Injection (SFI) fuel injector is a solenoid operated device controlled by the Powertrain Control Module (PCM). The PCM energizes the solenoid which opens a valve to allow fuel delivery onto the director plate. The fuel is injected under pressure in a conical spray pattern at the opening of the intake valve. Excess fuel not used by the injectors passes through the fuel pressure regulator before being returned to the fuel tank. A fuel injector which is stuck partly open will cause a loss of fuel pressure after engine shut down, causing long crank times to be noticed on some engines.

Fuel Pressure Regulator

The fuel pressure regulator is a diaphragm-operated relief valve with fuel pump pressure on one side and manifold pressure on the other. The function of the fuel pressure regulator is to maintain the fuel pressure available to the fuel injectors at 3 times barometric pressure, adjusted for engine load. The fuel pressure regulator is mounted on the fuel rail and may be serviced separately. If fuel pressure is too low, poor performance and a Diagnostic Trouble Code (DTC) may be set. If the pressure is too high, excessive odor and another DTC may result.

Fuel Rail

The fuel rail assembly mounts the fuel injectors and also the fuel pressure regulator. It mounts to the lower portion of the intake manifold and distributes fuel to the cylinders through the individual injectors. Fuel is delivered to the fuel inlet tube and goes through the fuel rail to the fuel pressure regulator. The pressure regulator maintains a constant fuel pressure at the fuel injectors. Remaining fuel is then returned to the fuel tank.

Fuel Sender/Pump Assembly

The fuel metering system starts with the fuel in the fuel tank. The fuel sender assembly consists of the following major components: a fuel sender, a fuel pump, a fuel pump strainer, a fuel pulse dampener and a roll-over valve. The fuel sender consists of the float, the wire float arm, the rheostat and the roll-over valve. The fuel level is sensed by the position of the float and float arm, which operate the 90 ohm rheostat. As the float position changes, the amount of current passing through the rheostat varies, sending the signal to the fuel quantity gauge or the Body Control Module (BCM), thus changing the gauge reading on the instrument panel. The roll-over valve is pressed into the EVAP pipe of the fuel sender and is not service separately. The roll-over valve prevents fuel from entering the engine compartment if the vehicle rolls over, by shutting OFF the EVAP pipe to the evaporative emission canister.

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Fig. This typical fuel sender combines the fuel gauge float and the electric fuel pump in one assembly

A high pressure fuel pump is mounted to the sender. Again, use care ordering replacement parts. Some W-Body vehicles use a roller-vane pump and others use a gerotor fuel pump. Both have the same job, to supply gasoline under high pressure to the system. The fuel is pumped to the engine at a specified flow and pressure. A fuel pressure regulator in the fuel rail keeps fuel available to the injectors at a constant pressure. Excess fuel is returned to the tank by a return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel condition and aggressive vehicle maneuvers. A woven plastic fuel pump strainer is attached to the lower end of the fuel pump. It is designed to filter the fuel and protect the pump from foreign matter. Servicing the in-tank fuel strainer when troubleshooting a low-fuel pressure complaint could prove to be a challenge. On some W-Body vehicles, the fuel module is accessible through an access panel in the trunk floor. On other W-Body models, the fuel tank must be removed to service the fuel sender module assembly. The life of the fuel pump strainer is generally considered to be that of the fuel pump. The fuel pump strainer is self-cleaning and normally requires no maintenance. Fuel stoppage at this point indicates that the fuel tank contains an abnormal amount of sediment or water, in which case the tank should be removed and thoroughly cleaned. If the fuel pump strainer is plugged, replace it with a new one. The fuel pulse dampener is installed between the fuel pump and the fuel sender in order to dampen fuel pulsation and to reduce noise generated by the fuel pump.

A fuel filter is used in the fuel feed pipe ahead of the fuel injection system. The fuel filter is mounted directly in front of the fuel tank, on the underbody. The fuel filter is constructed of steel with a quick-connect fitting at the inlet and a threaded fitting at the outlet. The threaded fitting is sealed with a replaceable O-ring. The filter element is made of paper and is designed to trap particle suspended in the fuel that may damage the injection system. There is no service interval for in-pipe fuel filter replacement. Only change the filter if it is restricted or troubleshooting indicates low fuel pressure.

When the key is first turned ON , with the engine OFF , the PCM energizes the fuel pump relay for two seconds to quickly build fuel pressure. If the engine is not started within two seconds, the PCM shuts the fuel pump off and waits until the engine is cranked. When the engine is cranked and the RPM signal has been detected by the PCM, the PCM supplies 12 volts to the fuel pump relay to energize the electric in-tank fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. An inoperative fuel pump would cause a No-Start condition. A fuel pump which does not provide enough pressure can result in poor performance.

As a back-up system to the fuel pump relay, the fuel pump can also be energized by the engine oil pressure indicator switch. The normally open switch closes when oil pressure reaches about 4 psi. If the fuel pump relay fails, the engine oil indicator switch will close and run the fuel pump.

Fuel Tank

The fuel tank, naturally enough, is used to store fuel for the vehicle. The tank also houses the fuel pump, the primary fuel strainer and the instrument panel fuel gauge sending unit, all packaged together in one module. The tank is located in the rear of the vehicle and is held in place by two metal straps that are attached to the underbody. The fuel tank is made of steel and is coated internally with a special corrosion inhibitor. Due to the internal coating of the fuel tank, the tank is not repairable. The fuel tank shape includes a reservoir to maintain a constant supply of fuel around the fuel pump strainer during low fuel conditions and aggressive vehicle maneuvers. A fuel tank filler pipe check-ball tube is attached to the fuel tank and extends from the fuel tank inlet to the reservoir. The fuel tank filler pipe check-ball is located inside the fuel tank filler pipe check-ball tube and prevents fuel from splashing back out of the fuel tank filler pipe during refueling.

The tank is designed as part of a sealed system, to help prevent fuel vapors from entering the atmosphere. The fuel tank cap is an important part of this system. This threaded-type cap requires several turns counterclockwise before it can be removed. A built-in torque limiting device prevents over-tightening. To properly install, turn the cap until at least three clicking noises are heard. The clicking noises signal that the correct torque has been reached and that the fuel tank filler cap is properly seated. This is important. If a fuel tank filler cap requires replacement, use only the correct fuel tank cap. Failure to use the correct cap can result in a serious malfunction of the fuel system. The Enhanced EVAP fuel tank pipe cap may have a tether connected to the fuel filler door. To prevent refueling with leaded gasoline, which will ruin the catalytic converter, the fuel tank filler pipe has a built-in restrictor and deflector.

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Fig. Typical fuel system supply schematic

Intake Air Metering Components

Idle Air Control (IAC) Valve

The purpose of the Idle Air Control (IAC) valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle plate. By moving a conical valve, known as a pintle, IN (to decrease air flow) or OUT (to increase air flow), a controlled amount of air can move around the throttle plate. If RPM is too low, the PCM will retract the IAC pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If RPM is too high, the PCM will extend the IAC pintle, allowing less air to be bypassed around the throttle plate, decreasing RPM. The IAC pintle moves in small steps, called Counts.

During idle, the proper position of the IAC pintle is calculated by the PCM based on battery voltage, coolant temperature, engine load and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed (TP sensor voltage is between 0.20-0.74), the PCM senses a near stall condition. The PCM will then calculate a new IAC pintle position to prevent stalls.

If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset. The IAC resets when the ignition switch is cycled ON then OFF . When servicing the IAC, it should only be disconnected or connected with the ignition switch in the OFF position to keep from having to reset the IAC.

The position of the IAC pintle affects engine start up and the idle characteristics of the vehicle. If the IAC pintle is open fully, too much air will be allowed into the manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio. DTCs may be set. If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with possible hard starting and a rich air/fuel ratio. Another DTC may set. If the IAC pintle is stuck part way open, the idle may be high or low and will not respond to changes in engine load.

Mass Air Flow (MAF) Sensor

The Mass Air Flow (MAF) sensor is located between the throttle body and air cleaner assembly. The MAF sensor is used to measure the amount of air entering the engine. The PCM uses this information to determine the operating condition of the engine and to control fuel delivery. Refer to Section 4 of this manual for more information on the MAF sensor.

Throttle Body Unit

The throttle body has a throttle plate to control the amount of air delivered to the engine. The Throttle Position (TP) sensor and Idle Air Control (IAC) valve are mounted on the throttle body. Vacuum ports located behind the throttle plate provide the vacuum signals needed by various components. Engine coolant is directed through a coolant cavity in the throttle body to warm the throttle valve and prevent icing.

Throttle Position (Tp) Sensor

The Throttle Position (TP) sensor is mounted on the side of the throttle body opposite the throttle lever. It senses the throttle valve angle and relays that information to the PCM. Knowledge of the throttle angle is needed by the PCM to generate the required injector control signals (pulses). More information on the TP sensor is in Section 4 of this manual.