For 1990-2009 cars

### Fuel System Overview

The fuel system is a returnless on-demand design. The fuel pressure regulator is a part of the fuel pump module, eliminating the need for a return pipe from the engine. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal temperature of the fuel tank results in lower evaporative emissions.

An electric turbine- style fuel pump is attached to the fuel pump module inside the fuel tank. The fuel pump supplies high pressure fuel through the fuel feed pipe to the fuel injection system. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator, a part of the fuel pump module, maintains the correct fuel pressure to the fuel injection system. The fuel pump module contains a reverse flow check valve. The check valve and the fuel pressure regulator maintain fuel pressure in the fuel feed pipe and the fuel rail in order to prevent long cranking times.

### Fuel Tank

The fuel tank stores the fuel supply. The fuel tank is located in the rear of the vehicle. The fuel tank is held in place by 2 metal straps that are attached to the underbody. The fuel tank is molded from high-density polyethylene.

### Fuel Filler Cap

The fuel fill pipe has a tethered fuel filler cap. A torque-limiting device prevents the cap from being overtightened. To install the cap, turn the cap clockwise until the cap clicks audibly. This indicates that the cap is correctly torqued and fully seated. A fuel filler cap that is not fully seated may cause a malfunction in the emission system.

### Fuel Pump Module

The fuel pump module consists of the following major components:

• | The fuel level sensor

• | The fuel pump

• | The fuel strainer

• | The fuel pressure regulator

• | The fuel filter

### Fuel Level Sensor

The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor card. The position of the float arm indicates the fuel level. The fuel level sensor contains a variable resistor which changes resistance in correspondence with the position of the float arm. The ECM sends the fuel level information via the serial data circuit to the instrument panel cluster. This information is used for the instrument panel cluster (IPC) fuel gauge and the low fuel warning indicator, if applicable. The ECM also monitors the fuel level input for various diagnostics.

### Fuel Pump

The fuel pump is mounted in the fuel pump module reservoir. The fuel pump is an electric high-pressure pump. Fuel is pumped to the fuel injection system at specified rates of flow and pressure. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The ECM controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

### Fuel Strainer

The fuel strainer is attached to the lower end of the fuel pump module. The fuel strainer is made of woven plastic. The functions of the fuel strainer are to filter contaminants and to wick away fuel. Normally, the fuel strainer does not require maintenance. Fuel stoppage at this point indicates that the fuel tank contains an abnormal amount of sediment or contamination.

### Fuel Pressure Regulator

The fuel pressure regulator is contained in the fuel pump module near the fuel pump outlet. The fuel pressure regulator is a diaphragm relief valve. The diaphragm has fuel pressure on one side and regulator spring pressure on the other side. The fuel pressure regulator is not vacuum biased. Fuel pressure is controlled by a pressure balance across the regulator. The fuel system pressure is constant.

### Fuel Feed Pipes

The fuel feed pipe carries fuel from the fuel tank to the fuel injection system. The fuel pipe consists of 3 sections:

• | The rear fuel pump fuel feed hose runs from the top of the fuel tank to the chassis fuel pipe. The rear fuel hose is constructed of nylon.

• | The fuel feed intermediate pipe is located under the vehicle and connects the rear fuel pump fuel feed hose to the front fuel pump fuel feed hose. The intermediate fuel pipe is constructed of a combination of nylon and steel pipes.

• | The front fuel pump fuel feed hose connects the fuel feed intermediate pipe to the fuel rail. The front fuel hose contains the fuel pulse dampener and is constructed of a combination of nylon and steel pipes.

### Nylon Fuel Pipes

• | Replace all nylon fuel pipes that are nicked, scratched or damaged during installation, do not attempt to repair any sections of nylon fuel pipes

• | Do not hammer directly on the fuel harness body clips when installing new fuel pipes. Damage to the nylon pipes may result in a fuel leak.

• | Always cover nylon vapor pipes with a wet towel before using a torch near them. Also, never expose the vehicle to temperatures higher than 115°C (239°F) for more than 1 h, or more than 90°C (194°F) for any extended period.

• | Apply a few drops of clean engine oil to the male pipe ends before connecting fuel pipe fittings. This will ensure proper reconnection and prevent a possible fuel leak. (During normal operation, the O-rings located in the female connector will swell and may prevent proper reconnection if not lubricated.)

Nylon pipes are constructed to withstand maximum fuel system pressure, exposure to fuel additives, and changes in temperature.

Heat resistant rubber hose or corrugated plastic conduit protect the sections of the pipes that are exposed to chafing, high temperature, or vibration.

Nylon fuel pipes are somewhat flexible and can be shaped around gradual turns under the vehicle. However, if nylon fuel pipes are forced into sharp bends, the pipes may kink and restrict the flow of fuel. Also, once exposed to fuel, nylon pipes may become stiffer and are more likely to kink if bent too far. Exercise special care when working on a vehicle with nylon fuel pipes.

### Quick-Connect Fittings

Nylon fuel pipes are somewhat flexible and can be shaped around gradual turns under the vehicle. However, if nylon fuel pipes are forced into sharp bends, the pipes may kink and restrict the flow of fuel. Also, once exposed to fuel, nylon pipes may become stiffer and are more likely to kink if bent too far. Exercise special care when working on a vehicle with nylon fuel pipes.

### Fuel Pulse Dampener

The fuel pulse dampener is a part of the front fuel pump fuel feed hose. The fuel pulse dampener is diaphragm-operated with fuel pump pressure on one side and with spring pressure on the other side. The function of the dampener is to dampen the fuel pump pressure pulsations.

### Fuel Rail Assembly

The fuel rail assembly is attached to the cylinder head. The fuel rail assembly performs the following functions:

• | Positions the injectors in the intake ports of the cylinder head

• | Distributes fuel evenly to the injectors

### Fuel Injectors

The fuel injector assembly is a solenoid device controlled by the ECM that meters pressurized fuel to a single engine cylinder. The ECM energizes the high-impedance, 12 Ω, injector solenoid to open a ball valve, normally closed. This allows fuel to flow into the top of the injector, past the ball valve, and through a director plate at the injector outlet. The director plate has machined holes that control the flow of fuel, generating a spray of finely atomized fuel at the injector tip. Fuel from the injector tip is directed at the intake valve, causing the fuel to become further atomized and vaporized before entering the combustion chamber. This fine atomization improves fuel economy and emissions. The fuel pressure regulator compensates for engine load by increasing fuel pressure as the engine vacuum drops.

### Fuel Metering Modes of Operation

The ECM monitors voltages from several sensors in order to determine how much fuel to feed to the engine. The ECM controls the amount of fuel delivered to the engine by changing the fuel injector pulse width. The fuel is delivered under one of several modes.

### Starting Mode

When the ECM detects reference pulses from the crankshaft position (CKP) sensor, the ECM will enable the fuel pump. The fuel pump runs and builds up pressure in the fuel system. The ECM then monitors the manifold absolute pressure (MAP), intake air temperature (IAT), engine coolant temperature (ECT), and accelerator pedal position (APP) sensor signals in order to determine the required injector pulse width for starting.

### Clear Flood Mode

If the engine is flooded with fuel during starting and will not start, the clear flood mode can be manually enabled. To enable Clear Flood Mode, press the accelerator to wide open throttle (WOT). The ECM will completely turn OFF the fuel injectors and will maintain this mode as long as the ECM detects a WOT condition with engine speed below a predetermined value.

### Run Mode

The run mode has 2 conditions referred to as open loop and closed loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins open loop operation. The ECM ignores the signal from the heated oxygen sensor (HO2S). The engine ECM calculates the air/fuel ratio based on inputs from the engine coolant temperature (ECT), the manifold absolute pressure (MAP), and accelerator pedal position (APP) sensor. The system stays in open loop until meeting the following conditions:

• | The heated oxygen sensor (HO2S) has varying voltage output, showing that the heated oxygen sensor (HO2S) is hot enough to operate properly.

• | The ECT sensor is above a specified temperature.

• | A specific amount of time has elapsed after starting the engine.

Specific values for the above conditions exist for each different engine, and are stored in the programmable read-only memory (EEPROM), which may be erased electrically. The system begins closed loop operation after reaching these values. In closed loop, the ECM calculates the air/fuel ratio, injector ON time, based upon the signal from various sensors, but mainly from the heated oxygen sensor (HO2S). This allows the air/fuel ratio to stay very close to 14.7:1.

### Acceleration Mode

The ECM monitors the changes in the accelerator pedal position (APP) sensor. and the manifold absolute pressure (MAP) sensor signal in order to determine when the vehicle is being accelerated. The ECM will then increase the injector pulse width in order to provide more fuel for increased performance.

### Deceleration Mode

The ECM monitors changes in accelerator pedal position (APP) sensor and manifold absolute pressure (MAP) sensor signals to determine when the vehicle is being decelerated. The ECM will then decrease injector pulse width or even turn OFF injectors for short periods to reduce exhaust emissions, and for better (engine braking) deceleration.

### Battery Voltage Correction Mode

When the battery voltage is low, the ECM compensates for the weak spark delivered by the ignition system in the following ways:

• | Increasing the amount of fuel delivered

• | Increasing the idle RPM

• | Increasing the ignition dwell time

### Fuel Cutoff Mode

The ECM cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:

• | The ignition is OFF. This prevents engine run-on.

• | The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.

• | The engine speed is too high, above red line.

• | The vehicle speed is too high, above rated tire speed.

• | During an extended, high speed, closed throttle coast down, this reduces emissions and increases engine braking.

• | During extended deceleration, in order to prevent damage to the catalytic converters

### Fuel Trim

The ECM controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage while in closed loop and regulates the fuel delivery by adjusting the pulse width of the injectors based on this signal. The ideal fuel trim values are around 0% for both short and long term fuel trim. A positive fuel trim value indicates the ECM is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) signal voltage. These changes fine-tune the engine fueling. The long term fuel trim makes rough adjustments to fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The ECM selects the cells based on the engine speed and engine load. If the ECM detects an excessively lean or rich condition, the ECM will set a fuel trim diagnostic trouble code (DTC).