Fuel System (Cab Chassis)

FUEL SYSTEM DESCRIPTION (CAB CHASSIS)

FUEL SYSTEM OVERVIEW
The fuel tank stores the fuel supply. The electric fuel pump sends fuel through an in-line fuel filter to the fuel rail assembly. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injectors. The fuel pressure regulator keeps fuel available to the injectors at a regulated pressure. A separate pipe returns unused fuel to the fuel tank.

FUEL TANK
The fuel tanks store the fuel supply. The front fuel tank is located on the left side of the vehicle. On duel-tank applications, the secondary fuel tank is located in the rear of the vehicle. The fuel tanks are each held in place by two metal straps that attach to the frame. The fuel tanks are molded from high density polyethylene.

FUEL FILL PIPE
The fuel fill pipe has a built-in restrictor in order to prevent refueling with leaded fuel. Once the fill vent is obstructed, fuel backs up the fill pipe and trips the dispensing nozzle. The front fuel tank vent runs into the rear tank to the top of the filler pipe assembly, which in turn vents to atmosphere. The fuel tank vent valves are connected and route to the canister to collect hydrocarbon emissions during operation of the vehicle.

FUEL SENDER ASSEMBLY




The fuel sender assembly consists of the following major components:
- The fuel sender assembly (2)
- The fuel strainer (3)
- The fuel pump (4)

FUEL LEVEL SENSOR
The fuel level sensor is a part of the fuel sender assembly. 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 amount of fuel in the fuel tank. On models that are equipped with a single fuel tank, the fuel level sensor is wired directly to the instrument panel (I/P) cluster. On models equipped with dual fuel tanks, both fuel level sensors are wired to the powertrain control module (PCM). The PCM calculates the total fuel level in both tanks. The instrument panel (I/P) fuel gage displays the total fuel level in both fuel tanks.

FUEL PUMP




The fuel pump is mounted in the fuel sender assembly. The fuel pump is an electric high pressure pump. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injectors. Excess fuel from the fuel rail assembly returns to the fuel tanks through the fuel return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions. The powertrain control module (PCM) 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.

SECONDARY FUEL PUMP - DUAL TANKS
The secondary fuel pump is a low pressure electric transfer pump. The secondary fuel pump is mounted outside the fuel tank on the left side of the body crossmember. The secondary fuel pump transfers fuel from the secondary fuel tank to the primary fuel tank when commanded by the powertrain control module (PCM). The PCM grounds the secondary fuel pump relay control circuit. Fuel level is continuously monitored in both tanks by the PCM. When the fuel level in the primary tank is approximately 2 gallons lower than that in the secondary tank, the secondary fuel pump is energized and approximately 4 gallons of fuel is transferred to the primary tank. The above actions continue until the primary tank fuel level is low enough that it will accept all of the remaining fuel in the secondary tank, at which time the secondary fuel pump is energized until the secondary tank is empty.

FUEL STRAINER




The fuel strainer attaches to the lower end of the fuel sender. The fuel strainer is made of woven plastic. The functions of the fuel strainer are to filter contaminants and to wick fuel. The fuel 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.

FUEL FILTER




The fuel filter is located on the fuel feed pipe, between the fuel pump and the fuel injectors. The paper filter element (2) traps particles in the fuel that may damage the fuel injection system. The filter housing (1) is made to withstand maximum fuel system pressure, exposure to fuel additives, and changes in temperature. There is no service interval for fuel filter replacement. Replace a restricted fuel filter.

FUEL FEED AND RETURN PIPES
The fuel feed pipe carries fuel from the fuel tank to the fuel rail assembly. The fuel return pipe carries fuel from the fuel rail assembly back to the fuel tanks. The fuel pipes consist of 2 sections:
- The rear fuel pipe assemblies are located from the top of the fuel tank to the chassis fuel pipes. The rear fuel pipes are constructed of nylon.
- The chassis fuel pipes are located under the vehicle and connect the rear fuel pipes to the fuel rail pipes. The chassis fuel pipes are constructed of steel.

NYLON FUEL PIPES
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 formed around gradual turns under the vehicle. However, if nylon fuel pipes are forced into sharp bends, the pipes kink and restrict the fuel flow. Also, once exposed to fuel, nylon pipes may become stiffer and are more likely to kink if bent too far. Take special care when working on a vehicle with nylon fuel pipes.

QUICK-CONNECT FITTINGS
Quick-connect fittings provide a simplified means of installing and connecting fuel system components. The fittings consist of a unique female connector and a compatible male pipe end. O-rings, located inside the female connector, provide the fuel seal. Integral locking tabs inside the female connector hold the fittings together.

FUEL PIPE O-RINGS
O-rings seal the connections in the fuel system. Fuel system O-ring seals are made of special material. Service the O-ring seals with the correct service part.

FUEL RAIL ASSEMBLY




The fuel rail assembly attaches to the engine intake manifold. The fuel rail assembly performs the following functions:
- Positions the injectors (3) in the intake manifold
- Distributes fuel evenly to the injectors
- Integrates the fuel pressure regulator (2) with the fuel metering system

FUEL INJECTORS




The Multec 2 fuel injector assembly is a solenoid device, controlled by the powertrain control module (PCM), that meters pressurized fuel to a single engine cylinder. The PCM energizes the high-impedance (12.0 ohms) injector solenoid (2) to open a normally closed ball valve (3). 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 2 machined holes that control the fuel flow, generating a spray of finely atomized fuel at the injector tip. Fuel from the injector tip is directed at the intake valve, causing it to become further atomized and vaporized before entering the combustion chamber. This fine atomization improves fuel economy and emissions.

FUEL PRESSURE REGULATOR ASSEMBLY




The fuel pressure regulator is a vacuum operated diaphragm relief valve. The diaphragm has fuel pressure on one side and regulator spring pressure and intake manifold vacuum on the other side. The fuel pressure regulator compensates for changes in intake manifold vacuum by changing the fuel pressure. In this way, the fuel pressure regulator maintains a constant pressure differential across the fuel injectors under all operating conditions.

FUEL METERING MODES OF OPERATION
The powertrain control module (PCM) monitors voltages from several sensors in order to determine how much fuel to give the engine. The PCM 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 ignition is first turned ON, the powertrain control module (PCM) energizes the fuel pump relay for 2 seconds. This allows the fuel pump to build pressure in the fuel system. The PCM calculates the air/fuel ratio based on inputs from the engine coolant temperature (ECT), mass air flow (MAF), manifold absolute pressure (MAP), and throttle position (TP) sensors. The system stays in starting mode until the engine speed reaches a predetermined RPM.

CLEAR FLOOD MODE
If the engine floods, clear the engine by pressing the accelerator pedal down to the floor and then crank the engine. When the throttle position (TP) sensor is at wide open throttle, the powertrain control module (PCM) reduces the fuel injector pulse width in order to increase the air to fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the PCM returns to the starting mode.

Run Mode
The run mode has 2 conditions called 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 powertrain control module (PCM) ignores the signal from the heated oxygen sensor (HO2S). The PCM calculates the air/fuel ratio based on inputs from the engine coolant temperature (ECT), mass air flow (MAF), manifold absolute pressure (MAP), and throttle position (TP) sensors. The system stays in Open Loop until meeting the following conditions:
- Both HO2S have varying voltage output, showing that they are 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 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 (injector on-time) based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode
When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the powertrain control module (PCM) increases the pulse width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The PCM determines the amount of fuel required based upon the throttle position, the coolant temperature, the manifold air pressure, the mass air flow, and the engine speed.

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The powertrain control module (PCM) monitors the corresponding changes in throttle position, manifold air pressure, and mass air flow. The PCM shuts off fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts off in order to prevent damage to the catalytic converters.

Battery Voltage Correction Mode
When the battery voltage is low, the powertrain control module (PCM) 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 PCM 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 powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM monitors the heated oxygen sensor (HO2S) signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the fuel injectors based on this signal. The ideal fuel trim values are around 0 percent for both short term and long term fuel trim. A positive fuel trim value indicates the PCM is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the PCM 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 short term and long term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to the fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short term 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 PCM selects the cells based on the engine speed and engine load. If the PCM detects an excessive lean or rich condition, the PCM will set a fuel trim diagnostic trouble code (DTC).