Fuel Metering System Component Description. Pontiac Bonneville 1998

For 1990-2009 cars only

Makes 🢒 Pontiac 🢒 1998 🢒 Bonneville 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Fuel Metering System Component Description

The fuel metering system is made up of the following parts:

•	The accelerator controls

•	The throttle body

•	The throttle position (TP) sensor

•	The idle air control (IAC) valve

•	The mass air flow (MAF) sensor

•	The fuel rail

•	The fuel injector

•	The fuel pressure regulator

•	The Powertrain Control Module (PCM)

•	The crankshaft position sensor

•	The camshaft position sensor

•	The idle air control valve (IAC)

•	The fuel pump

Basic System Operation

The fuel metering system starts with the fuel in the fuel tank. An electric fuel pump, located in the fuel tank with the gauge sending unit, pumps fuel to the fuel rail through an in-line fuel filter. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator in the fuel rail keeps fuel available to the fuel injectors at a constant pressure. Unused fuel is returned to the fuel tank by a separate pipe. For further information on the fuel tank, in-line filter, and fuel pipes, refer to Fuel Supply Component Description .

Accelerator Controls


(1)	Accelerator Control Cable
(2)	Accelerator Cable Strut Tower Clip
(3)	Accelerator Controls Bracket

The accelerator control system is cable type. There are no linkage adjustments. Therefore, the specific cable for each application must be used. The accelerator cable is routed through the groove in the throttle body lever.

When work has been performed on accelerator controls, always make sure that all components are installed correctly and that linkage and cables are not rubbing or binding in any manner. The throttle should operate freely without bind between full closed and wide open throttle.

Throttle Body Unit


(1)	MAF Sensor
(2)	Throttle Body Air Inlet Screen
(3)	Throttle Position Sensor
(4)	IAC Valve

The throttle body has a throttle plate to control the amount of air delivered to the engine. The TP sensor, MAF sensor, and IAC valve are also 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 TP sensor is a potentiometer that is mounted on the throttle body and provides the PCM with information on throttle valve angle. The PCM provides a 5 volt reference signal and a ground to the TP sensor and the sensor returns a signal voltage that changes with throttle valve angle. At closed throttle (close to 0°) the TP sensor output signal is low (below 1 volt) and at WOT (greater than 80°) the TP sensor output signal is high (above 4 volts). Because the TP sensor is not adjustable, the PCM must account for build tolerances that could affect the TP sensor output at closed throttle. The PCM uses a learning algorithm so that it can correct for variations of up to 6° of throttle angle.

The PCM uses TP information to modify fuel control based on throttle valve angle. For example, power enrichment occurs when the throttle angle approaches WOT. Acceleration enrichment occurs when the throttle angle increases rapidly (similar to an accelerator pump on a carburetor). A faulty TP sensor may cause various driveability conditions and should set a DTC.

Refer to Powertrain Control Module Description for description and operation.

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 .20 -.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 key is cycled ON then OFF.

When servicing the IAC, it should only be disconnected or connected with the ignition OFF in order 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. DTC P0507 may 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. DTC P0506 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 in the MAF sensor housing between the air snorkel and the throttle body. It senses the amount of air entering the intake manifold. This information is required by the PCM to control fuel and emissions.

The MAF sensor used on this vehicle is a hot wire type and is used to measure air flow rate (mass/unit time). The MAF sensor output is a variable frequency square wave based on air mass traveling through the sensor. A low frequency output signal from the MAF sensor indicates low air flow and high frequency indicates high air flow. The MAF sensor has two wires crossing the air stream through it. One senses ambient temperature and the other, the hot wire, is kept at a fixed temperature above the ambient temperature. The MAF sensor senses air flow by measuring the power necessary to keep the hot wire hot, which is proportional to air flow, and converts it to a frequency modulate output signal.

Fuel Rail (VIN K)


(1)	Fuel Rail Assembly
(2)	Fuel Pressure Regulator
(3)	Fuel Pressure Gauge Connection
(4)	Fuel Injector

Fuel Rail (VIN 1)


(1)	Fuel Pressure Gauge Connection
(2)	Fuel Injector
(3)	Fuel Pressure Regulator
(4)	Fuel Rail Assembly

The fuel rail is mounted to the top of the engine and distributes fuel to the individual fuel injectors. Fuel is delivered to the fuel inlet tube of the fuel rail by the fuel pipes. The fuel then goes through the fuel rail to the fuel pressure regulator. The fuel pressure regulator maintains a constant fuel pressure at the fuel injectors. Remaining fuel is then returned to the fuel tank. Fuel pressure can be observed use a fuel pressure gauge at the fuel pressure gauge connection.

Fuel Injector


(1)	Ball Valve
(2)	Director Plate
(3)	Injector Coil

The Sequential Multi-port Fuel Injection (SFI) fuel injector is a solenoid operated device controlled by the PCM. The PCM energizes the solenoid which opens a valve to allow fuel delivery.

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.

When the PCM energizes the injector coil, a normally closed valve ball opens, allowing fuel to flow past a director plate to the injector outlet.

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


(1)	Regulator Assembly
(2)	Pressure Regulator Spring
(3)	O-Ring -- Backup
(4)	O-Ring -- Large
(5)	Filter Screen
(6)	Relief Valve
(7)	O-Ring -- Small

The cartridge type fuel pressure regulator is a diaphragm-operated relief valve with fuel pump pressure on one side, and pressure regulator spring, and intake manifold vacuum on the other side. The regulators function is to maintain a constant fuel pressure across the injectors at all times. The pressure regulator compensates for engine load by increasing fuel pressure as engine vacuum drops.

The cartridge regulator is serviced as a separate component. When servicing the fuel pressure regulator, insure that the O-ring backup, large O-ring, filter screen, and small O-ring, are properly placed on the pressure regulator.

If the fuel pressure is too low, poor performance and a DTC P0171, could result. If the pressure is too high, excessive odor and a DTC P0172, may result. Refer to Fuel System Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) for information on diagnosing fuel pressure conditions.

(3X) Reference PCM Input (CKT 430)

From the electronic ignition module, the PCM uses this signal to calculate engine speed and crankshaft position. The PCM compares pulses on this circuit to reference low CKT 453. The PCM also uses the pulses on this circuit to initiate fuel injector pulses. If the PCM receives no pulses on this circuit, no fuel injection pulses will occur and the engine will not run.

CAM Signal (CKT 630)

The PCM uses this signal to determine the position of the # 1 piston during its power stroke, allowing the PCM to calculate true Sequential Multi-port Fuel Injection (SFI). A loss of this signal will set a DTC P0341. If the CAM signal is lost while the engine is running, the fuel injection system will shift to a calculated sequential fuel injection based on the last fuel injection pulse and the engine will continue to run. The engine can be restarted and will run in the calculated sequential mode as long as the fault is present with a 1 in 6 chance of being correct. Refer to DTC P0341 Camshaft Position (CMP) Sensor Performance .

Fuel Pump Electrical Circuit

When the key is first turned ON the PCM energizes the fuel pump relay for two seconds to build up the fuel pressure quickly. 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 a no start condition.

An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.