Fuel Metering System Component Description. Chevrolet Chevrolet C-Series Medium Duty Truck B7 1998

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 1998 🢒 Chevrolet C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 6.0L and 7.0L 🢒 Fuel Metering System Component Description

Throttle Body Fuel Injection (TBI) Unit

An eight digit part identification number (1) and date (2) is stamped on the throttle body. Refer to this number for service needs or the part replacement.

TBI unit (1) Model 295 consists of these major assemblies:

•	The fuel meter body assembly (2) includes the following items:

-	The fuel injectors

-	Then fuel pressure regulator (7)

•	The throttle body assembly (4) includes the following items:

-	The two throttle valves to control air flow into the engine

-	The Throttle Position (TP) sensor (5)

-	The Idle Air Control (IAC) valve (6)

-	The engine speed governor motor and throttle drive assembly (3)

-	The vacuum ports located at, above, or below the throttle valves to generate vacuum signals to various controls and accessories

Fuel Injectors

The Model 295 TBI features 2 injectors designed with the flow capacity required for the engine application.

The fuel injectors are solenoid-operated device, controlled by the ECM. The ECM turns ON the solenoid, which lifts a normally closed ball valve off a seat. The fuel, under pressure, is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve.

The fuel which is not used by the injectors passes through the pressure regulator before being returned to the fuel tank.

Pressure Regulator


(1)	TBI Injector Assembly
(2)	Fuel Meter Body Cover
(3)	Fuel Meter Body Assembly
(4)	Fuel Inlet
(5)	Fuel Outlet
(6)	Throttle Body Assembly
(7)	Pressure Regulator Cover Assembly
(8)	Pressure Regulator Spring
(9)	Pressure Regulator Diaphragm Assembly

The pressure regulator is an integral part of the fuel meter assembly, and contains an air chamber and a fuel chamber that are separated by a diaphragm-operated relief valve and a calibrated spring. The function of the regulator is to maintain a constant fuel pressure drop across the injectors at all times. Fuel pressure is regulated by the difference between fuel pump pressure acting on one side of the diaphragm and the force of the calibrated spring (and air cleaner pressure) acting on the other side.

The pressure regulator includes an internal constant bleed feature that relieves system pressure when the engine is turned OFF. The bleed also helps to purge vapors from the injector and regulator after a hot restart. Fuel in excess of injector needs is returned to the fuel tank by a separate line.

Throttle Position (TP) Sensor

The Throttle Position (TP) sensor (2), mounted on the side of the throttle body opposite the throttle lever assembly, senses the current throttle valve (3) position and relays that information to the ECM (1). The throttle position information allows the ECM to generate the required injector control signals (base pulse).

If the TP sensor senses a wide open throttle, a voltage signal indicating this condition is sent to the ECM. The ECM then increases the injector base pulse width, permitting increased fuel flow.

As the throttle valve rotates in response to movement of the accelerator pedal, the throttle shaft transfers this rotation movement to the TP sensor. A potentiometer (variable resistor) within the TP sensor assembly changes its resistance (and voltage drop) in proportion to throttle movement.

By applying a reference voltage (5.0 volts) to the TP sensor input, a varying voltage (reflecting throttle position) is available at the TP sensor output. For example, approximately 2.5 volts results from a 50 percent throttle valve opening (depending on TP sensor calibration). The voltage output from the TP sensor assembly is routed to the ECM for use in determining throttle position.

Idle Air Control (IAC) Valve


(1)	Fuel Meter Body Assembly
(2)	Throttle Body Assembly
(3)	Engine/Emission Control Vacuum Ports
(4)	Throttle Valve
(5)	Idle Air Control (IAC) Valve Assembly
(6)	Pintle
(7)	Filtered Air Inlet

The ECM, through the IAC Valve, controls the engine idle speed. The Idle Air Control (IAC) valve is on the throttle body. The ECM sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move IN (retract) or OUT (extend) a given distance (number of steps) for each pulse, (called counts).

This movement controls airflow around the throttle plate, which in turn, controls engine idle speed. The IAC valve pintle position counts can be seen using a scan tool. 0 counts corresponds to fully closed passage, while 140 counts or more (depending on the application) corresponds to full flow.

•	The ECM positioning the IAC valve pintle obtains the actual or controlled idle speed. The total idle air flow (IAC/passage + PCV + throttle valve + calibrated vacuum loss through accessories) generates the resulting idle speed is generated from.

•

Controlled idle speed is always specified for normal operating conditions. The normal operating condition is the coolant temperature in the operating range, the A/C is OFF, the manual transmission is in neutral or the automatic transmission is in drive. A high or low coolant temperature, or the A/C clutch engaged may signal the ECM to change the IAC counts.

The minimum idle air rate is set at the factory with a stop screw to permit enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts) from the seat during normal controlled idle operation.

Engine Speed Governor System

Governor System Block Diagram


(1)	Network of Engine Sensors
(2)	A3
(3)	Ignition
(4)	Engine Speed Governor Module
(5)	GRN
(6)	Engine Control Module (ECM

The ECM through the engine speed control governor module (governor motor driver circuit) electronically actuates and controls the DC governor motor. Governed operation prevents excessive engine speed under light loads, yet allows complete throttle opening for full power under heavy loads.

The components of the engine speed governor system are the ECM, the engine speed governor module, and the governor throttle drive assembly. The throttle body assembly also contains the throttle and governor parts that interface with the governor throttle drive assembly.

The model TBI 295 throttle body assembly is unique because the throttle lever cam is not attached directly to the throttle shaft. Instead, the throttle lever cam is attached to its own shaft with a throttle drive lever on the opposite end that engages a fixed lever permanently attached on the main throttle shaft. This split arrangement allows the throttle lever cam to close the throttle valves, but not to open them directly. The throttle lever cam drive only limits the maximum opening of the throttle valves. Based on engine speed and engine load, the governor controls the actual position of the throttle valves, within that range of possible opening.

Governor Motor and Throttle Drive Components


(1)	Throttle to Motor Link
(2)	Fixed Throttle Shaft Lever
(3)	Idle Stop Screw
(4)	Throttle Drive Lever
(5)	Governor Drive Lever
(6)	Shock Absorbers
(7)	Throttle Drive Motor
(8)	Governor Drive Lever Spring

A network of engine sensors, such as the Throttle Position (TP) sensor, the Manifold Absolute Pressure (MAP) sensor, and the distributor reference pulses (RPM) provides continuous feedback information on engine operating conditions to the ECM. The ECM uses the feedback information on the throttle valve position and the engine RPM in order to determine whether or not the governed operation is needed.

At low engine speeds, below approximately 3800 RPM, the governor drive motor (#2) is not energized and the governor drive motor lever (#4) returns, by spring force, to the parked position. That position is determined by contact between the governor motor drive lever and the shock absorber stop (#3). This allows the throttle valves to cycle with the throttle lever cam position, and no governing action takes place. At higher engine speeds, about 3800-4000 RPM, where the governing is needed, the ECM increases the current flow to energize the governor drive motor (#2) through the engine speed control governor module. Control is based on TP sensor feedback and engine speed input to the ECM. When the engine load increases and the RPM drops, the electrical current supply to the motor is reduced by the ECM through the engine speed control governor module, thus allowing the throttle valves to open farther. If a problem is detected in the governor system, the fuel cutoff will limit the engine speed and the vehicle speed.

Vacuum Ports

The throttle body may contain ports located above or below the throttle valves. These ports generate the vacuum signals for the Exhaust Gas Recirculation (EGR) valve, the Manifold Absolute Pressure (MAP) sensor, and the canister purge of the EVAP system.

Accelerator Control

The accelerator control system is a control cable type, attached to a accelerator pedal assembly.

Evaporative Emission (EVAP) Control System

The system transfers the fuel vapors from the fuel tank into a vapor canister and then vapors are purged into the intake manifold air flow and consumed in combustion.