Page 223 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 207

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Figure 2.261 Throttle body assembly
Figure 2.260 Exhaust gas recirculation (EGR) system

Service and on-board diagnostic (OBD) plugs are used for diagnostic and
Key fact
corrective actions with scan tools, dedicated test equipment and other test
equipment. If faults are detected the system malfunction indicator lamp (MIL) Service and on-board diagnostic
will come on. Alternatively, it will fail to go out after the preset time duration after (OBD) plugs are used for diagnostic
and corrective actions with scan
switching on the engine. All faults should be investigated as soon as possible.
tools, dedicated test equipment and
Many electronic systems have a limp-home or limited operation strategy (LOS)
other test equipment.
program, which allows the vehicle to be driven to a workshop for repair.
2.5.2.2 Air supply
The air supply components consist of ducting and silencing components
between the air intake and the inlet manifolds. This will also include an air fi lter, a
throttle body, throttle plate assembly and idle control components. The air supply
components must provide suffi cient clean air for all operating conditions. The air

fl ow into the engine would be noisy and unbalanced between cylinders without
the use of resonators and plenum chambers. A plenum chamber is a large
volume air chamber that can be fi tted either in front of or behind the throttle plate
housing.
Air fi lters on most modern petrol (gasoline)-engined vehicles consist of a plastic
casing with a paper fi lter element. Air fl ow into the fi lter is upwards so that dust
and dirt particles drop into the dust chamber, or rotary so that dust and dirt are
thrown out before the air enters the engine. Crankcase ventilation and the air
supply or pulse air exhaust emission systems are also connected to the fi lter
assembly.
The throttle is a conventional circular plate in an air tube ( Fig. 2.261 ). For fast
idle and warm up, an auxiliary air valve is fi tted to bypass the throttle plate, or an
electromechanical link is made to the throttle plate spindle. An auxiliary air valve,
idle air control (IAC) or idle speed control (ISC) valve is operated from signals
from the ECM.
The auxiliary air valve is often a rotary air valve. This has a special electric
motor to move and hold the valve in position. The position is based on the
electrical signals supplied by the ECU. Two electric windings in the motor work
in opposition to each other so that the motor is variable over a 90° arc. Other
designs have graduated opening values based on the signal supplied from
the ECU. This type consists of a solenoid valve with a spring-loaded armature