AIR
INTAKE
INJECTOR
FUEL PRESSURE
REGULATOR
FUEL
SUPPLY
FROM TANK
FUEL
RETURN
TO TANK
THROTTLE BODY
(FUEL CHARGING
ASSEMBLY)
THROTTLE
PLATE
INJECTION
Fig. 3-24.
Throttle body fuel injection assembly.
Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
67
THROTTLE BODY FUEL INJECTION
Throttle body, or single point, fuel injection is a common
feature on many domestic engines. The throttle body
assembly houses the throttle plates and provides the
mounting point for the fuel injectors, idle speed con-
troller, and throttle position sensor, figure 3-24. A fuel
pressure regulator and temperature sensor may also be
incorporated into the unit. Most of these items can be
serviced without removing the throttle body from the ve-
hicle.
If the internal passages become gummed up, the throt-
tle body can be removed, disassembled, cleaned, and
resealed. Remove the unit from the vehicle and disas-
semble it. Once the components have been removed
from the casting, it can be cleaned with a soft-bristle
brush and an aerosol cleaner, or submerged in carbure-
tor cleaner if necessary. Reassemble using new gaskets
and O-rings, then reinstall it on the intake manifold.
There are no adjustments to make during reassembly.
Adjusting the throttle position sensor, idle speed, and
fuel mixture are covered later in this book.
AIR INTAKE SYSTEM SERVICE
The air intake system consists of:
• Air intake ductwork
• Air cleaner and filter element
• Intake manifold
Air Intake Ductwork
A typical late-model air intake system is a complex sys-
tem of ducts, filters, meters, valves, and tubes, figure
3-25. When servicing, make sure the intake ductwork is
properly installed and all connections are airtight. Any
air leakage downstream of the airflow sensor can create
a lean condition and affect engine performance.
Air Filter Service
The air filter element is the only part of the intake air sys-
tem that requires periodic replacement. Although
change intervals vary by manufacturer, the air filter ele-
ment on most late-model engines is replaced every
30,000 miles. In particularly dusty areas, or when oper-
ated under severe service, the filters should be checked
and changed more frequently. An accumulation of en-
gine oil on the element or in the housing indicates me-
chanical problems. Oil entering the air filter is the result
of a crankcase ventilation system malfunction or exces-
sive blow-by. A paper element cannot be cleaned, and
must be replaced if it is dirty or torn.
Carbureted and throttle body injected engines have the
air filter mounted directly to the carburetor or throttle-
body. To replace the filter element, remove the fasteners
holding the housing cover, lift off the cover, and take out
the element. Check top and bottom seals for dust leak-
age and check the filter material for breaks.
Most fuel injected engines use a flat filter element in-
stalled in a remote air cleaner housing. Some of the
ductwork may have to be disconnected and moved
aside or removed to access the filter housing. Unfas-
tenthe clips holding the housing together, then lift out
the filter element.
Intake Manifold
During service, the intake manifold should be checked
for secure mounting and vacuum leaks. Be aware, a
vacuum leak normally influences the engine more at idle
because of higher intake vacuum than at higher engine
speeds when the percentage of air entering is less. Di-
agnostic time can be decreased by understanding what
to expect of the fuel system when there is a vacuum
leak.
Carbureted engines with a vacuum leak affecting one
cylinder will typically lean the fuel mixture only of that
one cylinder, causing the engine to run worse at idle.
The same engine with a centrally located leak will lean
the fuel mixture for all cylinders. Note that a “feedback”
carbureted system may be able to compensate for the
leak after entering the closed loop operational mode.
Summary of Contents for ASE-A8
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