Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
68
Most TBI engines react differently than carbureted en-
gines because they use a speed density method of fuel
injection. A vacuum leak into one cylinder may also lean
the fuel mixture for that one cylinder. A centrally located
vacuum leak will not influence the fuel delivery, but will
raise idle RPM. Vacuum leaks at the vacuum signal to
the MAP sensor will enrich the fuel mixture.
Multiport fuel injected (MFI) engines of the speed den-
sity type will raise idle RPM when a leak occurs, except
if the leak appears at the MAP sensor itself. MAP signal
leaks will enrich the mixture.
Multiport fuel injected engines that use the mass air flow
(MAF) or volume airflow methods of fuel injection lean
the fuel mixture to all cylinders with any type of vacuum
leak. This is because the air induced through the leak is
not being metered by the airflow sensor.
EXHAUST SYSTEM SERVICE
Servicing the exhaust system consists of inspecting the
pipes, brackets, mufflers, and catalytic converters, as
well as inspecting and testing the manifold heat control
valves. Check each component for damage, exhaust
leakage, loose connections, and loose mounting. Tight-
en where required. The converter or muffler generally is
not repaired if damaged, but is replaced with a new one.
The replacement procedure essentially is the same for
both.
Exhaust Manifold Service
Inspect the exhaust manifold to cylinder head area. If
signs of leaking exist remove the manifold for further in-
spection.
Check the manifold and head sealing surfaces for:
• Warping or distortion
• Erosion
• Cracks
• Pitting
If any problems are found, refer to the Service Manual
for applicable repair procedures. Also inspect the seal
between the manifolds and the exhaust pipe. Replace
or repair as necessary.
Exhaust System Inspection
Inspect the following components for signs of damage,
leaking, corrosion, or other abnormalities:
• All exhaust piping
• Muffler(s)
• Resonator(s)
• Heat shields
Repair or replace any components that are defective,
figure 3-26, following procedures outlined in the appro-
priate Service Manual.
FRESH
AIR
INLET
AIR PREHEAT
CONTROL VALVE
(VACUUM MOTOR)
INTAKE AIR
THROTTLE BODY
AIR OUTLET
TUBE
VANE AIRFLOW
METER
AIR FILTER
AIR CLEANER
HOUSING
HOT AIR
INLET TUBE
Fig. 3-25.
The intake ductwork delivers fresh air to the combustion chambers.
Summary of Contents for ASE-A8
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