Chapter A: General Diagnosis
11
Fuel System Diagnosis
Fuel system malfunctions can cause an assortment of
driveability problems, such as engine surging, stalling
and misfiring, hard or no starting, and poor fuel mileage.
A restriction somewhere in the fuel or air supply is often
the cause. Pressure testing is the most effective way to
locate fuel supply problems. Test procedures are cov-
ered in Chapter Three of this book.
The results of engine diagnostic tests, such as manifold
vacuum, cylinder power balance, oscilloscope analysis,
exhaust gas analysis, and onboard diagnostics, may in-
dicate fuel system problems.
When checking manifold vacuum, look for an oscillat-
ing needle with a reading slightly below normal at idle.
This indicates an incorrect fuel mixture. A defective port
fuel injector can cause a cylinder with good compres-
sion to show poor power balance test results. Onboard
diagnostic tests can reveal faults in the electronic cir-
cuits. For oscilloscope patterns that indicate fuel prob-
lems, refer to Ignition System Diagnosis discussed pre-
viously.
Electrical System Diagnosis
Hard starting, no start, and slow cranking speed can in-
dicate an electrical problem. Check the battery, starting
system, and charging system. Specific testing proce-
dures are covered in Chapter Six of this book. Most
minor electrical and electronic system malfunctions can
be detected by self-diagnostic and scan tool testing. In-
dividual components are tested with a digital multime-
ter (DMM), or oscilloscope. Component testing is de-
tailed in the subsequent chapters of this book.
Accessing and interpreting stored computer codes will
be discussed there as well.
Onboard Diagnostic Systems
The computers used in late-model engine control sys-
tems are programmed to check their own operation, as
well as the operation of each sensor, actuator, and cir-
cuit, figure 1-11. Whenever the computer recognizes a
Fig. 1-11.
The onboard diagnostic system, which monitors sensor and actuator signals, records a DTC when a circuit abnormality occurs.
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