Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
94
faulty circuit or component. Look for unusual circuit ac-
tivity, such as high or low signals, voltage dropout, and
lack of switching. When a problem is indicated on one
circuit, be sure to check for unusual activity on all of the
related circuits.
Diagnosis Of Emissions And Driveability
Problems
No Start Diagnosis
To run, an engine requires four things: air, fuel, com-
pression and ignition, all at the right time. Perform the
following tests to find what the problem is:
• Observe the engine's cranking speed; if it is too
slow check the battery and starting system.
• Check fuel pressure and volume
• Verify the electrical signal to the injector with a 12V
test light, depending on the OEM's recommenda-
tion
• Use a properly gapped spark tester to check for
spark
• Check compression by performing a cranking vac-
uum or compression test
• Check the ignition timing
• Verify camshaft drive integrity and valve timing
Hard Start Diagnosis
A variety of sensor or physical conditions may result in
a hard start condition without setting a diagnostic trou-
ble code (DTC). In order to determine if any of these
conditions exist, perform the following actions:
• Inspect for an engine coolant temperature (ECT)
sensor that has shifted in value.
• Inspect the mass air flow (MAF) sensor for proper
installation.
• Inspect the camshaft position (CMP) sensor for
proper mounting and/or a bad connection. An ex-
tended crank occurs if the engine control module
(ECM) does not receive a CMP signal.
• Verify proper operation of the manifold absolute
pressure (MAP) sensor.
• Inspect the exhaust gas recirculation (EGR) system
for proper sealing/connections and operation.
Engine Misfire Diagnosis
• Inspect the engine control module (ECM) grounds
for being clean, tight, and in the proper locations.
• Inspect the heated oxygen sensors (HO2S). The
HO2S should respond quickly to different throttle
positions. If they do not, inspect the HO2S for sili-
con or other contaminates from fuel or the use of
improper RTV sealant. The sensors may have a
white, powdery coating and result in a high but false
signal voltage rich exhaust indication. The ECM will
then reduce the amount of fuel delivered to the en-
gine causing a severe driveability problem.
• Inspect the air intake ducts for being collapsed,
damaged, loose, improperly installed, or leaking es-
pecially between the mass air flow (MAF) sensor
and the throttle body.
• Test the exhaust gas recirculation (EGR) system for
Fig. 5-9
. The above data can be accessed by the technician using the OBD II scan tool and the Data Link Connector (DLC).
Summary of Contents for ASE-A8
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