Chapte
r E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
95
proper operation.
• Inspect for proper operation of the manifold ab-
solute pressure (MAP) sensor.
• Inspect for an engine coolant temperature (ECT)
sensor that has shifted in value.
• Inspect the MAF sensor and intake air system for
proper operation.
Engine Hesitation Diagnosis
Momentary lack of response as the accelerator is
pushed down. Can occur at any vehicle speed. Usually
more pronounced when first trying to make the vehicle
move, as from a stop. May cause the engine to stall if
severe enough.
• 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 PCM 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
proper operation.
• Inspect for proper operation of the manifold ab-
solute pressure (MAP) sensor.
• Inspect for an engine coolant temperature (ECT)
sensor that has shifted in value.
• Inspect the MAF sensor and intake air system for
proper operation.
Poor Fuel Economy Diagnosis
Fuel economy, as measured by an actual road test, is
noticeably lower than expected. Also fuel economy is
noticeably lower than the economy was on this vehicle
at one time, as previously shown by an actual road test.
• Inspect the engine control module (ECM) grounds
for being clean, tight, and in the proper locations.
• Discuss driving habits with the owner.
• Is the A/C ON or the Defroster mode ON full time?
• Are the tires at the correct pressure?
• Are the wheels and tires the correct size?
• Are there excessively heavy loads being carried?
• Is the acceleration rate too much, too often?
• Remove the air filter element and inspect for dirt or
for restrictions.
• Inspect the air intake system and crankcase for air
leaks.
• Inspect the crankcase ventilation valve for proper
operation.
• Inspect for an inaccurate speedometer.
Engine Surges Diagnosis
Engine power variation under steady throttle or cruise.
Feels like the vehicle speeds up and slows down with
no change in the accelerator pedal position.
• 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 mass air flow (MAF) sensor for any con-
tamination on the sensing element.
• Inspect the air intake ducts for being collapsed,
damaged, loose, improperly installed, or leaking es-
pecially between the MAF sensor and the throttle
body.
• Test the exhaust gas recirculation (EGR) system for
proper operation.
• Inspect for proper operation of the manifold ab-
solute pressure (MAP) sensor.
• Inspect for an engine coolant temperature (ECT)
sensor that has shifted in value.
Rough, Unstable or Incorrect Idling and Stalling
Diagnosis
Engine runs unevenly at idle. If severe, the engine or ve-
hicle may shake. Engine idle speed may vary in RPM.
Either condition may be severe enough to stall the en-
gine.
Inches of
Mercury Absolute
Inches of
Mercury Vacuum
MAP Sensor Signal
Voltage (volts)
31.00
0.5 psi
4.80
29.92
27.00
25.00
23.00
20.00
15.00
10.00
5.00
0.00
2.92
4.92
6.92
9.92
14.92
19.92
24.92
4.60
4.10
3.80
3.45
2.92
2.09
1.24
0.24
Fig. 5-10.
Sample MAP sensor voltage chart.
Summary of Contents for ASE-A8
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