The Diagnostic Process

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Chapter 1
The Diagnostic Process
Advanced Engine Performance Diagnosis 3rd Edition
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Chapter 1
The Diagnostic Process

• CHAPTER OBJECTIVES
• Prepare for Engine Performance (A8) ASE
  certification test content area A (General
  Engine Diagnosis).
• List the steps of the diagnostic process.
• Describe the preliminary simple tests that should
  be performed at the start of the diagnostic
  process.
• List six items to check as part of a thorough
  visual inspection.
• List the six troubleshooting fundamental
  principles.

Advanced Engine Performance Diagnosis
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• The 8-Step Diagnostic Procedure
• Step 1 Verify the Problem

Advanced Engine Performance Diagnosis
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• The 8-Step Diagnostic Procedure (continued)
• Step 1 Verify the Problem (continued)
• Is the malfunction indicator light (MIL or
  check engine) on?
• What was the temperature outside?
• Was the engine warm or cold?
• Was the problem during starting, acceleration,
  cruise, ect.?
• At what distance?
• Were any dash warning lights on? If so, which
  one?
• Has there been any service or repair work
  performed on the vehicle lately?

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• The 8-Step Diagnostic Procedure (continued)
• Step 2 Perform a Thorough Visual Inspection
  and Basic Tests
• The visual inspection is the most important
  aspect of diagnosis.
• Most experts agree that between 10 and 30 of
  all engine performance problems can be found
  simply by performing a thorough visual inspection.

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• The 8-Step Diagnostic Procedure (continued)
• Step 2 Perform a Thorough Visual Inspection
  and Basic Tests (continued)
• Check for obvious problems
• Fuel leaks
• Vacuum hoses disconnected or split
• Corroded connectors
• Unusual noises, smoke, or smell
• Check the air cleaner and air duct (squirrels,
  etc., can build nests or collect dog food)

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• The 8-Step Diagnostic Procedure (continued)
• Step 2 Perform a Thorough Visual Inspection
  and Basic Tests (continued)
• Check everything that does and does not work.
  This step involves actively turning things on and
  observing that everything is working properly.
• Look for evidence of previous repairs. Anytime
  work is performed on a vehicle, there is always a
  risk that something will be disturbed, knocked
  off, or left disconnected.

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• The 8-Step Diagnostic Procedure (continued)
• Step 3 Retrieve the Diagnostic Trouble Codes
• Obviously, if a diagnostic trouble code (DTC) is
  present in the computer memory, it is displayed
  by illuminating a malfunction indicator lamp
  (MIL), commonly labeled check engine or service
  engine soon.

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• The 8-Step Diagnostic Procedure (continued)
• Step 4 Check for Technical Service Bulletins
  (TSBs) (continued)
• According to studies performed by automobile
  manufacturers, as high as 30 of the vehicles can
  be repaired following the information,
  suggestions, or instructions concerning
  replacement parts found in a service bulletin.
• DTCs must be known before searching for service
  bulletins because bulletins often include
  information on solving problems that involve a
  stored diagnostic trouble code.

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• The 8-Step Diagnostic Procedure (continued)
• Step 5 Look at Scan Tool Data (continued)
• The best way to look at scan data is with a
  definite sequence and specific selected bits of
  data that can tell the most about the operation
  of the engine such as
• Check that engine coolant temperature (ECT) is
  the same as intake air temperature (IAT) after
  the vehicle sits for several hours.

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• The 8-Step Diagnostic Procedure (continued)
• Step 5 Look at Scan Tool Data (continued)
• Check that the MAP sensor reading is the same as
  the BARO sensor reading (if equipped).
• Check that idle air control (IAC) valve is being
  commanded to an acceptable range.
• Check for proper operation of oxygen sensor
  (O2S).
• Should read below 200 mV at times
• Should read above 800 mV at times
• Transitions between rich and lean should occur
  between 1 and 5 times per second

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• The 8-Step Diagnostic Procedure (continued)
• Step 6 Narrow the Problem to a System or
  Cylinder (continued)
• Narrowing the focus to a system or individual
  cylinder is the hardest part of the entire
  diagnostic process.
• The following items must be checked and
  interpreted
• Study long-term fuel trim to see if the computer
  had to add or subtract fuel.
• Use exhaust analysis to help find the cause of
  the problem.
• Perform a cylinder power balance test.
• If a weak cylinder is detected, perform a
  compression and a cylinder leakage test to
  determine the probable cause.

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• The 8-Step Diagnostic Procedure (continued)
• Step 7 Repair the Problem and Determine the
  Root Cause (continued)
• Follow manufacturers recommended repair
  procedures and methods.
• If the problem still exists, reexamine the
  original concern.
• Study the vehicle systems.
• Use background knowledge and help from other
  service technicians.

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• The 8-Step Diagnostic Procedure (continued)
• Step 7 Repair the Problem and Determine the
  Root Cause (continued)
• Determine the fault (causes).
• Ask yourself the 5 whys.
• Why did that fault occur?
• Why did that fault occur?
• Why did that fault occur?
• Why did that fault occur?
• Why did that fault occur?

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• The 8-Step Diagnostic Procedure (continued)
• Step 8 Verify the Repair and Clear Any Stored
  DTCs (continued)
• Test drive to verify that the original problem
  (concern) is fixed.
• Verify that no additional problems have occurred
  during the repair process.

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• The 8-Step Diagnostic Procedure (continued)
• Step 8 Verify the Repair and Clear Any Stored
  DTCs (continued)
• Clear all DTCs. (This stpe ensures that the
  computer will not make any changes based on any
  stored DTC.)
• Return the vehicle to the custom and double check
  that
• The vehicle is clean.
• The radio is turned off.
• The clock and radio stations have been restored
  if the battery was disconnected during the repair
  procedure.

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• The 3 Cs
• Whenever working on a vehicle, always remember to
  record the 3 Cs on the repair (work) order.
• Complaint
• Cause
• Correction

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• Troubleshooting Fundamentals
• Principle 1 A lean air-fuel mixture is more
  likely to cause a driveability problem than a
  rich air-fuel mixture.
• Principle 2 Always check for the output of a
  component or sensor first.
• Principle 3 Use a commonsense approach to
  every problem.

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• Troubleshooting Fundamentals (continued)
• Principle 4 Electrical components,
  connectors, and grounds are the cause of many
  engine performance problems (especially
  intermittent problems).
• Principle 5 The power and ground of each
  electrical component should always be checked.
• Principle 6 All engines require an adequate
  supply of good clean fuel under pressure.

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• CHAPTER SUMMARY
• Funnel Diagnostics Visual Approach to a
  Diagnostic Procedure
• Step 1 Verify the Problem (Concern)
• When does the problem occur?
• Is the engine cold or normal operating
  temperature?
• Does the problem occur during starting,
  acceleration, cruise, etc.?
• Are any dash warning lights on when the problem
  occurs? If so, which ones?
• Check with the customer about any previous
  repair work done on the vehicle.
• Perform a thorough test-drive under similar
  conditions to verify the complaint (problem or
  concern).

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• CHAPTER SUMMARY (continued)
• Step 2 Perform a Thorough Visual Inspection
  and Basic Tests
• Check for obvious problems, including
• fuel leaks
• vacuum hoses disconnected or split
• corroded connectors
• unusual noises, smoke, or smell
• air passage restriction
• Check everything that does and does not work
• Look for evidence of previous repairs
• Perform a paper test to check for smoothness of
  engine operation
• Perform a dipstick test for low oil level or
  gas-contaminated oil
• Check adequate fuel level ¼ to ½ tank
• Check battery and charging voltage (13.5 to
  15.0 volts at 2000 RPM)
• Check that the ignition system is able to fire
  a spark tester (at least 25,000 volts)
• Check fuel pump pressure

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CHAPTER SUMMARY (continued) Step 3 Retrieve
the Diagnostic Trouble Codes (DTCs) Step 4
Check for Technical Service Bulletins
(TSBs) Step 5 Look Carefully at Scan Tool
Data Step 6 Narrow Focus of Problem to a
System or Cylinder Step 7 Repair the Problem,
Determine and Correct the Root Cause Step 8
Verify the Repair and Clear Any Stored DTCs
Advanced Engine Performance Diagnosis