Technical Service Bulletins And Scan Tool Data

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• Technical Service Bulletins
• Technical service bulletins (TSBs) are written by
  the vehicle manufacturers.
• A TSB is generated whenever one or both of the
  following conditions exist
• A part change is released to solve a problem or
  potential problem.
• A service procedure has been developed to solve a
  problem or a potential problem.
• Due to the cost involved of the creation of a
  typical TSB, many are unpublished yet available
  to authorized dealer personnel through the
  technical assistance office

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• Vehicle Identification (continued)
• The first number or letter designates the
  country of origin.
• The model of the vehicle is commonly the fourth
  or fifth character.
• The eighth character is often the engine code.
  (Some engines cannot be determined by the VIN
  number.)

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• Vehicle Identification (continued)
• The tenth character represents the year on all
  vehicles.

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• Vehicle Safety Certification Label
• This label indicates the month and year of
  manufacture as well as the gross vehicle weight
  rating (GVWR), the gross axle weight rating
  (GAWR), and the vehicle identification number
  (VIN).

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• VECI Label
• The vehicle emissions control information (VECI)
  label under the hood of the vehicle.

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• VECI Label (continued)
• The VECI label usually includes the following
  information
• Engine identification
• Emissions standard that the vehicle meets
• Vacuum hose routing diagram
• Base ignition timing (if adjustable)
• Spark plug type and gap
• Valve lash
• Emission calibration code

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• Calibration Codes
• Calibration codes are usually located on power
  train control modules (PCMs) or other controllers.

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• Casting Numbers
• Whenever an engine part such as a block is cast,
  a number is put into the mold to identify the
  casting.

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• Scan Tool Testing
• A data stream means that information regarding
  the sensor values and other parameters such as
  engine speed can be displayed on a scan tool.
• These scan tool values are also called parameter
  identification (PID).

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• Scan Data Retrieval Procedure
• Check for any stored DTCs.
• Check ECT and IAT temperatures.
• Both should be the same because both the air
  temperature and the coolant temperature should be
  at the ambient (surrounding) air temperature.
• If the two temperatures do not match, the sensor
  (or wiring) giving the reading that varies most
  from the ambient air temperature is at fault.

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• Scan Data Retrieval Procedure (continued)
• 3. Check the idle air control (IAC) position.
• If the IAC position is too low, this indicates
  that the computer is commanding less air or a
  lower idle speed. The most common reason for
  this is a vacuum leak caused by a defective
  vacuum hose, PCV valve, or gasket.
• If the IAC position is too high, this indicates a
  partially clogged throttle plate or a misadjusted
  throttle linkage. If the idle speed is also too
  high, the computer is commanding a higher than
  normal idle, usually to compensate for low
  battery voltage or an engine that is running too
  hot.

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• Scan Data Retrieval Procedure (continued)
• 3. Check the idle air control (IAC) position.
  (continued)
• NOTE Mass air flow engines may react
  differently, depending on a vacuum leak affecting
  engine operation. For example, a vacuum leak on
  an engine equipped with a MAF sensor may cause
  the engine to slow due to the false air causing
  the IAC counts to increase rather than decrease,
  which normally occurs on all engines that use the
  speed density of fuel control.

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• Scan Data Retrieval Procedure (continued)
• The fourth step is to check the oxygen sensor(s)
  (O2S) for proper operation and see if the vehicle
  computer can control the air-fuel mixture.
• Check the O2S for range (minimum of 200
  millivolts and maximum of 800 millivolts).
• Look at long-term fuel trim (LTFT).
• It should be zero, indicating that the computer
  is neither adding nor subtracting fuel.
• If the LFTF exceeds 20 - either the positive or
  the negative side a possible fuel or engine
  problem is indicated.

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• Injector Pulse Width Problem Diagnosis
• The fuel injector pulse is the amount of time the
  injector(s) is being pulsed, measured in
  milliseconds (ms).
• The larger the pulse width, the greater the
  amount of fuel being supplied to the cylinder.
• The pulse width can also be easily checked on
  some vehicles using a scan tool, meter, or scope.

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• Injector Pulse Width Problem Diagnosis
  (continued)
• 1.5 to 3.5 ms A normal reading for idle or 2500
  RPM with no load.
• 0.5 to 1 ms The low pulse width indicates the
  computer is attempting to reduce the amount of
  fuel delivery. The engine is likely to be
  receiving fuel from another source such as
• Through a defective fuel pressure regulator.
• A defective charcoal (carbon) canister control
  valve.
• The PCV system with gasoline in the engine oil.
• Stuck open injector.

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• Injector Pulse Width Problem Diagnosis
  (continued)
• 8 ms or more A higher than normal pulse width
  indicates that the computer is attempting to
  supply more fuel than normal to compensate for a
  lean air-fuel mixture. This could be caused by
• An intake manifold leak.
• A vacuum hose leak.
• An engine condition problem such as a misadjusted
  intake valve(s), broken valve spring, or other
  faults that could cause an intake valve to remain
  open.

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• Oxygen Sensor Activity
• The oxygen sensor has to be at operating
  temperature above 600F (315C) before accurate
  data from O2S is possible.
• Look for the following at 2500 engine RPM
• At least 800 mV or higher
• At least 200 mV or lower

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• Oxygen Sensor Activity (continued)
• Rapid changes of the reading when the throttle is
  held steady and switching from rich to lean and
  lean to rich one to five times per second (1 to 5
  Hz.).

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• Oxygen Sensor Activity (continued)
• If the reading stays high most of the time, the
  fuel mixture is rich or the O2S is defective.
• If the reading stays low most of the time, the
  mixture is lean or the sensor is detecting a lean
  condition.

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• The Need For Fuel Trim
• The purpose of fuel trim is to provide the
  catalytic converter with a rich and a lean
  air-fuel mixture.
• A rich mixture (lack of extra oxygen) is needed
  by the catalytic converter to reduce NOx exhaust
  emissions.
• The catalytic converter, however, must also
  receive a lean mixture (excessive oxygen) to help
  oxidize HC and CO into harmless carbon dioxide
  (CO2) and water (H2O) vapor.

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• Short-Term Fuel Trim
• Short-term fuel trim (STFT) is a percentage
  measurement of the amount the computer is adding
  or subtracting from a calculated value.
• Electronic fuel injector systems use the oxygen
  sensor (O2S) to determine whether the exhaust is
  rich or lean.
• Fuel trim is expressed as a percentage () either
  positive () or negative (-) and represents the a
  mount of fuel different from the anticipated
  amount.

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• Short-Term Fuel Trim (continued)
• A short-term fuel trim of 20 indicates that 20
  additional fuel had to be added to be able to
  achieve the proper air-fuel mixture.
• A -20 short-term fuel trim indicates that fuel
  had to be removed by shortening the injector
  pulse width to achieve the proper air-fuel
  mixture.
• Short-term fuel trim represents actions by the
  computer over a relatively short time.

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• Long-Term Fuel Trim
• Long-term fuel trim (LTFT) is designed to add or
  subtract fuel for a larger amount of time than
  short-term fuel trim.
• When the LTFT makes an adjustment, the STFT can
  still make short and quick changes in the
  air-fuel mixture needed to provide the catalytic
  converter with an alternating rich, then lean,
  then rich, etc. exhaust.

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• Using Fuel Trim As A Diagnostic Aid
• A scan tool will display both short-term and
  long-term fuel trim.
• Vehicle 1
• STFT 5, LTFT 20
• Explanation The computer is responding to a
  lean condition. The LTFT indicates that the
  programmed amount of fuel had to be increased by
  20 to achieve the proper air-fuel mixture to the
  level where the STFT could toggle the mixture
  rich and lean for most catalytic converter
  efficiency. Look for a vacuum leak or low fuel
  pressure.

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• Using Fuel Trim As A Diagnostic Aid (continued)
• Vehicle 2
• STFT 10, LTFT 0
• Explanation Readings are perfect. It is normal
  for the STFT to add or subtract up to 20 to
  achieve the proper air-fuel mixture.

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• Using Fuel Trim As A Diagnostic Aid (continued)
• Vehicle 3
• STFT -10, LTFT -30
• Explanation The engine was rich because the
  LTFT had to remove 30 of the anticipated amount
  of fuel to achieve the proper air-fuel mixture.
  Look for a defective (stuck open) injector,
  defective fuel pressure regulator, or a
  restriction in the intake air passage.

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• Fuel Trim Cells
• Both STFTs and LTFTs react to oxygen sensor
  voltage to modify fuel delivery.
• Most vehicles set aside different cells for each
  combination of engine speed (RPM) and load.
• The computer can then correct for slight
  differences in fuel mixture separately for each
  cell.

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Fuel Trim Cell Diagnosis (continued)
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• Generic And OEM Data
• Generic OBD II data includes fuel trim and other
  major system component information that is
  required to be able to be read by a generic scan
  tool.
• Original equipment manufacturer (OEM) data
  usually includes more information and additional
  diagnostic trouble codes (DTCs) not displayed by
  a generic scan tool.

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Scan Tool Diagnosis Photo Sequence
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Advanced Engine Performance Diagnosis