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Diagnostic Procedure

WARNING: Wrap a shop towel around the fuel pressure connection in order to reduce the risk of fire and personal injury. The towel will absorb any fuel leakage that occurs during the connection of the Fuel Pressure Gauge. Place the towel in an approved container when the connection of the fuel pressure gauge is complete.
NOTE: Fuel Composition Tester (J-44175), Fuel Pressure Gauge (J-34730-1A) and Fuel Pressure Gauge Adapter (J-42242) are required to perform this procedure. See Figure -Figure .
NOTE: The numbers in parenthesis in the following steps refer to numbered components in figures.
  1. Turn ON Fuel Composition Tester.
  2. Verify the Fuel Composition Tester is operational by measuring the AC frequency output with a DVOM. A frequency without a fuel sample in the test cell indicates that the tester is working correctly.
  3. Install the Fuel Pressure Gauge. See FUEL SYSTEM PRESSURE RELEASE  .
    4. NOTE: It will be necessary to bleed the Fuel Pressure Gauge a few times in order to obtain an accurate fuel sample.
  4. Close the bleed valve on the Fuel Pressure Gauge.
  5. Place bleed hose (1) of the Fuel Pressure Gauge into 3.4 oz. (100 ml) beaker (2). See Fig 1 .
  6. Command the fuel pump ON with a scan tool.
  7. Slowly open the bleed valve on the Fuel Pressure Gauge, until an adequate fuel sample is obtained.
  8. If water appears in the fuel sample, clean the fuel system and replace the fuel in the vehicle. See FUEL SYSTEM CLEANING under FUEL SYSTEMS in REMOVAL & INSTALLATION - CTS article.
    9. NOTE: Do not allow any substances other than gasoline, ethanol/gasoline blends, air, or acetone into the test ports of the Fuel Composition Tester. Contaminants in the Fuel Composition Tester could result in misdiagnosis.
  9. Pour the fuel sample from the beaker (1) into the Fuel Composition Tester (3), until the level of the fuel is at the top of each fuel test port (2). See Fig 2 .
  10. Observe the diagnostic LEDs on the Fuel Composition Tester. If the Red fuel diagnostic LED is illuminated, a fuel contamination condition exists. See FUEL SYSTEM CLEANING under FUEL SYSTEMS in REMOVAL & INSTALLATION - CTS article.
  11. Measure the output frequency of the Fuel Composition Tester.
  12. Subtract 50 from the reading on the DVOM to obtain the percentage of alcohol in the fuel sample. See examples in FUEL COMPOSITION TEST EXAMPLES  table.
  13. If the fuel sample contains more than 10 percent ethanol, replace the fuel in the vehicle.
Fig 1: Bleeding Fuel Into Beaker
G00196157Courtesy of GENERAL MOTORS CORP.
Fig 2: Pouring Test Sample Into Fuel Composition Tester
G00196158Courtesy of GENERAL MOTORS CORP.
FUEL COMPOSITION TEST EXAMPLES

Example Frequency - Hz Subtract 50 Ethanol Percent
A 50 -50 0
B 65 -50 15
C 129 -50 79
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