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HOMESERVICE MANUALSCHEVROLET1998SUBURBAN K2500, 7.4 J, PART TIMEREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 173 (ENGINE CONTROL SYSTEM TESTING & ENGINE COMPONENT TESTING)FUEL SYSTEMS (2.2L - CAVALIER CNG)FUEL SYSTEM UNWANTED SWITCH TO GASOLINEDIAGNOSTIC AIDS
1998 Chevrolet Suburban K2500, 7.4 J, Part Time

Diagnostic Aids

1998 Chevrolet Suburban K2500, 7.4 J, Part TimeSECTION Diagnostic Aids
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Diagnostic information should be utilized only if concern of vehicle operator can be duplicated. An unwanted switch may occur only during specific vehicle operation. Examples of unwanted switch are as follow:

  • Vehicle has an intermittently leaking Low Pressure Lock-Off (LPL) solenoid.
  • After turning engine OFF and waiting several minutes, all CNG purges from fuel lines.
  • During next ignition ON key cycle, High Pressure Lock-Off (HPL) solenoid is opened for approximately one second. At the same moment HPL is opened, AF ECM monitors fuel pressure sensor. If LPL is still leaking, CNG pressure cannot build within the system and low fuel pressure is observed by AF ECM. Detecting low fuel pressure in the system, AF ECM performs a switch-over to gasoline.
  • Vehicle may start and run on CNG during any ignition cycle in which LPL does not leak. Customer concern would be an intermittent unwanted switch to gasoline with no DTCs stored and possibly a no-start flooded condition.

If an unwanted switch only occurs during certain driving conditions, perform fuel system diagnosis and verify that fuel pressure can be maintained during all driving conditions. See FUEL SYSTEM DIAGNOSIS .

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.