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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 220 (ENGINE CONTROLS - BASIC DIAGNOSTIC PROCEDURES - 6.0L W/BiFuel & CNG ENGINES) >> Fuel Systems >> CNG Fuel Tank Venting Procedure >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Savana, 2003 Chevrolet Silverado, and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
  1. Release fuel system pressure. See CNG TANK FUEL PRESSURE RELEASE . Disconnect negative battery cable. Remove fuel tank cover. In order to vent rear tanks on cargo/passenger vans, tanks must be removed from vehicle.
  2. Disconnect fuel outlet line from the tank. Install a cap supplied from the CNG Tank Vent Kit (J-42435) to fuel line fitting of fuel outlet port. Tighten cap to 27 ft. lbs. (37 N.m).
  3. Leave Fuel Tank Pressure (FTP) sensor installed, if equipped. Disconnect fuel inlet line from tank.
    4. CAUTION: When venting CNG fuel tank, use proper grounding procedure in order to prevent a build-up of static electricity which can lead to an electrical discharge. Use CNG Tank Vent Kit (J-42435) with a vent stack. Vent CNG fuel tank in a well ventilated work area. Keep sparks, flames and smoking material away from the area. Wear appropriate hearing protection during the venting procedure. A sudden pressure loss from an uncontrolled gas leak can cause hearing damage.
  4. Connect female end of vent hose to fuel line fitting. Connect male end of vent hose to a vent stack. Connect the ground strap to one of the fittings and to vent stack ground.
  5. Ensure that manual lock-down screw of the manual shut-off fitting is open by rotating the Allen screw counterclockwise until fully seated. See Figure. If tank begins venting when the manual lock-down screw is opened, do not proceed to the next step until tank stops venting.
  6. Remove manual shut-off fitting from the HPL. Obtain manual vent tool supplied from the CNG Tank Vent Kit (J-42435). Rotate screw on manual vent tool counterclockwise in order to fully retract plunger. Install manual vent tool into HPL port with plunger fully retracted. Tighten manual vent tool to 108 INCH lbs. (12 N.m).
  7. Slowly turn the manual vent tool screw clockwise until the plunger is fully extended. Open vent kit in-line valve. The tank should always contain some pressure and begin to vent.
  8. Do not leave tank unattended during venting. Allow CNG fuel tank to vent until the pressure gauge reads zero psi. When tank stops venting, inspect vent line or HPL for a frozen fitting. Fitting will eventually thaw and fuel may continue to vent.
  9. When tank is completely vented, remove vent line from fuel fitting. Remove fuel fitting from fuel outlet port. Remove manual vent tool. If tank is to be reused, replace the HPL.
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