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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel Tank Inspection

Fuel Tank Inspection Type 4Tank

       Important: Only properly trained personnel should perform the compressed natural gas (CNG) fuel tank inspection procedure. Refer to the GM Training Course guide and ensure you have successfully completed the current inspection training course for the tank type as required. The label on the CNG fuel tank indicates what type of tank is installed in the vehicle.

    • Inspect the CNG fuel tank and brackets every 3 years or 60 000 km (36,000 miles), whichever comes first. Record the inspection information in the CNG Fuel Tank Inspection Record. The CNG Fuel Tank Inspection Record is located in the owner's manual supplement.
    • Do not remove the tank from the vehicle for general inspections. Tank removal is only necessary if the following conditions are present:
       - The tank has been exposed to corrosive chemicals such as acid or alkali.
       - Unusual or excessive corrosion is observed in the bed area.
       - Obvious damage is observed in the bed area.
       - The tank straps are loose or damaged.
       - The vehicle has been involved in a collision and damage is observed in the tank area.
       - The vehicle has been involved in a fire.
    • Ensure the tank cover is in place and intact. Never operate or store the vehicle outside with the tank cover removed. The CNG tank wrap can be damaged by extended exposure to ultra-violet (UV) radiation. Extended exposure to UV radiation can cause the following conditions:
       - Loss of label legibility
       - Loose surface fibers as the external finish degrades
       - Chalking of the external finish
    • If the tank is replaced, perform the following steps:
        1. Obtain a new underhood CNG tank expiration label.
        2. Obtain a new fill area CNG tank expiration label.
        3. Record the new tank "Do Not Use After Date" service expiration date onto both labels by copying the expiration date as printed on the replacement tanks label.

Tank Label Inspection Procedure

Verify that the CNG fuel tank label is in place.

Contact the GM Technical Assistance Center (TAC) if any of the following conditions exist:

    • The tank label is illegible.
    • The tank label is peeling.
    • The tank label is missing.

Verify that the expiration date indicated on the tank matches the underhood and fill area labels. If needed, replace the underhood and fill area labels so the expiration date matches the tank label.

Verify that the pressure relief device (PRD) Caution label is intact and legible. Replace a missing or illegible PRD Caution label.

Tank Wrap Inspection Procedure

Important: Gaps of the fiber wrap around the dome area of the tank may occur during manufacture. Also, resin and paint runs may appear as a hard circle or spot on the tank surface. This is normal. Tank strength is not affected and does not require repair.

A 1.27 mm (0.050 in) sacrificial external wrap protects the CNG fuel tank. Inspect and resurface abrasions or cuts as follows:

Level 1 Cut or Abrasion:

    • Level 1 cut, scratch or abrasions are minor cuts or abrasions that are less than 0.25 mm (0.010 in) deep.
    • It is not necessary to resurface Level 1 abrasions.

Level 2 Cut or Abrasions:

       Important: Only hand tools should be used during the repair process. Never exceed Level 2 specifications in making a repair. It is not required that the tank be removed from the vehicle during the repair process.

    • Level 2 cuts or abrasions have some exposed fibers or have flat spots with a depth between 0.26-0.89 mm (0.011-0.035 in). It may be necessary to remove loose fibers in order to accurately gage the depth of the cut or abrasion.
    • Resurface Level 2 cuts or abrasions before returning the tank to service by performing the following:
        1. Remove any loose fibers.
        2. Sand or hand file the cut or abrasion in order to taper the affected area.
        3. Paint small areas or scratches with black touch-up paint 12345155 or equivalent. If painting the entire tank or large areas, use Sherwin Williams Polane HS Plus® polyurethane paint or equivalent. Ensure the tank label remains legible.

Level 3 Cut or Abrasions:

    • Level 3 cuts or abrasions are cuts or abrasions with a depth that exceeds 0.89 mm (0.035 in). It may be necessary to remove loose fibers in order to accurately gage the depth of the cut or abrasion.
    • Remove from service any tank that has sustained Level 3 damage. Refer to Removing Tank from Service.

Heat or Chemical Exposure

    • Inspect the tank for heat exposure such as darkening or charring/sooting of the surface.
    • Inspect for the following indications of chemical exposure:
       - blistering
       - swelling
       - softening
       - resin removal
       - fracturing
    • Any tank with indications of heat or chemical exposure must be removed from service.

Weathering Exposure

    • Inspect for excessive weathering exposure such as discoloration or light flaking of the surface coating.

       Important: Only hand tools should be used during the repair process. Never exceed Level 2 specifications in making a repair. It is not required that the tank be removed from the vehicle during the repair process.

    • Repair weathering exposure by performing the following:
        1. Sand or hand file the affected area.
        2. Paint small areas with black touch-up paint 12345155 or equivalent. If painting the entire tank or large areas, use Sherwin Williams Polane HS Plus® polyurethane paint or equivalent. Ensure the tank label remains legible.

Natural Gas Leakage

Important: Bubbles can be observed on the tank surface for up to 4 hours after re-pressurization of an empty tank. The bubbles are caused by air trapped between the liner and the composite fiber wrap. This condition is referred to as Trapped Air Migration and does not indicate gas leakage.

Inspect for gas leakage with a liquid leak detector such as Snoop® or equivalent. Always rinse the liquid leak detector from the tank and fittings with clean water after inspection. The electronic combustible gas detector J 45878 is not recommended for leak detection on or near the Type 4 CNG fuel tank for several hours after re-pressurization. The detector can falsely indicate a leak as the trapped air is expelled. Any tank that does exhibit gas leakage must be removed from service.

Tank Impact Inspection

Impact damage differs from cuts or abrasions. More severe damage may occur within the wall of the tank and may not be readily apparent on the surface. A tank with localized surface deformation that does not exceed 0.89 mm (0.035 in) can be returned to service as long as internal damage of the wall is not present. Internal wall damage can be determined by the following:

    • Permanent deformation of the tank surface resembling denting that exceeds 0.89 mm (0.035 in).
    • Softness or deflection of the tank wall. The tank wall should be rigid and not easily deformable.
    • Localized areas of surface cracking. The cracks will have a localized circular, oval or linear pattern. Test these areas for internal wall damage by performing the following tap test:

        Important: The tap test can only be performed in the cylinder portion of the tank. The dome ends of the tank cannot be tested by sound.

        1. Tap the surface of the tank with a small metal object such as a large coin.
        2. Compare the sound emitted in the suspect area to the sound emitted in an area where impact is not suspected.
        3. Internal wall damage is present when the suspect area emits a sound that is different than other portions of the tank cylinder.
    • Remove from service any tank with internal wall damage.

Mounting Bracket Inspection Procedure
  1. Verify that the rubber isolator is properly seated on the tank brackets and straps.
  2. Inspect the rubber isolator for cracks, deterioration, or other damage.

    3. Important: Do not verify the tank strap nut torque on a fully pressurized tank. A composite tank will expand and contract as the internal pressure increases and decreases. In order to compensate for expansion and contraction the tank strap rubber isolator is designed to deform slightly. The rubber isolators will eventually settle slightly causing tank strap nut torque to vary from the initially higher installation torque.

  3. Replace the rubber isolators whenever the tank straps are removed or loosened.
  4. Inspect the tank brackets, straps, mounting frame, and mounting hardware for cracks, corrosion, deformation, or other damage.
  5. Replace if necessary. Refer to Fuel Tank Bracket and Mounting Replacement .

External Pressure Relief Device (PRD) Inspection Procedure
    • Inspect all external surfaces of the PRD for the following:
       - PRD body should be straight and true without surface dents or gouges.
       - Surface finish should be undisturbed without showing the brass base-metal color. Slight tool marks on the hex flats are acceptable.
    • Inspect for leaks at the fitting O-ring.

       Caution: The compressed natural gas (CNG) fuel tank internal pressure must be completely vented before attempting to remove the external pressure relief device (PRD). Refer to Fuel Tank Venting Procedure. Failure to perform the venting procedure could result in CNG fuel tank damage and personal injury.

    • Vent the internal tank pressure and replace the O-ring or PRD if necessary.
    • Inspect that the PRD Caution label is intact and legible. Replace a missing or illegible PRD Caution label.

Removing Tank From Service
    • The CNG fuel tank has a fifteen or twenty year service life from the date of manufacture.
    • The CNG fuel tank manufacture date and expiration date appears on a label on the tank.
    • The fuel tank expiration date is also on the underhood and fill area labels.
    • The CNG fuel tank must be removed from service at the expiration date with the following procedure:
        1. Completely vent the CNG fuel tank internal pressure . Refer to Fuel Tank Venting .
        2. Clearly mark the tank as "CONDEMNED".
        3. Disconnect the vent hose.
        4. Remove the tank from the vehicle. Refer to Fuel Tank Replacement .
        5. Place the tank outside in a well ventilated area.
        6. Allow the tank to purge for 24 hours with the high pressure lock-off (HPL) solenoid valve removed from the fuel tank.
        7. With a non-electric drill, create two or more 1/2 inch diameter holes completely through the tank wall and inner liner.
        8. Dispose of the tank in a safe and approved manner.