CNG Bus No-Start After Refueling
A transit agency operates 40 dedicated CNG buses. Driver reports bus #12 cranks but will not start after refueling at the depot fast-fill station. Outside temperature is 95°F. The bus was running fine before refueling. Technician checks fuel gauge reads 3/4 full. When the ignition is turned on, the fuel pressure gauge at the regulator shows 0 psi. The manual shutoff valve at the cylinder is open. The high-pressure filter is new. The bus has a 3600 psi storage system. The technician hears a faint hiss near the fill receptacle when the fuel door is opened. No DTCs are stored. The bus has a quarter tank of fuel.
1. What is the most likely cause of the no-start condition, and what immediate safety step should the technician take?
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The faint hiss near the fill receptacle indicates a leak in the high-pressure fill line or check valve. After refueling, the fill receptacle check valve may be stuck open or the fill line may have a loose fitting, causing pressure to bleed off. The fuel gauge reads 3/4 because the gauge is likely reading the cylinder pressure via a sender, but the actual pressure at the regulator is 0 psi due to the leak. Safety step: Do not start the engine. Evacuate the area, eliminate ignition sources, and follow CNG leak procedures: shut off the manual valve at the cylinder if accessible, ventilate the area, and use a combustible gas detector to confirm the leak. Wear PPE and avoid breathing CNG. Notify supervisor and repair the leak before further operation.
2. How should the technician verify the leak location and what repair is likely needed?
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Use a combustible gas detector or soap bubble solution on all high-pressure connections from the fill receptacle to the cylinders, including the fill line, check valve, and fittings. Since the hiss is at the fill receptacle, focus there. The fill receptacle contains a check valve that prevents backflow. If the check valve is leaking, it must be replaced. Also inspect the fill line for damage. After repair, pressurize the system and leak-test again. Replace any O-rings or seals as needed. Use only CNG-approved parts. Purge the line before reconnecting. Verify no leaks with a gas detector before returning to service.