A lumber yard reported that a Hyster S80FT LPG forklift would start and idle perfectly, but when the operator tried to lift a full load of timber (approx. 3,500 lbs), the engine would shudder and stall. The machine could drive forward and reverse without issue.
The technician suspected a fuel delivery problem under high load. LPG (Liquefied Petroleum Gas) systems require a liquid-to-vapor conversion. The system consists of a fuel tank, a shut-off valve, and a vaporizer (regulator) that mixes the gas with air. The technician connected a manometer to the vaporizer outlet. At idle, the pressure was correct. When the lift lever was actuated, the vacuum dropped significantly, indicating the engine was "starving" for fuel.
The inspection focused on the liquid LPG line between the tank and the vaporizer. A flexible rubber hose section near the vaporizer was found to be internally delaminated. The inner lining of the hose had collapsed, creating a flap that restricted flow. At idle, the low fuel demand was met, but when the throttle was opened for lifting, the restriction prevented enough liquid LPG from reaching the vaporizer. This caused the mixture to become too lean (too much air, not enough gas), leading to the stall.
Additionally, the vaporizer's "idle mixture screw" had been tampered with, further leaning out the system. The technician replaced the compromised fuel hose with a new, braided stainless steel line, and reset the idle mixture to the factory specification (usually 1.5 turns out from lightly seated). The engine now maintained RPM under full lifting load, and the stall was resolved.
