A lumber yard reported that a Hyster S80FT LPG forklift would start and idle fine, but would stall immediately when trying to accelerate or lift a load. The engine would crank over strongly, but the exhaust smelled heavily of raw propane, indicating a rich or flooded condition.
The technician first checked the spark plugs and ignition timing, which were correct. The issue was isolated to the fuel delivery system. The Hyster S80FT uses a vaporizer (converter) to turn liquid propane into gas. The converter requires engine coolant to flow through it to prevent the unit from freezing up during the vaporization process.
The vaporizer was removed and inspected. The internal diaphragm was torn, and the metering valve was stuck in the open position. This caused liquid propane to pour directly into the intake manifold rather than being vaporized into a gas, flooding the engine and causing the stall.
However, the underlying cause was a lack of heat. The rubber coolant hoses running from the engine to the vaporizer were old and collapsed internally. When the engine warmed up, the hoses softened and kinked, cutting off the hot coolant flow to the vaporizer. Without heat, the converter iced over, causing the internal rubber components to freeze and crack.
Furthermore, the main fuel line from the LPG tank to the converter was routed too close to the exhaust manifold. The heat from the exhaust had melted the outer sheath of the fuel line, causing the liquid propane to boil and create vapor lock bubbles before it even reached the converter.
The repair involved replacing the vaporizer with a new unit, replacing the collapsed coolant hoses with molded, high-temperature silicone hoses, and rerouting the fuel line away from the exhaust manifold with a new heat shield. After the repair, the forklift started instantly and accelerated smoothly under a full load without stalling or icing up.
