A busy lumber yard reported that a Clark C30D LPG-powered forklift was experiencing a severe engine stumble under load. The machine idled relatively smoothly, but when the operator revved the engine or attempted to lift a heavy pallet of timber, the engine would bog down, backfire through the intake, and nearly stall. The maintenance team had replaced the spark plugs and ignition coils, but the power loss persisted.
Because the symptoms only appeared under high load, the technician focused on the LPG fuel delivery system. Propane is stored as a liquid under pressure in the tank but must be vaporized into a gas before entering the engine manifold. This vaporization process requires significant heat, which is why the LPG vaporizer (converter) is plumbed into the engine's coolant system.
The technician removed the vaporizer cover. The internal diaphragm was intact, but the unit was heavily frosted over on the outside, despite the engine being at full operating temperature. A coolant line inspection revealed that the small rubber hose supplying hot coolant to the vaporizer was severely kinked and partially collapsed internally, restricting the flow of hot engine coolant. Without adequate heat, the liquid propane was freezing the vaporizer, preventing complete fuel vaporization and causing the engine to run dangerously lean under demand, leading to the stumble and backfire.
However, once the coolant hose was replaced and the vaporizer thawed, the engine still lacked full power. The technician removed the air intake hose to inspect the throttle body. The throttle butterfly valve was heavily caked with a thick, oily, black carbon buildup. Propane burns very cleanly, so this carbon was a mystery.
A closer look at the closed-crankcase ventilation (PCV) system revealed a clogged PCV breather. Because the PCV was blocked, the engine crankcase pressure was forcing oil vapor backward through the intake system, coating the throttle body. The carbon buildup was physically choking the airflow, preventing the engine from drawing enough air to match the LPG fuel delivery under heavy load.
The repair involved rebuilding the LPG vaporizer with a new diaphragm kit and replacing the kinked coolant hose with a molded, high-temperature silicone hose. The throttle body was chemically cleaned to remove the carbon glaze, ensuring the butterfly valve opened fully without obstruction, and the PCV valve was replaced. After bleeding the LPG lines, the Clark C30D lifted a maximum capacity load smoothly, and the engine revved freely without a single stumble or backfire.
