A beverage distribution warehouse reported that a Clark CMP50 5,000-pound LPG counterbalance forklift had developed an intermittent fuel-delivery fault over a two-week period. The truck would start normally on a cold engine, but after one to two minutes of operation the engine would begin to hesitate, lose power, and eventually stall - behaving as though it had run out of fuel despite the LPG tank being full. Restarting the truck was difficult, and the operator reported visible frost forming on the LPG vaporizer's housing during operation - a condition that had not been present when the truck was running normally. The truck also produced a distinctive "popping" sound from the intake during the hesitation periods.
The technician first understood the CMP50's LPG fuel architecture. LPG forklifts store fuel as a pressurized liquid in the tank; before the fuel can be burned in the engine, it must be converted from liquid to vapor. This conversion happens in the "Vaporizer-Regulator" - a device mounted on the engine that uses engine coolant circulated through its internal passages to supply the heat required to vaporize the liquid LPG. Coolant from the engine's cooling system flows through the vaporizer's internal heat-exchange passages, warming the incoming liquid fuel and causing it to flash to vapor as its pressure is reduced across the regulator's internal diaphragm stages. When the vaporizer's coolant supply is restricted - by a clogged coolant line, a failed coolant-pump, or air trapped in the coolant circuit - the vaporizer cannot supply sufficient heat, and the liquid LPG does not fully vaporize before reaching the engine's intake. The partially liquid LPG chills the vaporizer's housing (producing the frost the operator observed) and delivers a lean, inconsistent fuel mixture to the engine - producing the hesitation, stalling, and intake popping the operator described.
He verified the symptom by starting the truck and observing the vaporizer's housing during a cold-start test. Within two minutes, visible frost formed across the vaporizer's exterior, and the engine began to hesitate - confirming the fuel was not being fully vaporized. He then checked the vaporizer's coolant supply and return lines for temperature difference: the supply line was warm, but the return line was cold, indicating coolant was not flowing through the vaporizer's internal passages. He disconnected the return line and confirmed minimal coolant flow. He then inspected the vaporizer's coolant-supply line, which showed a visible internal restriction - the line's interior was coated with a hardened coolant-deposit scale that had narrowed the flow path to roughly 30 percent of its original diameter.
He further inspected the LPG fuel filter, which is installed between the tank and the vaporizer to remove particulate from the fuel. The filter element was visibly saturated with a dark, tar-like residue - a common LPG contaminant formed from heavy-end hydrocarbons in low-quality or long-stored fuel. The saturated filter had restricted liquid LPG flow to the vaporizer, compounding the fuel-delivery restriction and requiring the vaporizer to work harder on a reduced fuel flow with inadequate heat.
The root cause was traced to a clogged vaporizer coolant line compounded by a saturated LPG fuel filter. The hardened coolant-deposit scale had restricted the coolant flow through the vaporizer, reducing the heat available for vaporization; the saturated fuel filter had further restricted fuel delivery, compounding the lean-mixture condition. The combination had produced the hesitation, stalling, intake popping, and frost formation the operator described.
The repair involved replacing the vaporizer's coolant supply line with a new length of the correct specified coolant hose (the scale buildup was not field-cleanable), back-flushing the vaporizer's internal coolant passages to remove any residual scale, and replacing the LPG fuel filter with a new element. The engine's cooling system was checked for coolant level and condition (both correct), and the coolant was topped up after the new hose was installed. The LPG tank and its connections were inspected for damage (none found), and the tank's service valve and coupler were leak-tested with soapy water (no leaks detected). After reassembly and a full cold-start test, the truck ran smoothly through a 30-minute loaded operational cycle with no hesitation, stalling, or frost formation on the vaporizer, and the intake popping was eliminated. Vaporizer coolant-line condition and LPG fuel filter replacement were added to the truck's 500-hour preventive maintenance checklist, and the warehouse was advised to source LPG from a supplier providing consistent fuel quality to reduce future heavy-end contamination.
