An industrial-gas distribution facility reported that a CASE 821G wheel loader fitted with an LPG-converted auxiliary engine option had developed an intermittent fuel-delivery fault over a three-week period. During normal operation, the loader's auxiliary engine ran smoothly and produced rated power. However, after 20 to 30 minutes of continuous operation under load, the engine would begin to hesitate, lose power, and produce a distinctive "raw fuel" odor from the exhaust - indicating unburned LPG passing through the combustion process. The operator also noted a faint "popping" sound from the intake during the hesitation periods, and the loader's engine temperature would climb noticeably during the fault events.
The technician first understood the loader's LPG fuel architecture. The LPG fuel system stores 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. The vaporizer also includes a "Drain Valve" at its lowest point, designed to drain accumulated heavy-end LPG contaminants and condensate from the vaporizer's internal cavity during scheduled service. When the drain valve becomes clogged, heavy-end contaminants and liquid LPG accumulate within the vaporizer, eventually reaching a level where they are carried into the engine's intake - producing the raw-fuel odor, hesitation, and intake popping the operator described.
He verified the symptom by operating the loader under load and monitoring the engine's exhaust and fuel-system behavior. Within 25 minutes of loaded operation, the engine began to hesitate, the raw-fuel odor became noticeable, and the intake popping occurred - confirming the fuel was not being fully vaporized before reaching the engine.
He inspected the vaporizer's "Drain Valve." The valve showed heavy external corrosion and was visibly seized in the closed position - its drain orifice was completely blocked by accumulated tar-like LPG residue. He then opened the vaporizer's lower inspection port (after ensuring the LPG supply valve was closed and the system was depressurized) and found a significant accumulation of tar-like heavy-end LPG contaminants and liquid residue within the vaporizer's internal cavity. The accumulated liquid was at a level where it was being periodically drawn into the engine's intake during high-demand operation, producing the hesitation, raw-fuel odor, and intake popping the operator described.
He further inspected the LPG tank's "Service Valve Packing" - the sealed packing assembly around the service valve's stem that prevents LPG from leaking externally around the valve's operating mechanism. The packing showed visible wear and a faint external leak when tested with soapy water - allowing small amounts of LPG vapor to escape around the valve stem during operation, contributing to the fuel smell the operator described.
The root cause was traced to a clogged vaporizer drain valve compounded by a leaking service valve packing. The drain valve's accumulated heavy-end LPG residue had blocked the vaporizer's drain path, allowing liquid contaminants and condensate to accumulate within the vaporizer and eventually migrate into the engine's intake. The leaking service valve packing had contributed to the external fuel odor. Both faults had developed gradually over extended service in a high-cycle distribution environment.
The repair involved cleaning the vaporizer's drain valve with solvent (restoring its free-draining function), thoroughly flushing the vaporizer's internal cavity to remove all accumulated heavy-end contaminants and liquid residue, and replacing the LPG tank's service valve packing with a new packing assembly. The vaporizer's coolant supply and return lines were inspected (both in good condition, with full coolant flow verified), and the LPG fuel filter was replaced with a new element. The vaporizer's diaphragm and regulator stages were inspected (all within service limits). The system was leak-tested with soapy water across all LPG connections (no further leaks found), and the vaporizer's drain valve was lubricated with high-temperature lubricant to prevent future corrosion seizure. After reassembly and a full loaded operational cycle, the engine ran smoothly with no hesitation, no raw-fuel odor, and no intake popping - and the vaporizer's housing temperature remained stable throughout. Vaporizer drain-valve service and LPG service valve packing inspection were added to the loader's 250-hour preventive maintenance checklist.
