A beverage distribution warehouse reported that its Toyota 8FD25 LPG forklift ran perfectly for the first 20 minutes of each shift, then progressively lost power, stumbled under load, and finally stalled. After sitting for 15 minutes, it would restart and run normally-until the cycle repeated. Someone had noticed frost forming on the outside of the LPG converter body during the failures.
The technician recognized the frosting immediately. LPG is stored as a liquid under pressure, and the vaporizer/regulator must boil that liquid into gas before the engine can consume it. Boiling absorbs heat-roughly 8 to 10 kW at full engine load-so the vaporizer is jacketed in engine coolant to supply that heat. Frost on the housing is the classic signature of insufficient coolant flow through the jacket: the converter freezes internally, the diaphragm and secondary valve ice up, gas flow chokes off, and the engine leans out and dies. The 15-minute recovery time was simply the time needed for the ice to melt.
The question was *why* coolant flow was restricted. He traced both heater hoses from the engine to the converter. Both were warm at the engine end, but the outlet hose at the converter was almost cold at operating temperature, confirming minimal circulation. He back-flushed the converter's coolant jacket with a hand pressure pump: barely a trickle came through, followed by a plug of gelatinous orange sludge. This machine had previously been on standard green coolant; someone had topped it off with an incompatible organic-acid coolant years earlier, and the two chemistries had reacted, forming the jelly that slowly accumulated in the lowest-flow passage in the cooling system-which on this truck is precisely the small vaporizer jacket.
The repair involved removing the converter and hot-tanking it to dissolve the sludge from the jacket, replacing both heater hoses (the interior rubber had begun to delaminate from the coolant breakdown), and draining the entire cooling system of the contaminated coolant mixture. The system was refilled with a single-specification OAT coolant, thoroughly bled at the highest hose point. On the vaporizer side, the primary and secondary regulator diaphragms were inspected-one had stiffened and cracked at the freeze line from repeated icing cycles, so the converter rebuild kit was installed as well. After the repair, the forklift ran an eight-hour continuous shift with the converter housing staying warm to the touch and no frost, no stumbling, and stable fuel mixture under full-load lifting.
