A propane (LPG) powered forklift was running perfectly at the start of the shift. But after about forty-five minutes of heavy lifting in a humid environment, the engine began to lose power, eventually bogging down and dying completely. If the operator let it sit for twenty minutes, it would restart and run fine for another half hour. The shop had replaced the fuel filter and the propane tank, assuming they had bad fuel or a restricted line.
To understand this failure, you have to understand how a propane system works. LPG is stored as a liquid under pressure in the tank. The forklift engine cannot burn liquid propane; it must be a gas. To turn the liquid into a gas, it passes through a component called a vaporizer (or converter). The vaporizer is a small aluminum block bolted to the engine. It uses hot engine coolant flowing through it to heat the liquid propane, causing it to boil and turn into a vapor.
When propane boils from a liquid to a gas, it undergoes massive evaporative cooling-it absorbs a huge amount of heat. If the engine coolant flow through the vaporizer is restricted-often by a clogged coolant hose, a stuck thermostat, or just an air pocket in the cooling system-the vaporizer cannot provide enough heat to keep up with the demand. The evaporative cooling wins. The aluminum block drops below freezing, and any moisture in the ambient air immediately freezes to the outside and inside of the vaporizer, forming a block of solid ice. This ice physically chokes off the propane supply, starving the engine of fuel until it dies.
When the operator lets the machine sit, the ice melts, the coolant catches up, and the cycle repeats. We replaced the degraded coolant hose leading to the vaporizer, bled the air out of the cooling system, and the forklift ran hard for the rest of the shift without freezing. It's a critical rule: on an LPG forklift, the fuel system is cooled by the engine cooling system. If the engine is losing fuel, check the water hoses first.
