A heavy-duty LPG (propane) forklift was losing power badly after 20 minutes of heavy use in a cold warehouse. It would start fine and run perfectly, but as it heated up under load, the engine would begin to bog down, miss, and eventually stall. After sitting for 15 minutes, it would start right up and run fine again.
The shop replaced the fuel filters and the propane carburetor, assuming the mixture was off. It didn't fix the issue. We had to look at the physics of liquid propane. LPG is stored as a liquid under pressure. To burn in the engine, it must convert to a gas. This phase change (boiling) requires a massive amount of heat.
To provide this heat, forklifts route hot engine coolant through a small heat exchanger called a vaporizer (or regulator). The liquid propane travels through the hot coolant jacket, absorbing heat and vaporizing before it reaches the carburetor. We felt the coolant hoses going into the vaporizer. One was hot; the other was barely warm.
The coolant passage inside the vaporizer was completely plugged with rust and silica scale from the engine block. Without adequate coolant flow, the vaporizer couldn't provide enough heat to boil the liquid propane during heavy use. To compensate, the vaporizer began absorbing heat from the surrounding air, quickly dropping the temperature of the unit below freezing. The ambient moisture in the air froze onto the outside of the vaporizer, forming a thick layer of white ice. The inside of the unit was so cold that the liquid propane stopped vaporizing, flooding the carburetor with liquid and stalling the engine. We removed the vaporizer, rodded out the coolant passages with a wire, flushed the engine block, and the forklift ran at full power all day without freezing.
