A cement plant reported that a Hyster S80FT LPG forklift would crank over strongly but fail to start 50% of the time. When it did start, it ran rough and lacked the power to climb ramps. The operators had to crank the engine for 10-15 seconds before it would fire.
The technician first checked for spark, which was strong and consistent. The issue was isolated to fuel delivery. The machine uses a liquid withdrawal propane system with a vaporizer (converter) to turn liquid propane into gas. A fuel pressure gauge was installed in the line between the vaporizer and the mixer carburetor.
When the key was turned on, the gauge should have jumped to approximately 1.5 psi. It remained at zero. The LPG Lock-Off Solenoid (a safety valve that opens to allow fuel flow when the ignition is on) was located on the tank line. The solenoid was removed and tested with a 12V battery source. It clicked but did not fully retract the plunger. The internal spring was weak, and the plunger was dirty, preventing the valve from opening fully to allow liquid fuel to reach the vaporizer.
However, even when the solenoid was manually bypassed, the engine still ran rough. The technician disassembled the vaporizer unit. The large rubber diaphragm inside was cracked and hardened. This diaphragm creates the vacuum necessary to draw the propane gas into the engine. The cracks caused a vacuum leak, preventing the proper fuel/air mixture from forming during cranking.
The repair involved replacing the LPG Lock-Off Solenoid and installing a complete diaphragm repair kit in the vaporizer. The fuel lines were blown out to ensure no debris was blocking the liquid flow. After the repair, the forklift started instantly on the first crank and pulled strong up the inclines with no power loss.
