A lumber yard reported that a Toyota 8FGU25 LPG forklift was surging at idle and stalling immediately when trying to lift a load. The engine ran smoothly when the propane tank was disconnected and the machine was running on gasoline (dual-fuel setup, though typically LPG only), but ran poorly on LPG.
The technician identified this as a lean fuel condition on the LPG circuit. The air-fuel mixer (vaporizer) on an LPG system relies on engine vacuum to draw the gas into the intake manifold. A visual inspection of the vacuum lines connecting the mixer to the intake manifold revealed a large crack in the main vacuum elbow hose. This crack allowed un-metered air to enter the intake, leaning out the mixture. At idle, the engine computer could compensate slightly, but under load, the vacuum leak diluted the propane gas, causing a stall.
However, the lean condition was compounded by a second fault. The LPG lock-off solenoid, which opens the fuel line to the vaporizer, was removed. The internal plunger was sticky and coated in black tar residue from contaminated propane. The solenoid was not opening fully, restricting the flow of liquid propane to the vaporizer. When the engine was under load, the demand for propane exceeded the restricted flow, causing the engine to starve for fuel and surge.
The repair involved replacing the cracked vacuum hose with a new, reinforced silicone elbow. The LPG lock-off solenoid was disassembled, cleaned with solvent, and a new plunger spring was installed. The vaporizer diaphragm was also inspected and found to be in good condition. After reassembly, the forklift ran smoothly on LPG, lifting maximum capacity loads without a hint of surging or stalling.
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