A building supply yard reported that a Hyster H50FT (Mazda 2.2L LP gas engine) forklift backfired through the intake, and the exhaust manifold glowed cherry red after 10 minutes of operation. When the operator tilted a heavy load of drywall back, the engine would violently shudder and stall completely.
The technician first ruled out fuel starvation, as the LP gas vaporizer was delivering consistent pressure. The red-hot manifold and intake backfire pointed to severely retarded combustion timing or an exhaust restriction. He performed a cylinder leak-down test. Cylinders #3 and #4 showed 30% leakage, with air hissing out of the exhaust pipe, indicating burnt or bent exhaust valves.
He removed the cylinder head and found the "Exhaust Valve Guides" were severely worn and ovalized. Because LP gas burns very hot and dry, the cylinder head lacked the lubrication of older diesel engines. The worn guides allowed engine oil to leak down the valve stems, burning instantly and leaving hard carbon deposits on the valve seats. The carbon buildup prevented the exhaust valves from seating fully. The extreme exhaust gas heat caused the valve heads to warp and tulip (expand).
The warped exhaust valves were blocking the exhaust ports. The exhaust gas had nowhere to go, turning the manifold red-hot. When the operator tilted a heavy load, the hydraulic pump placed a massive load on the torque converter. The engine, already suffocating from the exhaust restriction, could not pull enough air through the intake to maintain torque under the pump load, causing the violent shudder and stall.
The repair involved completely rebuilding the cylinder head. The valve guides were replaced with bronze replacements, and new exhaust and intake valves were installed and ground. The exhaust manifold and catalytic converter were removed and de-carbonized to clear the exhaust restriction. After reassembly and valve adjustment, the forklift tilted heavy loads without stalling, the manifold maintained normal temperatures, and the backfiring ceased.
