A lumber yard reported that a Hyster H80FT LPG forklift was overheating after just 20 minutes of operation and lacked the power to climb the yard ramp. The coolant level was correct, and the radiator fins were clean. The exhaust smelled strongly of unburnt fuel.
The technician first checked the ignition system. The spark plugs were black and sooty, indicating a rich fuel mixture. The ignition timing was checked with a timing light and found to be severely retarded (delayed). This caused the fuel to burn late in the cycle, wasting energy and heating up the exhaust valves and coolant. The timing was reset to the manufacturer's specification, but the engine still struggled.
The final diagnosis was the catalytic converter. The LPG fuel system had been running rich for some time, coating the catalytic converter's honeycomb with carbon deposits. This created a blockage in the exhaust system, building up back-pressure that prevented the engine from expelling exhaust gases. This back-pressure increased the engine's thermal load. The catalytic converter was removed and found to be clogged solid. The converter was replaced, the ignition timing was reset, and the LPG mixer was adjusted to the correct air-fuel ratio. The forklift regained its power and the operating temperature stabilized.
