While starting the engine, noticed a mild knocking noise. When on load, the noise disappeared. On inspection, found one unit’s exhaust temperature is not rising. Suspected fuel valve to be faulty, same replaced, but no improvement. Fuel pump checked and renewed. Timings checked and found satisfactory. Still the exhaust temperature was not coming up. Finally the cylinder head was opened up and found the exhaust valve partially broken. Compression pressure was not building up and hence there was no combustion in that unit. Cylinder head was replaced with an overhauled one. The engine was tried out, found satisfactory.