the manufacture is recommending a cool down period of about 3 minutes to prevent overheating of the combustion chamber. the following is from the manufacture on shut down procedure.
3.2.8Check Stopping of Generating Set
(1)Disconnect the load from the generating set.
(2)Turn off the air switch of the generating set.
(3)Set the speed lever at RUN position, run the generating set without load for about 3 minutes. Do not stop the engine suddenly, otherwise the temperature will increase abnormally, the nozzle will blocked and the generating set will be damaged.
Press down the stop lever. Concerning the electric starter, turn the key to OFF. Turn the fuel **** lever to S position.
Pull out the recoil starter handle until you feel resistance (at this position, both intake/exhaust valve are closed.) and leave the handle in this position. This prevents the engine from rust.
If the engine keep on running even after the speed lever is placed at the STOP position
, either turn the fuel **** to the CLOSE position or loosen the nut of high pressure fuel pipe on the pump side to stop the engine.
Do not stop the engine with the decompression lever.
Do not stop the generating set with load. Stop it after the load removed.
the following link provides the complete maintenance manual for the diesel generator