Injector nozzles normally require coming when residual fuel is used. Interruption of the fuel valve cooling supply quickly results in vapour lock of the cooling passages within the nozzle and then blockage by scale. When fuel valve cooling is restored after a failure it is essential to check flow from all returns and to clear any with no return flow immediately. Loss of cooling will result in the formation of carbon deposits(trumpets) around the nozzle in a very short time