Indeed shut the engine down if it needs to be. In BA (I do not know about other airlines' policies) If a B747 has a dodgy engine, which under normal circumstances would be shutdown in flight, the crews are encouraged to keep it ticking over at idle. The reason being that their B767s have the same engine type and any in flight shutdown on a B747 effects the ETOPS status of the B767. More than a certain number of in flight shutdowns then ETOPs is degraded, say from 180 mins to 120mins.