240 is on the high side for normal running but ok for a warning level. Check everything is OK - oil coolers not blocked with grass etc. Check your mixture settings too.
I dont like repeated 'false' warnings from an engine monitor - they get switched off and ignored. If your engine wont run below 230 (some wont!) even though everything is right set the limit to 240 then if something does go wrong you'll get a warning you take note of.
If you're serious about understanding your engine - and I guess you are- google John Deakin's Pelicans Perch articles on AVweb. There are a lot of irrelevant articles but the engine management ones are gold! They approach engine management from a fixed wing perspective but the knowledge can equally be applied with care to helicopter engines too.