Exhaust Mixing Elbow Inspections

[Dreaming Yachtsman]

Today I replaced the exhaust mixing elbow on one of my Yanmar 3GM30-FC engines. The old elbow was only five years old but had developed a crack in the external weld between the outer sleeve and the flange that bolts to the exhaust manifold. A local mechanic at Bahia del Sol, El Salvador, ruined the piece when he welded a bead all the way around the outer pipe to stop the leak - the heat from the weld warped the flange and opened up the seam between the inner and outer sleeves. After a stutter step trying to get a new elbow through an El Salvador company, I gave up and ordered one from the U.S. The part arrived four days later on DHL but took another four days to get through customs. Close examination of the old elbow revealed pits on the inside of the inner sleeve that look like they are nearly half way through the wall thickness. The exhaust line was pretty heavily caked with carbon build up as well.

My questions:
1. Does anyone know the recommended frequency for inspecting the elbows for internal pitting? The owner's manual does not specifically address the issue but one could argue that such an inspection would be included in the cleaning specified every 300 hours. I just don't like breaking open perfectly good joints for no good reason.

2. Will the carbon deposits now in the exhaust manifold eventually burn out if I run the engine more heavily loaded than I have been? We have in the past cruised on both engines at about 2000 RPM for fuel economy but have recently started running only one engine at 2600 RPM, the cruising speed listed in the boat owner's manual, not the engine manual. Max RPM listed in the boat owner's manual is 3600.