through Steve
It looks like a very poor weld repair job.
I wonder though - is that not part of the water jacket that we are seeing? Would not an insert allow water to leak out?
Based on years of weld repairs around black liquor recovery boilers it is very important that you get a "full penetration" weld to obtain a proper repair. Steve knows.

Jim
'85 300D - no check engine light here
'95 E320
'97 CRV