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Volvo MD-11C Exhaust manifold
If you can get hold of a pump that has a pipe input (not a standard bilge pump) and a pipe output too, then you can runa super-solution of CLR or similar chemical product on a constant circuit through the engine''s water jacket. It will disolve mountains of salt, lime and general crud in about an hour of circulating. A standard second hand impellor driven pump can be picked up for about $20-50 from junk sales or anyone trying to get rid of a spa-bath or jacuzzi.
Either remove your normal waterpump from the engine and replace with one that has been stripped of its impellor, or alternately remove the drive belt from the normal functional one so the impellor can free spin and aid circulation. This way you will not need to have he boat engine running.
You WILL need to remove the thermostat and replace the cover.
That is the way we cleaned out our Yanmar and it worked brilliantly! I left i circulating for about two hours and then ran a fresh solution that wasn''t as strong through for another twenty minutes, then checked wwhat colour it was. It came out pretty clear, so I figured the job was done (The first batch came out as the grottiest hard grey colour).
Flush the system with fresh water thourally after the CLR, and even leave some fresh water circuilating in your nifty pump system for a while. This just makes sure the CLR does not stay in prolongued contact with aluminium parts (It is not supposed to do any damage....but I would rather not take chances)