Join Date: Feb 2004
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Bruno - I had a Universal 5411 diesel that had the same problem (and was raw water/seawater cooled). After a lot of reading and suggestions from other boaters I used phosphoric acid (which removes rust) to flush through my engine block. While it removed some of the rust - it wasn't doing much for the salts that had closed off a number of the smaller block/head water passages. I wound flushing then neutralizing the acid, removing the head, then manually opened up the salt-closed passages using smal screw drivers, files, etc. to break through the salt. Then I started a hot-water flush to help remove the salts and was going to follow up with phosporic acid again for the rust. At this point, I "found" a Universal M3-20 for cheap, which was what the hp level the boat really needed - so I pulled the 5411 and sold it.
The flushing can be done by routing discharge water hose into 5 gal plastic buckets with wet towels to minimize acid splash-out.
If I was to do it again - I would pull the head first - then pull the engine - tranport it home to work on in driverway- or, do a strip down of the block and send block and heads to a rebuild company for "hot tanking".
You can do it in the boat, but expect to spend some time (at least a full weekend) especially if the boat is seriously overheating.