question about old volvo penta MD7a

[bruno schmit]

I have an 30 year old VP MD7a in my boat and goes into the "red" zone when running 30 minutes at 1100 rpm.
heat transducer and thermostate are newly replaced
A mechanic told me that this is due to accumulation of rust, salt and sand in the water passages around the cilinder (direct cooling with seawater (no intercooling system).
(I had also impression he wanted to sell me a second hand engine).
I believe this may be true, but is there a way to clean these water corridors without removing the engine?
Thanks for any help.

[TomTilley]

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.

TomT