Yanmar 3GMF 20 coolant flush has rusty metal flakes

[illanrob]

This 1985 Morgan 36 with a Yanmar 3GMF20 had it for 8 months and sailed it 30 times (~60 recent hrs on the motor now with 970hrs total). Noticed some water/oil spray in the cabin (the engine cover has been off for several weeks) after motoring at 3000rpms (normally only run to 2800rpms or so) up river to the dock from the front of engine.
Did the overdue coolant flush and it was very cruddy and there were many rusty metal flakes. Ran motor for 30mins after changing the 'L' shape top coolant hose ( had a cracking sound when squezzed) rpms at 2800 3000 did not notice any spray.
Took a sample of the coolant (from the cap overflow hose) as the engine was running engine temp went up to 180F from the normal 150F. The not so cruddy now coolant sample ( all water) settled and the water is nearly clear. Didn't see any trace of oil in the coolant sample.

The rusty flakes in the coolant are from?
A. The heat exchanger baffles
B. ??

[a64pilot]

The cast iron block most likely

[rognvald]

Rob,
If the PO did not service the coolant system as per schedule, the coolant lost its rust/corrosion inhibitors and created rust in the block as Pilot mentioned or calcification that appeared to be rust after it had settled to the bottom of the heat exchanger/hoses/block and was continually mixed every time the motor was operated. I would suggest a serious engine flush with Prestone and then 3 rinses with distilled water which should eliminate 98% of your sediment issues. Texaco is the recommended antifreeze although I have used Prestone for the last two changes with good results. It is my opinion that in both cars and boat engines, antifreeze/coolant fluid change is one of the most important service schedules and sadly one of the least performed. Hope this helps. Good luck and safe sailing. P.S. Since you stated that you recently performed an engine flush, I would run your engine for the next 250 hours and repeat the process. The cost is minimal and the rewards are great.

[Liam Wald]

Engines with aluminum blocks require special antifreeze/coolant.
Get coolant that is specified for Asian cars.
Zerex 675130 Asian Vehicle Antifreeze / Coolant is a good choice.