8.1 Cooling system flush questions

Bwardz50

Member
Jul 3, 2018
31
Lutz, FL
Boat Info
2006 340 Sundancer
2005 Mastercraft X2
1995 16' Carolina Skiff
Engines
8.1 Mercruiser Horizons with V-Drives
MCX 350 Indmar with V-Drive
Yamaha F40TLRC
Hi there, thanks in advance for reading...

I took my new (to me ) '06 340DA out on New Years (water was cold) and once under load, my port engine started overheating. I shut it down at 203° to avoid damaging anything. Long story short, I discovered that the fresh water cooling system had chunks of material around the thermostat from the PO's attempt to fix a leak with a stop-leak product. He ended up replacing the heat exchangers on both engines before selling it to me but there's still residual gunk in the system. I've flushed it with water and a radiator flush & fill kit but there's still crusty stuff stuck to the areas I can reach with my fingers. Temps are now back to normal (155°) under load so I'm happy about that but I'd really like to get this crusty stuff out of my cooling system.

Does anyone have a recommendation of a product to use that will break this stuff down without damaging the seals, hoses etc?

Thanks again,
Brian
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Did you try flushing it by removing the 2 hex bolts on the block? I would run a hose in the thermostat and flush through the block drains. I would try removing the plug, stick the hose in the thermostat hole and flush. Then put the plug back in , fill and run the engine a bit( idle) , remove the plug and drain. Repeat. When/if it's clean, fill with antifreeze.
 
Did you try flushing it by removing the 2 hex bolts on the block? I would run a hose in the thermostat and flush through the block drains. I would try removing the plug, stick the hose in the thermostat hole and flush. Then put the plug back in , fill and run the engine a bit( idle) , remove the plug and drain. Repeat. When/if it's clean, fill with antifreeze.

I did that to drain the block but this stuff is crusted on. It softens when wet but still sticks around. Need a chemical that I can flush through that will break this up without damaging seals and hoses.
 
I guess that's possible. I'd really like to figure out what chemical to run through the system that won't damage anything. Thanks for the interest thus far.
 
Prestone makes a “Flush” product. Maybe give that a try?
 
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Prestone makes a “Flush” product. Maybe give that a try?

I think you need to be cautious about introducing new chemicals into the system to flush it out. Mixing antifreeze solutions may have cause this although that is speculation based upon what the true facts may be. If antifreeze incompatibility caused this, who is to say a flushing chemical would not make it worse by causing some other chemical compound to precipitate out of solution. If it were my boat, I would contact someone from Dexcool to learn how the mess can be reversed, if in fact in can be reversed.
 

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