Black sludge in closed cooling ststem

speakrdude

Well-Known Member
Feb 21, 2009
1,214
Northwest Arkansas
Boat Info
2002 Sea Ray 340 DA
Raymarine C80 Tri Data
Engines
twin 6.2 MX MPI
Checking fluids recently on 2002 merc 6.2mx mpi’s v drives, noticed black ugly sludge forming in cooling resivior. Flushed and changed fluids last year.
What may causing?
 
Did you replace the coolant with orange dexcool when you changed it last year?
 
It has been told that mixing Dexcool(orange) and "the green stuff" is a recipe for the black sludge.

However, I had a 2500 Duramax diesel that slowly started doing the same thing. It bugged me to no end. I dumped the coolant, flushed everything, and started over with new Dexcool. Same thing started developing. One day, oil pressure light comes on and gauge shows little to no oil pressure. I stop, open the hood, and find brownish sludge foaming out of my coolant resivoir. Truck is towed to shop where I am told that the oil cooler had a pin hole in it and finally ruptured. As oil pressure is more than coolant pressure, oil is pumped into the coolant. Replaced the cooler and hooked the coolant system to a circulating machine for 3 days and let it flush and filter. Never did get all of the oil out of the coolant.

Not sure this could be applicable on a boat like yours, but likely you have one of these two conditions most likely.

Bennett
 
Last edited:
How certain are you that you flushed the system perfectly? Less than 5% Dexcool left in the system can turn it to sludge.
It's not clear what he started with. Was it green to begin with? If so, not an issue with adding back green.
 
Let me rephrase that.
Without reaching under the engine and creating a huge mess,
Is there a better way to evacuate the coolant, flush, replace?
Jim
 
If it ever had orange Dex cool in it:
Drain what you can, fill with fresh water, run for short period of time until T stat opens, drain, filll with fresh water, run again until T stat opens, etc..
Keep repeating until it is clean. Drain again and fill with green coolant as specified.
You may have to remove the heat exchanger and get that cleaned out too while you’re at it.
The orange Dex cool is notorious for causing sludge. If it was ever in there it may take several flushes to clean it all out.
Picked up a nice little 05 Grand Prix 2 years ago for my son to use to learn to drive on and go back and forth to high school in.
I had to swap the radiator, go through the process, repeatedly back flush the heater core, and scrub out the overflow to get all that crap out of there. The old radiator weighed substantially more than the new one because of the sludge in it.
 

Forum statistics

Threads
112,945
Messages
1,422,750
Members
60,928
Latest member
rkaleda
Back
Top