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Grey / Silver Sludge

Kit Sigma 41

New member
Any help would be appreciated.

Having issues with a Sigma 41 coolant issue.

Issue is thick silver / grey sludge in coolant reserve / heat exchange.

History of boat:
Not used much over the winter, engine left open for a few weeks whilst fuel injector was being replaced.
Water pump leaking (white sludge indicating mix with oil).
Fresh water cap on heat exchange tiny leak.

unfortunately overlooked checking coolant reserve in pre engine checks.

Over heated on first sea trial of the year, returned to port with marina help.

Checked impeller
Opened release tap to find nothing dripping out
stuck a hose in coolant reserve and started a flush
Took ages and a THICK grey sludge came out the release tap
Opened up the heat exchange to find it all coated in the sliver grey sludge
Cleaned out and flushed with a hose as much as possible
flushed 10x times.


Managed to get yacht to marina we could work on it. Whilst engine running temp stayed normal ( literally had the engine on for minimum time however the water pump was leaking tremendously.

Had water pump professionally serviced and fixed

More flushes later

Engine is running as normal for extended periods and under load.

Next leak occurred in heat exchange cap meant sea water started going into coolant system.

New cap purchased ready to drain and replaced

Drained about double the quantity supposed to be in.

Decided to open up to check sludge - still loads of the stuff.

Flushed again

Refilled with coolant and run for 1hr at idle engine temp has now for the first time since initial issue started to rise slightly.

To summarise around 2 months of sludge issue 10/15 flushes. Not much getting completely rid of it however it is now more liquid like and floating to the top of coolant reserve.

Personally don’t think it’s HG as no other signs.
Could be from previous mixes of coolant however have now switched to vetnus organic marine.

I though brown milkshake sludge was HG? With oil contamination. Not silver?

Have just ordered liquid moli radiator cleaner
And am planning to take reserve and pipes off and completely clean through.

Any suggestions would be more than welcomed.

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