Question for the diesel experts...

[jakkum]

Just fresh water in a steel system can turn very black.
Did you taste it?
Just put your finger in and taste. Coolant will be sweet. Soot will taste sooty. And you will taste both if they are both in there. They are both significant and very different.
Now spit, do not swallow.

An I.M.P.

[Cadence]

If you determine it to be soot you probably should give your heat exchanger some attention.

[High Pointer]

I have had the same problem for years on the Yanmar YSM8,a low compresison test resulted in removing the head and found it needed new valves,this restored the compression but the black soot in the coolent is still there untill warmed up,then it reduces but not completly.have been told the adjustment of the
governor link to the injection limiter could resolve an over supply of fuel that could cause the problem.

[Steven UK]

Someone may be more knowledgable than me but I believe deoxygenated water goes black especially with contact with ferrous metal. I remember messing with central heating systems in the uk would always be like that the water would leave a soot like residue on your hands. If it was a head gasket seal to the combustion the the pressure cap would be blowing. What is the low temp in winter where it is? It may just have been filled with water... Good luck.

[hamburking]

The first red flag is water instead of coolant. Especially if you are somewhere the temps go below freezing...expansion will damage the engine...likely in exactly the way you are describing. With the limited info you have given, it seems like somebody didn't want to spend a few bucks on antifreeze, and used water instead. Aside from the freezing issue, antifreeze contains other good stuff (according to the label) too.

Second red flag...running the engine with no water. Congrats, you just peeled all the rubber blades off the impeller by running it dry. Not only does it need a new impeller now, but you've got to find those rubber blades before they clog up the heat exchanger. You could have run the water intake to a bucket of water, or a water jug...its not a big deal. A pocket tool would have been sufficient. I'm surprised at the owner for allowing the engine to be run with no water.

Its a real shame though...the old yanmars are excellent engines. However, the only way to really know its a good engine is to go for a sea trial, get it up to temp, and see it working for real.

I think you did the right thing to walk away. When looking at boats myself, if the owner doesn't say "the engine run perfect" then I'm pretty skeptical and reluctant. After all, who wants to leave the dock with a sketchy engine?

[Donald Duque]

I agree that in Prof Mariner's circumstance it was best to walk away from the deal. I would like to stick my two cents in here, though, in hopes that it may help others in a similar circumstance.
Sea Breeze is on the right path. But to confirm his observation, there is a device and a product called a Block Checker that has been around for probably fifty years that bubbles the gas escaping from the coolant through a solution that changes color if the gas does, in fact, contain combustion gas. if it does, it is most likely a leaking head gasket, but could also be a cracked cylinder in a parent bore block, or a cracked liner. In more modern common rail engines it could also be a leaking injector bore liner or cup.
The Block checker can be found at most good automotive or truck parts stores. they are not too expensive, as they are very simple devices. you may also be able to borrow one from a shop as, again, they are not expensive or sophisticated.

[Sputnik]

My 10 peneth:
Water, or coolant (antifreeze mix) will go black if there is salt-water corrosion inside the cooling jacket. The most likely cause is an internal leak in the heat exchanger. This will allow the coolant and sea water to mix, the antifreeze will eventually be washed out of the cooling system, so you just have sea water in there.
If the engine bay has a sump below the engine: get a fresh water hose supply and feed it into the engine bilge for a water supply. Remove the hose from the intake side of the raw water pump and dip it into the fresh water. Now you can run the engine for a long period (obviously keep the bilge supply water topped up).
You are looking for the coolant system to pressurise and overflow with the "raw water" (the fresh water taken from the engine sump supply). Or pressurisation of the cooling system from a blown head gasket.....I don't think that is your problem, that is a lot of soot to make the coolant black.
Good luck,
Neil

[GeneM]

I had the exact same thing on my 5.9 Cummins. I got bad advice and kept running the engine. Basically, the head gasket was bad and let cumbustion gasses into the coolant. The coolant was black from dissolved soot, not oil. If I had good advice, I could have just replaced the gasket. I wound up replacing a cracked head and rebuilding the block.

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[AllAboutMe]

Unless it's a unique boat worth taking a chance on, just pass on it. There are too many boats out there for sale. That being said, do you have todrive 280 miles every time you want to look at a boat or is it just this one? A tool kit and some airtight jars for water/ oil samples left in the trunk would be handy.

[buzzstar]

[QUOTELife is sexually transmitted/QUOTE]


No always anymore. Times they are a changing.

[prof_mariner]

Wow, got a lot more replies than I anticipated...

Just to clarify a few bits:

First, I asked the seller if we could take the raw water intake hose and put it in a bucket of water so we could run water through the engine for the test. Wouldn't allow a very long test but better than running the engine dry. The seller didn't want to bother and ran it just long enough for it to kick over and run for maybe 2-3 minutes. I agree, by doing this the raw water impeller was probably damaged, but if I had bought the boat replacing the impeller wouldn't have cost much. I don't think it's likely the blades would go very far without water to move them.

I had never seen a fresh water cooled diesel engine that didn't use coolant of some sort. And yes, the area had a full week of sub freezing weather so the lack of coolant + cold weather + black/sooty water in the reservoir made me suspect the worst. I posted here mostly to confirm what I already suspected.

And lastly, I definitely walked away from the deal. Surprisingly, the seller didn't seem too surprised when I first showed him the black liquid in the coolant reservoir/heat exchanger. The boat had other issues, but the suspicious engine issues sealed this deal's fate. It's a shame, the boat was once really nice. But lack of proper maintenance over a long period of time resulted in a condition where fixing it would cost more than the boat is worth.