Question for the diesel experts...

[prof_mariner]

I've got a question for the diesel engine experts out there: Is black "soot" normal in the freshwater cooling side of a diesel engine?

Here's some background: I'm looking at a boat that has a Yanmar 2GM engine. The boat is 35 years old and the engine appears to be original. I'm semi familiar with this motor as I've had one like it before. This is the freshwater cooled model. Because the boat is out of the water and not anywhere near a water supply, I could only run it for a few minutes. Although the weather was rather cool and the boat had been sitting idle for several months, it cranked over and started within 30 seconds. It took a few seconds to begin to run smoothly, but when it settled down, it "sounded" pretty good. When I checked the oil, it was black but otherwise looked normal (no "milky" sign of water or smell of fuel in the oil). When I checked the coolant reservoir, instead of the ethylene glycol I was expecting, it appeared to be black-colored fresh water. The black appeared to be soot and not oil.

My first reaction was why didn't they use coolant? And how long had this engine gone without a coolant change? Am I correct in assuming that if the water isn't red-ish colored, there hasn't been excessive rusting inside the engine block?

At any rate, I'll have to decide whether to move ahead on this deal or walk away. The price is good enough that I'm almost willing to take a chance. But before I do, I just wanted to make sure that soot in the coolant wasn't a sign of a blown head or anything of similar gravity.

[misssherry]

I'm no expert, but my guess would be a head gasket leak between the combustion chamber and water jacket. You need to have the cylinders compression checked.

[brianlara 3]

Many brands of coolant are green!.
Unless an expert is there with you all opinions are going to be educated guesses.
You've already given this motor a pretty good check and I'm inclined to think that it will be ok with a flush and new coolant.
Re: head gaskets/ compression. Once the motor has settled into a smoothish idle a simple test for compression equality is to crack an injector line open. The motor will then labour on one cylinder. Take an aural note of speed and smoothness. Then after a few seconds of noting, close the fuel line and you will go back to both cyls. Then crack the other injector pipe and again note the similarities. A failed head gasket MAY be apparent if there are differences.
What you see may not be soot and different experts seeing what you have seem may disagree with each other but unless sighted all evaluations will be chancey.
Frankly, I think you have a pretty good handle already.
Do the cylinder isolation test next and good luck.

[prof_mariner]

Thank you for the quick replies. Unfortunately, the boat is 280 miles from me and I only had one opportunity to give it a look. I didn't have any tools and because it was out of the water we weren't able to run it for more than maybe 2 or 3 minutes. So given the circumstances all I can hope for is a best guess. Wish I was able to investigate this further, but at the very least I'm pretty sure the owner wasn't very vigilant about preventive maintenance. First time I've seen a freshwater cooled diesel engine that didn't use coolant. Especially considering the weather here can dip well below freezing this time of year. I've decided to walk away from this deal

[CSY Man]

No coolant should be black.
Orange, green or yellow.
Smell the coolant from your finger.
Run hot, look for leaks, smoke or steam.

[prof_mariner]

Originally Posted by CSY Man

No coolant should be black.
Orange, green or yellow.
Smell the coolant from your finger.
Run hot, look for leaks, smoke or steam.

I don't think you read my original post. I know it isn't coolant, it's fresh water with soot. Doesn't smell like diesel. Water isn't oily. It's definitely soot. Because the boat isn't in the water and there wasn't a source of water for the raw water cooling system (and it isn't my boat) I couldn't risk running the engine for more than a few minutes. No doubt if I was able to run the engine long and hard I would have been able to do more. But since it isn't my boat, I really didn't have much choice.

Thanks everyone for your input.

[a64pilot]

It almost certainly is a blown head gasket.
If you buy it, it almost certainly will need a new impeller for the raw water pump as well

[ramblinrod]

Originally Posted by prof_mariner

I've got a question for the diesel engine experts out there: Is black "soot" normal in the freshwater cooling side of a diesel engine?

Here's some background: I'm looking at a boat that has a Yanmar 2GM engine. The boat is 35 years old and the engine appears to be original. I'm semi familiar with this motor as I've had one like it before. This is the freshwater cooled model. Because the boat is out of the water and not anywhere near a water supply, I could only run it for a few minutes. Although the weather was rather cool and the boat had been sitting idle for several months, it cranked over and started within 30 seconds. It took a few seconds to begin to run smoothly, but when it settled down, it "sounded" pretty good. When I checked the oil, it was black but otherwise looked normal (no "milky" sign of water or smell of fuel in the oil). When I checked the coolant reservoir, instead of the ethylene glycol I was expecting, it appeared to be black-colored fresh water. The black appeared to be soot and not oil.

My first reaction was why didn't they use coolant? And how long had this engine gone without a coolant change? Am I correct in assuming that if the water isn't red-ish colored, there hasn't been excessive rusting inside the engine block?

At any rate, I'll have to decide whether to move ahead on this deal or walk away. The price is good enough that I'm almost willing to take a chance. But before I do, I just wanted to make sure that soot in the coolant wasn't a sign of a blown head or anything of similar gravity.

Black soot in the fresh water coolant is definitely not a good sign. Likely a blown head gasket or warped or cracked head (most likely from overheating). Replacing a head gasket is no big deal for one with decent mechanical aptitude / ability. Sometimes cracked heads can be repaired, but often require replacement. A replacement head gasket is likely around $100 bucks where a replacement head is likely around $1000.

Either way, the head gasket will have to be replaced.

[seabreez]

Originally Posted by prof_mariner

I don't think you read my original post. I know it isn't coolant, it's fresh water with soot. Doesn't smell like diesel. Water isn't oily. It's definitely soot. Because the boat isn't in the water and there wasn't a source of water for the raw water cooling system (and it isn't my boat) I couldn't risk running the engine for more than a few minutes. No doubt if I was able to run the engine long and hard I would have been able to do more. But since it isn't my boat, I really didn't have much choice.

Thanks everyone for your input.

No coolant should be black! Green or yellow for cast iron blocks and head. Red for aluminum heads. If there is fresh water in the cooling system.... that is an indicator that something is wrong. By running the engine for a few minutes... did you disconnect the water pump?

Now to confirm the head gasket leak, this is how you do it. Disconnect the water pump. Fill the water system to the top. Start the engine and observe the coolant level. If bubbles appear there is a head gasket leak and hence the soot in the coolant. Other indicator is white smoke coming out of the exhaust when running.

[prof_mariner]

Originally Posted by seabreez

No coolant should be black! Green or yellow for cast iron blocks and head. Red for aluminum heads. If there is fresh water in the cooling system.... that is an indicator that something is wrong. By running the engine for a few minutes... did you disconnect the water pump?

Now to confirm the head gasket leak, this is how you do it. Disconnect the water pump. Fill the water system to the top. Start the engine and observe the coolant level. If bubbles appear there is a head gasket leak and hence the soot in the coolant. Other indicator is white smoke coming out of the exhaust when running.

I didn't have tools with me and it wasn't my boat so I wouldn't have been able to do as you suggested, but I'll keep this tip for the next time I have to diagnose a leaky head / gasket.

[Cheechako]

I remember dipping a finger in radiators and it coming out with black sooty looking water, the black stays on the finger. I'm not sure what it is, but as people mentioned, it could be a leak. Or maybe it's just no antifreeze. It's possible old hoses start giving off that black stuff. Ever run your finger inside an old hose you remove? It comes out very black.
A compression test would be nice, but will require a fitting to be made unless your mechanic has one for the Yanmar. The GM series does have some issues with cracked pistons from time to time. Although sometimes this makes them a bit slow to start. You should be able to run that engine a good long time if there is a garden hose handy, open the top of the strainer and put the hose in letting it overflow a bit if necessary.
If you have lumps of black soot floating on the water, then that's definitely soot.

[seasick]

Blackened coolant may also indicate the need to replace the pencil zincs. Sounds like a good bargaining chip for you if you like the boat. Before replacing the head gasket or even going to the trouble of conducting a compression test, I recommend draining the coolant, flushing the freshwater system, replacing both pencil zincs and filling with the proper coolant. Checking the pressure in the cooling system is a great deal easier than a compression test and should be revealing.

Bottom line, Blackened coolant would not necessarily be a deal killer for me but rather an indication that maintenance in other areas is probably lagging. Document your concerns with photos and professional opinion and use this to negotiate a price that covers the worst case scenario, I.e. Blown HG and or cracked head.

[Cadence]

Originally Posted by misssherry

I'm no expert, but my guess would be a head gasket leak between the combustion chamber and water jacket. You need to have the cylinders compression checked.

SWAG also.

[Bill Seal]

Fresh water also goes through the exhaust manifold water jacket. (At least it does on my Green Monster)
Could be a cracked manifold or bad manifold gasket.

[McKenna]

A crank case oil sample sent in for a professional evaluation would be money well spent, it's quick, relatively inexpensive and informative.