Got the 2QM20 Black Oil Blues

[partingclouds]

While motoring long hours during a coastal passage the oil turned black very quickly (after an oil change) and the exhaust stream was puking lots of sooty black water. I pulled the exhaust mixing elbow only to find nothing wrong there.
Any advice from the hive would be welcome.

[rslifkin]

First thing to check would be the air filter. Restricted air flow on a diesel will cause incomplete combustion, resulting in black smoke (which also soots up the oil, turning it black).

[deblen]

What temp does engine run? Is thermostat working?
What is wide open throttle RPM when steaming?
What cruising RPM do you set?


https://sbo.sailboatowners.com/downl...20H-3QM30H.pdf


Cheers/Len

[alctel]

My old boat had a 2QM20

My experience was the oil would turn black almost instantly even being in the vicinity of the engine! On the plus side it was absolutely bulletproof - I had all the coolant leak out once and didn't notice till after the passage. The idiot light didn't even come on.

[partingclouds]

Thank you, sailors.

The air filter is clean. The thermostat is working perfectly. The engine temperature is around 68 Celcius running at 2200 RPM. Hull is clean. The mystery deepens...

[Wotname]

Originally Posted by partingclouds

While motoring long hours during a coastal passage the oil turned black very quickly (after an oil change) and the exhaust stream was puking lots of sooty black water. I pulled the exhaust mixing elbow only to find nothing wrong there.
Any advice from the hive would be welcome.

The 2QM20 is raw water cooled (unless it has been modified post manufacture) so it runs cool (by design). If you have not been running it at higher power setting for some time, it will have carbon deposits in the exhaust (and in the combustion chamber).

Very likely motoring long hours during a coastal passage is blowing out the exhaust carbon and some of the internal carbon is going into the oil (via the rings). This is a good thing!

In fact a long hard run (but staying within temperature/pressure limits) will be beneficial. Max continuous rpm is 2600 with max peak rpm is 2800 (1 hour limit) so say 12 to 24 hours at say 2400 to 2600. Assuming it is still raw water cooled, water temp should remain below 125F (52C). Oil pressure should be above 30psi.

If the engine starts easy when cold, does not consume large quantics of oil and will run for 12 / 24 hours above say 2400 with stable water temp and oil pressure, there isn't much to worry about.

[Wotname]

Originally Posted by partingclouds

Thank you, sailors.

The air filter is clean. The thermostat is working perfectly. The engine temperature is around 68 Celcius running at 2200 RPM. Hull is clean. The mystery deepens...

Presumably coolant cooled then otherwise 68C is way too high!

[partingclouds]

Thanks, Wotname. I'll run it wide open a while, burn out the carbon and see if it does the trick. Blackened oil in such a short time is incredible and had me worried. I will carry on as if l know what I'm doing.

[Wotname]

Originally Posted by partingclouds

Thanks, Wotname. I'll run it wide open a while, burn out the carbon and see if it does the trick. Blackened oil in such a short time is incredible and had me worried. I will carry on as if l know what I'm doing.

Black oil and old diesels go hand in hand! You will probably eventually get the exhaust to be clean but the oil is likely to always go black quickly (like in a couple of hours) depending on the state of the rings, power settings and load etc.

Does it start OK when cold?
Do you have an oil pressure gauge?

[partingclouds]

The engine starts easily when cold. It's been very reliable. Oil pressure at 45 psi when warmed.

[Wotname]

Originally Posted by partingclouds

The engine starts easily when cold. It's been very reliable. Oil pressure at 45 psi when warmed.

Perfect!

IMHO, the 2QM20x is the second best engine Yanmar made. Relativity low rpm, heavy construction and designed as a workhorse to run all day, not as a pony for an afternoon at the Showgrounds. The best was the 3QM30x but only because of the extra cylinder makes it slightly smoother.

Another performance check is to run the engine at near max load / power with new oil for a couple of hours, then pull it back to idle and check the drop in oil pressure.

[GordMay]

Originally Posted by Wotname

Black oil and old diesels go hand in hand! You will probably eventually get the exhaust to be clean but the oil is likely to always go black quickly (like in a couple of hours) depending on the state of the rings, power settings and load etc...

Heat cycles naturally darken motor oil.
A heat cycle represents each time your engine reaches its normal operating temperature, then cools down again.
Some additives in motor oil are more susceptible to darkening in the presence of heat than others.
In addition, normal oxidation can darken oil, too. Oxidation occurs when oxygen molecules interact with oil molecules, and cause chemical breakdown, just like how oxygen causes a cut apple to brown, or iron to rust. High heat accelerates oxidation.

While heat cycles cause oil to darken, soot causes oil to turn black [it's carbon].
Soot is a byproduct of incomplete combustion.
Since soot particles are less than one micron in size, they typically don’t cause engine wear. The problem with soot happens when the soot particles begin to agglomerate, into larger, wear-causing contaminants.

One of the ways quality engine oils deal with soot, is to hold the soot particles in suspension, preventing them from agglomerating [flocculating], and depositing themselves on engine parts, which would lead to the build up of sludge deposits.

The best way to avoid problems with soot is to use good quality oil, with dispersant additives.
Also consider utilizing a comprehensive oil analysis program, which will show the level of soot, the oil viscosity, and the indicate engine wear levels.

See “What Is Lubricant Oil Soot?” ➥ https://www.aei-tech.com/products/bt...cant-oil-soot/