Black Exhaust Stains On Transom

[eyebbober]

I have a 1976 Volvo MD21A 75hp 4cyl.diesel. I did several recent trips where I had to motor for 8 hours or so.I have noticed a black area above the water discharge on the transom. Anyone have any ideas on what is causing and if there is a rememdy?

Thanks in advance

[RichardElliott]

All diesels smoke to more or less of a degree. However, if you have visible smoke at normal operating temperature, you should look at valve adjustment. The smoke stain on the transom can be cleaned with any good degreasing detergent like 409 or Simple Green.

[e31]

In Bruce Bingham''s book "The Sailor''s Sketchbook" he has a page refering to a "magical" cleaning solvent for sailors. It ends up being brush cleaner which you can buy at any hardware store for a reasonable price. I have been using it for two seasons to clean the diesel residue around my exhaust port (like yours), as well as the brown waterline buildup that seems to resist everything else but "ON&OFF", which is too acidic for my liking.

Just remember to rinse with clean water when you are done.

[RichH]

Diesel residue at the exhaust port is a combination of two actions due to the ''porosity'' of gelcoat. This means that if one routinely seals the gelcoat surface with wax the cosmetic problems of black unburned carbon and the staining due to rust will not occur (as often). Once gelcoat becomes oxidized it also becomes porous (ultimately it will become ''alligotored''). So, no matter what you use to clean this be sure to wax the area after cleaning and you wont have such a problem as often. Colllinite "Fleet Wax'' seems to be the longest lasting wax in the NE USA.

Agressive caustic cleaners such as those contain sodium silicate will typically work best for the black staining due to attached carbon particles. Once the carabon migrated deep down into the porous gelcoat, you will have to sand/abrade away the upmost layer of gel to restore color. Rust is best removed by application of oxalic acid; oxalic acid obtained from a hardware or paint store (cheapest).

Oxalic is probably the best to remove the brown tannin, etc. stains on waterlines and bow (and engine exhaust ports) .... but if such cleaning/bleaching is needed, it really means that the gelcoat has become porous due to oxidation and should be sealed regularly with wax.
:-)

[garygroves]

Eyebbober: If I read your inquiry correcly, you are asking about what causes the black smoke in the first place - if not, please forginve the reponse. As earlier stated you might need to adjust the valves. Most engine black smoke is from lugging the engine, either from over-propping or (most cases) dirty bottoms. Engines don''t like to try and push a boat that''s covered with a shag rug instead of a clean bottom. You could also have a problem with a prop covered in barnacles. Check these out. If not those, then dirty air filter or an exhaust hose that''s partially blocked will also cause dirty exhaust.

[eyebbober]

Yes Gary you are correct. Although I appreciate the advice on removal my concern was what causes this in the first place.

I never run the engine at higher than 1800 rpm and you have to look closely to see any smoke coming out the discharge at all.

When I need power quickly, mainly to turn this full keel beast in tight spots, a plume if white smoke will come out.

So if you have any thoughts I would aprreciate them

Best Regards thanks for the response

Brian

[garygroves]

Eyebobber: When was the last time you checked your injectors? (admittedly I''m not familiar with your engine, but I am with Yanmars) White smoke is unburnt fuel caused by poor combustion. Possible causes may be poor injector spray or low compression. If it was steam, this is usually caused by lack of raw water cooling flow. Smoke tends to linger in the air while steam dissapers more quickly.

[hamiam]

Im not a mechanic but ive been told on several occasions that nothing is worse for a diesel than to run her with no or little load. most mechanics ive spoken to throw out an ideale rpm range of approx 75%-85% of max rpms. As for the white smoke, I had a episode of that myself. Much unlike Garygroves, I was told (and it made sense) that the white smoke was from the engine running too lean (ie too little fuel for the amount of air). As my fuel filter was clogged this would make sense. I would imagine that blue or black smoke is probably the result of unburnt fuel, no? If it was steam, that opens up paradora''s box of problems including a warped head, etc.

[eyebbober]

Thanks for the reply. Gary I have never checked the injectors etc, the boat is new to me this spring and I wouldnt know what im looking at anyways. The surveyor did a compression check which apparantly was good.

It sounds like steam is the issue I am having. Its much more visible at night with the stern light on. Also it dissipates and does not linger.

Would steam cause the black residue?

thanks

brian

[maxlynn]

A little carbon accumulation at the exhaust outlet is pretty much normal for long operation times. If it becomes excessive, have someone check the valve settings and/or injector timing. A lot of steam is not normal - a little steam is normal in cool temperatures. A lot of steam is indicative of overtemperature. Do you have a temp gauge or idiot light?