Ink black exhaust water - soot?

[TropicCat]

I own a boat with 2 Yanmars and would like to suggest something no one else has mentioned.

My engines are very sensitive when using old diesel fuel. In fact it causes the symptom you mention. I would suggest a cheap fix by tossing a couple of gallons of new diesel in the tank and add a stabilizer. I use 2 stabilizers.

Stabil to extend the fuel life and Diesel Clean to add the lubosity lost in our low sulfur diesel fuel at the pumps these days.

It can't hurt.

[sandycohen]

Hi TropicCat - Thanks for the input. Unfortunately I can report that I put about 5 gallons of new, marina grade diesel in with an additive last month, and the tank is only about 20 gallons so that's a pretty good share.

[SlowButSteady]

Tropic Cat may still be on the right track. Diesel fuel will polymerize as it ages. This makes it burn more slowly and often results in incomplete combustion (basically, the longer the molecular chains the slower a hydrocarbon will burn). Adding some new diesel will help, but if there is enough really old fuel in the tank the soot could be a result of this incomplete combustion of old diesel. Microbial growth in the tank will make things even worse.

I would take a look at a sample of the fuel from the tank and compare it to some fresh diesel. If the difference in clarity is dramatic (as in, one look makes you wonder how the engine ran at all), replacing ALL the fuel and cleaning the tank may be in order.