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Current Replies for fuel in exhaust
exclamation fuel in exhaust (MarkD)
Posted: 8:44:14 pm on 7/26/2014 Modified: Never
Lately I have noticed a film of diesel emanating from the stern of my MK V on start up.  It seems to only appear at that time.

The Yanmar diesel seems to be working properly in every other respect although it is the original engine for this boat.

Any Ideas?

Easy Does It - MK V

Mark Doucette
St Margaret's Bay
Nova Scotia

  Re: fuel in exhaust (Bill Salton)
Posted: 11:03:15 am on 8/10/2014 Modified: Never
Are you sure it's diesel? When we first got our boat (an atomic 4 admittedly), the previous owner was using a synthetic oil, and we were getting an oil slick behind the boat, especially at start up. I switched back to Castro SAE 30 and the problem disappeared, never to resurface.
  Re: fuel in exhaust (MarkD)
Posted: 2:53:06 pm on 8/10/2014 Modified: Never
I never considered that the slick could be caused by oil and not fuel.  It would be easy to switch to a heavier oil and see what happens.


Mark Doucette MKV "Easy Does It"
St Margaret's Bay
Nova Scotia

  Re: fuel in exhaust (oasis)
Posted: 2:20:32 pm on 8/16/2014 Modified: Never
I am no expert.  But I think this is fuel, and I think it is perfectly normal.

When the engine is running hot, you have hot exhaust pushing hot cooling water through a common exhaust/water hose pipe and through a water lift muffler and out the stern.  Hot exhaust, hot water - mix pretty well.

Stop the engine and everything gets cold.  Some water and fuel is left in the water lift muffler, and some in the hose before the lift muffler.  The exhaust gas, which will includes some unburned fuel, as these engines are not particularly clean burning, separate, as water and gas or diesel mixed will do.

So, when you start up the first thing to exit is the cold water and separated fuel, along with various chucks of carbon in the exhaust system.  So you get a small film of fuel and carbon on the water until the gases and water are hot enough to mix again and make it look clean.

Anyway, that is my theory.  But I'm no expert.

My 1GM10 has been doing this ever since I got it, and I wouldn't give it a second though.  Just annoying that it fouls the rudder and transom a bit as it warms up.

Barry, Oasis, 1987 Mk V
  Re: fuel in exhaust (pablorusjan)
Posted: 11:08:08 pm on 8/24/2014 Modified: Never
I have observed the same issue in my Yanmar this season (I have used it for 5 years). I wonder whether the issue can be related to the specification of the oils. I was shocked today reading the page 29 in the last version (June 2013) of the 1gm10 manual (
It says:
Never use API Service Category CG-4 or CH-4 oils.
I thought there was some kind of backward compatibility in oils and that the CC specification was obsolete. ( It is not easy to find oil for diesel that does not comply with CG-4 or CH-4.
Any feedback is welcome.
  Re: fuel in exhaust (JWente)
Posted: 8:59:54 pm on 8/25/2014 Modified: Never
they all do it
I was guessing it is dirty injector(a)
  Re: fuel in exhaust (JWente)
Posted: 7:13:35 pm on 9/10/2014 Modified: Never
we changed the Thermostat last year and although we still spit black exhaust at starting it is better now than in a long time
we think it is because the thermostat was stuck open and the engine was running cold
now it runs hotter and burns diesel better
or so we think
Distant Thunder