Cause of green/black streaking in engine bay

In the past I've had a nasty looking green/black residue on the hull in the engine bay below the large gray water exhaust muffler. It was streaking down to the bilge. I've always thought this to be water spun off the prop shaft as the streaking residue was in line with the SS exposed prop shaft.

Here's a a Posting ref for discussing this in April 2013.... http://www.tugnuts.com/viewtopic.php?f=18&t=3585&p=26421&hilit=green#p26421

When I had the USCG inspection done a month or two back the inspector did not like the look of it and said I should examine things to find the cause as it looked as if there was oil/grease/fuel in the green looking residue, and if that gets into the bilge I would have an oil spill issue when the bilge pump operated. Hmmmm.... not good at all.

Today, I believe I've found the cause.

Since the USCG inspection I had wiped up the green/black streaking and removed all of it. The idea was to see if it returned over time. It had not over a period of 2 months with but a few boat outings as well.

Today I was fiddling with my Yanmar's 4BY2-150 i5601E engine display and happened to start the engine several times in quick succession.

I had the engine hatch open and to my surprise I caught the green/black streaking culprit at work. This muck appeared to me to come from a small dark gray knob at the base of the large gray vertical water mixer muffler as shown below. I actually saw a quick squirt of liquid coming from this knob or a spot awfully close to it. I bent down and used my hand to feel if there was any water residual on and near the knob... and there was.

So having presumably found the culprit I now ask myself, just what is this knob's purpose in life ? Why does it allow water from the gray muffler to escape ? I hesitated trying to turn the knob in case I damaged something, or worse still not being able to screw it back in.

Anyone know what the dark gray knob is for ? BTW... is it likely the gray vertical water/muffler is under any pressure with the engine running, especially when first starting the engine ?



Here's a close up of the dark gray knob. It appears to be a screw cap rather than a plug screw. There are three small holes in the cap and wonder if water squirts out from them. The format of this cap tells me it can be unscrewed as threads are clearly visible. Maybe it's some kind of pressure-relief valve !!!!

Bill, the knob is the muffler drain plug used to fully drain the silencer for winter storage ....... to close the drain turn it fully clockwise ....... for winter storage (freezing) open it fully counterclockwise and let all the water drain. There should be very little back pressure on the muffler/silencer and the leak might indicate a worn "O" ring if your drain is fitted wit one ...... hope this is of some help ....... Fran 😉

fran uteg":354tx3br said:

Bill, the knob is the muffler drain plug used to fully drain the silencer for winter storage ....... to close the drain turn it fully clockwise ....... for winter storage (freezing) open it fully counterclockwise and let all the water drain. There should be very little back pressure on the muffler/silencer and the leak might indicate a worn "O" ring if your drain is fitted wit one ...... hope this is of some help ....... Fran 😉

Click to expand...

Thank you very much... So if I unscrew this cap (drain plug) will it come completely off to expose an O-ring ? I will turn it gently clockwise to see if it turns a bit and to stop any more possibility of water seeping out of it.

I honestly don't know if the cap comes completely off ...... on my 21EC the drain cap is very close to a frame member (the 21ECs are assembled by Keebler Elves with small hands) and I can't take it completely off. I assume there is either an "O" ring or a rubber washer in the top of the cap .... either way the cap should snug up and stop your leak and open enough to properly drain ....... that little cap is quite important. If it fails you will be pumping a lot of water into your bilge ....hope you have many hours of dry cruising ..... Fran

Was able to get 1/8 to 1/4 turn to tighten the drain plug and that seems to have cured the issue. Thanks for your help.