Diesel fumes from breather cap

For years we've been suffering from the bypass fumes of the engine in the boat.  I considered putting in a fan to vent the engine fumes outside but hesitant about cutting a hole in the boat.  I found a simple solution to the problem similar to emission controls in our cars.  I removed the oil filler cap and fitted a radiator hose with plug to fit over the tube and allow the attachment of a 3/8 ID rubber hose.  I ran the hose over the engine and inserted into the air breather allowing the engine to remove the fumes.  

Could you post some photos of your installation, please.

I am wondering if you drilled another hole in the air intake rather than just stuff the 3/8 hose in the existing intake opening. That would restrict the amount of air going to the engine and diesels like a lot of air.....

Sounds like a good fix though...

cheers,
George
WarBird

I agree these fumes in the boat are annoying. I'm just wondering about the method, however. The oil-fill cap is sort of made special with a reed valve on the top. I presume this allow has something to do with the internal oil pressure inside the crankcase. The modification you describe basically leaves the top of the oil-fill tube open to atmosphere preventing any internal pressure from developing at all. While I have actually re-built my Bukh myself, I admit to not knowing whether it relies on internal crankcase pressures or not. Bill? Somebody? If it works and does no harm, I will go with it for sure.

Crankcase pressure is equal and opposite to compression so when the piston hits BDC the crankcase has to be able to release that pressure and does so thru the vented oil fill cap. My observation anyways.

That's the shortcoming in it's design I suppose. I just tested a filler cap (nice clean dry one in my basement) and it allows crankcase pressure to exit out the cap but does not allow air into the crankcase. So this explains the engine smell we have when motoring.

Looking closely at what Dick did makes sense except I'm not sure how you dealt with the baffling that's part of the oil fill cap. The baffling fins on the cap prevent oil from getting out the filler neck. This is a problem when motoring when healed. Any stock engine will spew out oil thru the cap if motoring when healed..... most of us have experienced this at some point.

With the baffling removed and just a hose installed and led to the air cleaner. I would suspect you may be burning some of that escaping oil. Just a thought.... I would watch your oil level closely.

It doesn't take much healing for the oil to get past the baffles in the cap and spray the entire engine compartment with oil. I've seen it many times. Is there a way to use the cap so you can retain the baffling? Perhaps modify the cap to fit the hose. Just thinking out loud. You don't want to lose engine oil to the air filter for any reason if you can help it.

I love it! Thanks people!

The hose from the saildrive leg  is it's oil fill hose. It came with a plastic plug on that end.

You could always ty-rap and screw the 3/8" hose to the underside of the cockpit and drop it down by the air filter, leaving some wiggle room. Keeping that hose vertical until it hits the cockpit is probably desirable should oil ever get into it.

After sleeping on it I decided to go the Bondo and Aluminum tape route to finish out the summer.  Did not remove the oil residue from filler cap so Bondo can be easily removed.  I like Frank's idea.  Winter project.

Hi All. I finally got to the project of the re-breather for the oil cap and tested it earlier this week. It works.  

Edited by Bill Layton - 29 August 2014 at 4:05pm