Catch can instead of venting back into engine?

[loco]

Hi all,
Having run my engine flat out with the engine cover open, I've seen how much in the way of blow-back oil fumes are vented back into the engine. I have a 4.3 V6, with one vent going into the carb vacuum connector, and the second one going into the flame arrestor.

It occurs to me that
a) I'm losing some vacuum, and feeding the engine grubby fumes instead of fesh air/fuel mix
b) the fumes are going to be dirtying up the inside of the engine, and being burnt and going into the environment, which explains the small amount of oil sheen I see on the water after I've run the engine in the harbour.

Wouldn't it be best to put the manifold cover vents into a catch can, and in theory, vent the clean air to atmosphere. Or - plug the vacuum feed, and have both the vents feed into the flame arrestor, rather than the vacuum source?

Seems like a way of gaining power, and running cleaner for the environment, for little effort.

 

[alldodge]

When your motor was new there was next to no combustion vapors coming up the hose.

one vent going into the carb vacuum connector, and the second one going into the flame arrestor

If your motor uses a PVC valve then the motors vacuum will suck the fumes into the intake and burn. If the hose goes from the valve cover to the flame arrestor then its using the hose just to get it close so the motor will draw them in without vacuum.

a) Your not loosing vacuum because if there where fumes or not the motor will do the same, that is unless your PVC valve is defective. If you don't have a PVC valve and its just a hose there is no vacuum at the hose.

b) If the fumes were burning correctly there would be no sheen. My guess is your motor needs some work because either your rings are carboned up, or worm out, or the valve seals need replacing.

You could put the hose in a catch can but do not seal it up, excess pressure must come out of the motor. If you use a catch can which must also have a filter, the fumes would be pushed out of the can. So you need a filter to keep the oil from getting out.

In short, I think you need to check your motor over to see why your seeing oil, Compression and leak down test for starters

 

[GA_Boater]

Catch cans need to drained. What happens when the can is filled and the can vent starts spewing raw oil, which a larger environmental concern.

It sounds to me like you are going to cover up the problem instead of finding out the cause of oil on the water. I think your motor needs work.

 

[Bondo]

Ayuh,... I agree with the above,...

Either leave it be, or if the blow-by is so bad, rebuild the motor,...

 

[jimmbo]

At WOT throttle there is not a lot of Manifold Vacuum so almost all the blowby gases will be going thru the spark arrestor hose. If your engine has a PCV system, not all marine engine do, but Volvo does use them on a lot of engines, and you see lots of blowby gases at lower speeds such as below 2000 rpm coming out of the hose, then I would be concerned. The PCV system does help keep the engine much cleaner internally than other methods of venting the crankcase. The oil sheen can be caused by, leaky valve seals, worn rings, worn engine bearings, hydraulic fluid off the trim rams. a drop of oil can make a large sheen. Rich mixture can send unburnt fuel out the exhaust, and it too can make a sheen

 

[wrench 3]

As far as the environment goes, with a can you would still be venting hydrocarbons into the atmosphere. By sucking it back into the engine it gets re-burned and comes out the exhaust cleaner.

 

[loco]

Thanks for the replies. Sorry for the delay - I've been abroad for three weeks.

I think it's an absolutely miniscule amount of oil/fuel that comes out the exhaust after the engine is shut off. I don't think there's anything wrong with the engine as it only has ~120hours on it, but I'll give it the full test when I have the boat out the water in a few weeks.

As for the blow-by fumes, they do only seem to come out at higher RPM, though I'll check more closely next time I run the engine.
Looks like it's best to run them through the engine, then.
What about blanking off the vacuum feed that one of the vents go to, and welding a second attachment to the air cleaner? Is there benefit to that?

To be clear, I have one vent tube going to a vacuum attachment on the baseplate of the carb, and a second one going to an attachment on the air cleaner. I'm suggesting blocking up the vacuum attachment on the carb, and having both vents just go to the air cleaner. It seems to me, since blow-by only occurs at higher RPM, why not block whatever I can that would affect manifold vacuum, and hence carb performance?

 

[alldodge]

Opinion: Currently the vacuum line and the PVC valve is taking some of the combustion gasses produced and pulling them into the motor where they are burned. To remove the vacuum will cause all the gasses to be belched up on to the flame arrestor. You will see more oil on it and the outside of the motor then seeing now