I'm running Merc 500EFI intake systems on my 540 stroker engines. The PCV valve is installed between the plenum and the intake manifold and I installed filtered breathers on the valve covers. I notice on the Merc parts diagram that they plumb the valve covers into 90 deg elbows and hoses that run ( I think) to the back corner of the plenum where I have my non stock fuel pressure regulator vacuum hose plumbed. If the valve cover breather holes are plumbed to a vacuum source on the plenum how can they draw air into the crankcase? If this is how they're set up, is there any downside to my just using breathers on the valve cover?

The valve cover breather tubes "T" together and go to the flame arrester.

No flame arrester, EFI

Ok call it what you want, air filter, breather, throttle body screen

Not trying to be argumentative but I'm pretty sure there's no hose nipple anywhere near the throttle body. The only one I recall seeing is at the rear right corner of the air plenum where I plumed in my fuel regulator vacuum hose as I mentioned.
Thanks-

There isn't any nipple, it just aims at it. There is a stick on wire tie holder that is stuck to the thermostat housing, and the tube is tied to it and just sits with the open end against the flame arrester

^^^^^^ that's how mine are.

The two vale cover breather hoses T together and go the the FLAME ARRESTER, yes it is a flame arrester, you can still backfire with EFI. their is also intake vacuum line going to fule reg, and a pvc hose going to the bottom of the plenum. But to answer you question.. you could run valve cover breathers, and it would be better for the motor to not try and re-burn crank case fumes. your engine compartment might get a little dirty, but oh well

i do agree with what you say but is it cost guard approved to run that way? Not that I would give a good rats arse what they think, but could this be a issue while being boarded ?

Nope.. the 850 1075 motors don't re-burn crankcase fumes