Manual says every 100 hours, or once a year. Honestly, I think im still on the original ones from 2007. Engines run great (twin 525efis, 189 hours). Question, you guys changing yours often?

The manual is Mercury ? they sell parts, enough said.........if it still rattles, blows and don't suck, its fine...........

What's a PCV valve? :-)

Lol^^
Mostly the blue motors have them.

Ya they are used burn moisture off from the oil and blow by. No reason to have one in a boat motor....

Lol on the replies. Yep, Mercury Racing wanting all these things changed. 190 hrs on the motors today and running like a top. Guess I won't be too concerned with them then. Thx guys...

Positive crankcase ventilation. Its a one way check valve , It directs oil vapors generated by blowby from the valve covers into the aircleaner to be burned rather than vented to the air in the engine compartment.

I got rid of them and puny breather system on my tired 502 MPI's and went from burning a quart in 30 minutes of hard running to burning a quart maybe all season.