2ZR Turbo Yaris

[06YarisRS]

Originally Posted by tmontague

your breather shouldn't be the cause of any vacuum issues as it is not connected to the intake in any way. How you currently have it set up is smart as the PCV is normally closed under boost pressure and any blow by will not be able to vent - therein lies the solution of the valve cover breather.

The only way I could see this being a potential issue is during idle when the PCV in barely open and pulling mild vacuum on the crankcase. The breather technically allows fresh air to enter the valve cover and potentially out the PCV and into your intake. Since the PCV venting enters downstream of the MAF (IIRC) then it is unmetered air.

Easiest way to figure this out would be to block of your valve cover breather and see if the fuel trims correct themselves. If they do than a check valve on that breather tube to stop air from going back into the valve cover should solve that issue

You, sir, are a wonder! I was thinking about doing precisely what you mentioned with regard to blocking the breather hose and checking fuel trims. I hadn't thought about the check valve but may have eventually come to the conclusion to try the valve. So, you wouldn't anticipate any issues with, say, an extended period of idling and no way for fresh air to get back into the crankcase as it would stopped by the check valve. I guess I'm not sure I fully understand this system as the original valve cover vent was connected to the intake pipe before the MAF sensor, which, I assume, would also have vacuum on it. How would fresh air ever enter the engine if both the valve cover vent and PCV are under vacuum? Help me understand. LOL!

I am going to check my vacuum again this evening, but even with the open valve cover breather, I think I was getting around 25"/hg at idle. I'll monitor vacuum with the valve cover breather blocked off and not blocked off. BTW, I did test my oil catch can and it's sealed. When I started opening the ball valve, I could hear strong suction and the engine started to stumble. Close the check valve and idle went back to being smooth.