2ZR Turbo Yaris

[tmontague]

Originally Posted by 06YarisRS

The open-to-atmosphere breather is there to let blow by gasses out of the crankcase when the PCV valve is closed under boost. It would also draw fresh air into the crankcase when under vacuum. At least this is my understanding of how it should work. All that said, I'm having fuel trim issues and not sure if the two are related. I'll monitor trims after I have the F/IC hooked up and the car running- hopefully. LOL.

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