.....anything on a 5.4? I have one laying around, so if it works might aswell use it. Something like this.

 

Using that cap will allow unrestricted airflow through the crankcase and into the intake. Odds are you'll see an increase in oil consumption via the PCV due to the extra air drawn through the filter.

If you disconnect the PCV hose, the filter will serve to catch any outgoing oil mist... But again, you're not doing anyone any favors.

And, in some states that require emissions tests, that could qualify as tampering with the emissions system.... Not an argument I want to have to challenge.

Sell it on Ebay. It'd be more useful as cash in a PayPal account to be used for something that helps.

-Joe

 

It's a closed system, so you can't use this as a matter of course.
The PCM will detect it and it may upset idle and fuel tables a bit along with a code and a CEL.
Just like the complex fuel tank monitoring/testing that is done in it's closed system, all to keep fumes from leaving the system. It even knows when the cap is loose or leaking (not holdng pressure or tank vacuum).