Vacuum leak or poor ventilation

1988 XJS V12.

I’ve been chasing what I think is a vacuum leak. I also noticed oil in left air filter housing. So I took apart pcv system and noticed this big hole/tear in rubber cap that goes on pcv oil separator.

Would this create a vacuum leak AND/OR not put a vacuum on crankcase to help keep pressure buildup?

May have answered my own question. I fixed the rubber cap with the magic of duct tape until I can get a new one.

My warm idle went down 300Rpm (900 to 600) Had to back out idle screw quite a ways to bring it back up to 900.

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I am not smart enough to understand how fixing that rubber cap resulted in the need to adjust idle that much.


Agree with Paul; that torn boot shouldn’t affect idle.

I assumed at idle, the high vacuum is opening the PCV valve, which creates a vacuum in the crankcase.
But if the rubber boot is leaking, then it is sucking in outside air instead of crankcase fumes?

Oh, what may have happened, two weeks ago I pulled out my newish Fram pcv and put old one back in.
Together with patching rubber boot this morning, I also put back my newish Fram PCV this morning. Could that have changed the idle?

Yep, that’s what did it. Supposedly the Fram PCV valves function totally differently and not as effectively at preventing crankcase leaks. Advise you clean up the OEM PCV and reinstall.

But the OEM PCV was the one that idled too high, and leaked more oil. The Fram seemed to behave much better.

I guess I could soak the OEM PCV in gasoline overnight and try again? It does seem to stick a bit, a lot of black inside of it.

It needs to rattle freely.