I have been going thru my 1.8L engine in a attempt to rid all potential vacuum leaks. The L-jet is very sensitive to the slightest vacuum leak. Why is there a hole in my oil cap?

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Don't know, but I'd plug it with something like a small screw maybe.

1.8 usually needed to replace the O ring under the cap.

I already installed the secret seal kit for the oil cap, 2 new o-rings. The tiny hole looks original, almost like the factory intended it to be there.

Mine also has the little hole

It is a small vent to allow water vapor or even steam to vent so that the oil filler does not get that "white mung" that many 911 oil filler necks get due to moisture build up. Even though the PCV valve is there at the filter box there are times when no flow is taking place.

I'll admit this, I had the oil fill assy off, and I put my thumb over the oil hose port, and put my mouth over the bottom, to create a vacuum. I was surprised of how much vacuum leak I had while sucking on the filler neck! That tiny hole may look unsignificant, but being I have a Ljet system, it worry's me.