Suction at boxster oil filler?

[Dave Diamond]

How much suction is normal with the oil filler cap off in a boxster? After a check-engine light and codes P1128 and 1130 (usually a vacuum leak) I found the ribbed oil-filler tube was cracked and sucking a lot of air, creating a rough idle when cold and throwing the codes (at least I assume so). This tube was a PITA to replace, but I did so and now it idles fine. Not sure I've driven it enough to see if the codes are going to return, but I expect they won't. However, there is a fair amount of suction if I remove the oil cap while engine is running, and the idle immediately gets rough if engine is cold. I'm not sure if it was like this before or not. I recently replaced the AOS, too, but I am still getting occasional smoke on startup in the morning.

Any ideas on this?

Thanks.