Ive had this prob and did everything you just asked about doing. By pluging both ends of where that hose was and placing a filter cap on the valve cover you have just created a massive vaccume leak. My check engine light was always on but never knew why. Well heres why normally the pcv system sucks air from the intake pipe then through the crankcase and then through the pcv valve and into your upper intake plenium. This is fine because the air was sucked from the intake pipe after it was measured by the MAF sensor. Placing the filter cap instead will cause air UNMETERED by the MAF to enter your engine causeing a lean condition. there are a few good and very long threads on this subject. I think if you search for "PCV" or something like that youll see them.
SOrry so long but its been a big prob for a long time that i THINK ive just fixed
EP
