The plug blew off the nipple on my 16v, the replacement was somehing absurd like 10 dollars, so a bit of heat shrink and a nail is sufficing. Just to make sure we are talking about the same thing though......
If the 'nipple' is uncovered, start the engine, pop the hood, and cover it with your hand. If that improves the idle situation, i'd say it needs a cover. If it doesn't improve anything, try the test at different engine speeds and tempretures.
When I tested mine, it evened out a rough idle.