The rear plugs are replaced from underneath and are quite easy to get to. When you removed the plenum, did you install a new gasket? Saw one once that developed a 1/2" long gap right in the front where the gasket cracked. It had never been taken apart, but that gap was big enough to cause the engine to idle at over 3000 rpm.
If the gasket is new, look for cracked or disconnected vacuum hoses, but they won't usually cause the rpm to go that high.
Friday, June 19th, 2009 AT 10:40 PM