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Message: <blockquote data-quote="96blak54" data-source="post: 1586015" data-attributes="member: 14727">I just wrote this up on a different post and it fits here tooWe modular nuts are discovering the original plastic intake manifolds degrade with age, including the original intake manifold gaskets which in turn will allow a hidden vacuum leak. With this said if you have updated to a better original Ford intake manifold and new intake gaskets, a certain intake manifold gasket is required and not just some run of the mill 4.6l manifold gaskets. The required 4.6l intake manifold gasket for an original Ford manifold(npi or PI) made by Montaplast requires the thicker intake gaskets made by Fel-pro or Ford(same). These specific gaskets are roughly .040" thicker to compensate for manufacturing warpage of the original 1 piece plastic manifolds. In other words, if you ever have replace intake manifold gaskets with ebay, amazon, or run of the mill 4.6l intake gaskets, you most likely have a leak. How do I know this you may ask?......I worked for the intake manufacture Montaplast and this is well known within the factory. So long story short, hidden vacuum leaks with the original Ford manifolds and original gaskets can have you chasing problems due to age and/or overlooked incorrect replacement parts. Do you know the history of your engine? Has there ever been any manifold work done?Even the new multi shell replacement manifolds we find are vacuum leaking as well and this may not be the manifold them self, but might be surface preparation. The new manifolds have ring seals around each port. A good cleaned surface is needed to ensure a good seal. Also the npi heads have tiny egr type ports/dimples next to a few intake ports that should be associated with that ports ring seal. Some replacement manifold might not cover these egr ports well</blockquote>