I just found out just lately that this a common occurrence in the 5.4 Litre motor from our local mechanic in town ,he will not fix them ,he said take to Ford .Ford put in low grade manifold studs causing them to burn off warping the exhaust manifold at same time,then you replace the manifold studs with stainless studs which should have right from the beginning of the 5.4Litre ,also replace exhaust manifold .My feeling is Ford should come good for this because they cheaped out on their product.I have had Ford several years I am changing to something else if they don't come through.