this has always been a problem with the 4.6 Ford had a recall that went back 7 years for Crown Vic Police cars in service or not but now they are only warranting in service cars. go to
www.crownvic.net and serch for plastic intake and see all the talk about it , the CV has been using the 4.6 since 92 the town car has had it since 91 they have had plastic intakes since 95 IIRC.