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Which engine do you have? If it is a 302 there is good chance that your intake gasket that goes between the upper and lower pieces of the intake is blown out. I had the same problem on several different trucks with the 302, seems they have somewhat of problem with this. It is not very hard to change, but is a little time consuming. If this turns out to be the problem,and you replace it , make sure you use a good aftermarket gasket like Felpro as these seem to hold up better.
This Hennessey Takes the Expedition Tremor's Off-Roading Capability to the Next Level
Slideshow: The VelociRaptor Expedition gains a lift, upgraded suspension, Brembo brakes, and trail-ready equipment while retaining the stock 440-horsepower EcoBoost V6.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.