The plastic intake manifold used on these engines is not a one piece sealed design. It has an open center, which is sealed by a plastic plug on either end. The rear plug is closest to the throttle body and susceptable to being blown off in the event of a backfire or explosion event inside the intake manifold. Older cast aluminum or cast-iron manifolds did not have this problem, being one-piece castings with bolted or threaded fittings and plugs. The plastic plug ends of this manifold design are afixed with adhesive only, and are the weakest point in the design. Some later manifolds were revised to not have this plug, indicating that engineers knew this was a problem.
Backfiring in the intake manifold is not a normal operating condition, and since the 4.7L engine is a multi-injector design, not something that is easily produced without adding something through the throttle body. Ordinarily the manifold will have nothing but air passing through it. Using additives like SeaFoam or fuel injector cleaner directly through the throttle body should be greatly discouraged, as this adds explosive material inside the manifold itself and can easily lead to a backfire and blowing the rear off the intake manifold.
If the rear plug has failed, it can be difficult to see visually as the rear of the engine is close to the firewall. It will be readily evident though as the engine will accelerate out of control until it hits the speed limiter and/or fuel/electronic cutoff. This will happen every time the engine is run as too much air is rushing into the rear of the manifold and fuel is injected quickly to keep up with it. Ordinarily this event will throw no codes as it is similar to the throttle being held wide open.
If stranded, a temporary fix to drive to a repair location can be accomplished by sealing the rear of the manifold with tape. But for a more permanent fix, the plug must be replaced.
There is a hose that joins this plug from the side, and sometimes this hose will keep the plug from flying away and being lost. If this is the case, remove the plug and inspect it for cracks or breaks. If intact, it may be reused. Otherwise a new plug should be obtained to prevent replacing the entire intake manifold.
We recommend a combination of RTV and glue. A thin bead of RTV around the raised bead of the plug for an airtight seal, and a bead of superglue or plastic-bonding adhesive around the outside of the plug. Carefully place the plug into position and apply pressure to seal, then hold or tape while the adhesive dries. Any small air leak will create an increase in idle speed later. Once the adhesive has dried completely, replace the hose and any parts removed for access and the vehicle may be operated normally.
