I have a 2001 Expedition Eddie Bauer 4x4 with the 5.4L engine. The vehicle has 103K miles on it.
The problem is the fuel pressure drops off under load. Pressure starts at about 32 psi and drops to 18 psi under heavy load/high RPM (4000 and up). This causes low power and ping/detonation noise. Do get a P0300 code, random cylinder misfire.
A mechanic has looked at it and replaced the fuel filter with no change in behavior, he then changed the fuel pump. This did not fix the problem either. ***Correction the fuel pump change did fix the fuel pressure drop under load.
If you disconnect the fuel regulator the fuel pressure goes up to about 40 psi. However, when you drive the vehicle the same thing happens, pressure drops off under load and high rpm.
The same mechanic changed the intake manifold due to the COP's bolts being stripped out and there for was unable to remove them to change the plugs. A Ford OE intake was not use. An OEM manifold was used and came from these guys, http://www.stevensparts.com/EXHAUST%...DS%20INDEX.htm
I did check with two other people who used this OEM manifold from Steven's Parts before installing it on my vehicle. Both people said it worked just fine. Car runs well.
Not sure what to look at next.
Any help on the cause of this problem would be great.