Recommended Procedure:

WARNING: This page does not describe the selected car, but rather 9 other vehicles, including the 2004 Ford RV Cutaway, 2004 Ford Pickup, 2004 Ford F550 Super Duty, 2004 Ford F450 Super Duty, and 2004 Ford Excursion. However, it is still accessible from the selected car via links, so may be relevant.

Connect the scan tool. Access and monitor the EP and RPM PIDS.

Run engine at 3,800 RPM.
Record the EP PID value.
A low RPM or an EP PID greater than 234 kPa (34 psi) (F-Super Duty/Excursion - Early Build) or 244 kPa (35 psi) (E -Series or F-Super Duty/Excursion - Late Build) indicates a restricted exhaust condition.