Here is a awesome test to help us pinpoint this problem. It will tell us which way we should go in this diagnoses. Lean, Rich or a mechanical. Its called the flat raters test drive.
Get the engine to act up under wide open throttle and look at some of the parameters on your scan tool and compare them to the readings I have below.
Here is the test. ( have to be off line for an hour or two. I will check in with you later tonight.)
The flat raters test drive..........
Under safe road conditions, a wide open throttle acceleration is performed. Try to maintain WOT for at least 5 seconds. 10 to 20 seconds would be better.
What will the pre cats do?
Will indicate rich
Should the barometric pid change?
Should remain stable! Very important!
What will air flow do?
Max out. Volumetric efficiency. Engine load 100%
(Engine load Parameter on scan tool should go close to 100%)
Poor acceleration, rich exhaust, and good Barometric pressure = restricted exhaust or late ignition timing.
Poor acceleration, rich exhaust, low or decreasing Barometric pressure = intake path restricted
Poor acceleration lean exhaust and good barometric pressure = fuel delivery concern.
Poor acceleration, lean exhaust and low barometric pressure = air flow measurement error. (Ford has this problem with Mass air flow sensors)
Note: Some vehicles Barometric pressure is not updated until Wide open throttle.