My '94 Mustang GT 5.0L 5 speed has no power. Well, maybe 100 horsepower. It shows flashes of normal horsepower, but only briefly, after a hot soak in hot (100-degree-plus) weather. After a minute or so, it returns to its "normal," which is an extreme lack of power and unwillingness to rev over 3800 RPM.
Here is the deal: 160,000 miles on the original engine. Good compression, clean looking (tan) plugs, no excessive oil consumption, normal (mid teens) city gas mileage. Engine is dead stock. No internal or bolt on mods. TPS voltage appears normal, as does fuel pressure (39 psi).
New Parts: Fuel filter, fuel pump and screen, High-flow catalytic H-pipe, O2 sensors, EGR valve, computer, plug wires, cap, rotor, 195-degree thermostat.
Codes: Consistent "EGR lean" codes (171 and 176), no other codes.
Any body with some wisdom on this? We are baffled.