birdogz

Here is my problem: 1994 F-150, 5.0L, auto trans. The engine runs smooth then runs at reduced power. When it runs at reduced power, you can clear it out by depressing the gas pedal and making it shift down or you can wait and it will pick up and run again at full power again after 30 seconds without changing the current throttle setting, when you don't shift down to passing gear, it will stumble and backfire.
The check engine light flashes and I retrieve code 172- HEGO lean. With the cruise control set, it will hold 65 mph on the turnpike, it shifts down a good bit of the time and that clears it. It cycles constantly, running good for a minute or two then running at reduced power fo 30 seconds or so.
I have done a lot of things to correct the problem:
Fuel pressure regulator, PCV valve, mass air flow sensor, TPS because it had bare wires adjacent to one another, and O2 sensor. The old sensor and harness was wet with crankcase oil, I replaced the pan gasket to remedy that. The plugs, wires, cap and rotor look good.
The IAC looks newer and the idle changes when the electrical connector is removed. When the IAC is connecte, I hear a "moaning" sound in the airbox. Air filter is new.
I cleaned the throttle body, cleaned the EGR port and the crossover port between the two plenum tubes. The previous owner replaced the EGR valve but I don't think he cleaned the port.
I inspected the vacuum lines and sprayed carb cleaner to check for vac leaks especially in the area of the plenum gasket.
Vacuum is steady and responsive when measured with the gauge.
When I replaced the O2 sensor, it ran good for about 10 miles (20 minutes) then started acting up again.

One question I have is regarding the O2 sensor- what brand does Ford use from the factory? I used a Bosch but on our 2004 Dodge Caravan, the light won't reset with a Bosch sensor but it will with a Densa brand sensor which is what Dodge uses from the factory.

I don't know if it an issue with the O2 sensor or something else causing a problem and the O2 sensor is reporting it accurately. I am at a loss at this point- should I just replace the plenum gasket and hope for the best??? I would appreciate and input at this point.

I was a mechanic for over 20 years on Marine inboard engines and race motors, I am thorough on troubleshooting but in my line, we did not have all of the sensors ect.

Thanks for reading this saga...