I've got a 97 F-150 4x4 with a 4.6, 4R70W auto (no mods -All Stock) and about 175000 miles.

I've got no power under load and/or when I'm going up a hill at freeway speeds (55+). Here are the detailed symptoms: Driving along at 55-65 mph (engine at ~2k RPM in OD), as I start to drive uphill I start losing speed if I keep my foot stable so I kick it down, the TQ locks up (along with the RPM's to ~2600) and I can barely maintain speed. So I throttle up and it drops out of overdrive (engine at ~3000 RPM) and I can still barely maintain speed of 55-60 (with no prayer of actually accelerating). No strange misses, no rough idle.

*No Codes ever, since I've owned it (and I've tested the ODBii sys to verify that the CEL isn't dead)

Local Ford Dealer put it through ~2 hours of diagnostics and found nothing out of spec, i.e. no failures, no misfires, nada. They pulled the O2 sensors and found ~10 lb of backpressure on the passenger side Cats (driver side had 3 lb). They said, "that's it".

Replaced the pass. side cats (both) at a muffler shop. It seemed better at first (not like was originally), but now it has degraded to the previous no power condition. Still no codes and still no strange misses, no rough idle.

Before anyone says what a gutless block the 4.6 is, it was definitely not always this way... I understand that it's not a new car, but it's still got a lot of life left

So far I've done the following:

New (Motorcraft) Plugs (dealer)
New (Motorcraft) Plug wires
Ford Fuel Injector Cleaning (dealer)
New Fuel Filter
Tranny Service +TQ drain +Lucas treatment (me)
New TPS
Cleaned MAF
Cleaned TB and EGR Port
Cleaned IAC
New Air filter

Any ideas? (torque converter shot? other cat now plugged?)

 

Welcome to the forum

Have you checked the backpressure again? Perhaps maybe some loose converter stuff is lodged in the muffler.

 

Checked it again yesterday by pulling the O2 sensors one at a time. At 2500 RPM (Engine Warm) -No back pressure...

 

Mine was doing the same thing (99 F-150 4x4, 4.6l). I changed the pressure sensor on the EGR and it cleared most of the problem up. The check engine light wasn't throwing any codes, but I had noticed that the old pot metal housing on the sensor had allowed the ports to nearly corrode shut. Sure hope you get your issue figgered out.

 

Finally made some improvements!

After my last post I replaced the O2 sensors (all 4) then reset the PCM by doing the battery disconnect. I had read that (pretty much) every manufacturer recommended replacement at ~100,000 miles, so I figured that mine were on borrowed time at 178,000... I had to take it up through the Sierra's from Sacramento, CA to Reno, NV
yesterday and it actually had power in OD to maintain speed, and even accelerate after the TQ locked up. It actually had pretty darn good acceleration once I kicked it out of OD.

It still isn't performing like a new truck so I will definitely have a look at the pressure sensor (the DPFE right?). Thanks for the input.