No Acceleration Max 3,000 RPM's under load?
 

2001 Ram 5.9 Auto with 125,000

As title states truck accelerates fine under light throttle driving conditions. But if floored I get about max 3,000 rpm and just seems to fall on its face. So accelerating on an on ramp is useless. And once up to say 60 if I stand on it trans kicks down fine but rpm's stick about 3000 again and if I stay in it will stark to spark knock like no tomorrow.

Only codes I ever get is an off and on P0138 downstream O2.

Sounds like a plenum/clogged cat issue. Have you checked out the stickied plenum thread at the top of the list? Quick test would be to remove the pre-cat O2 sensor from the pipe and drive it like that to see what it does.

+1 ^ clogged cat

I was leaning towards clogged cat. Just wanted to see what others opinions were. As for plenum. Plan on doing that in spring as its way to cold right now.

Too Cold? Oh Come on. It might have made it to double digits today. :)

Right beats the - double digits its been when leaving for work last few days.

If you can find a reasonable place to work on it for a bit, yank the front O2 sensor, and take it for a drive. If it runs better, cat is more than likely the current problem.

Have you fixed your plenum yet?

I will definitely pull the font O2. Crossing fingers should not take to long. Haven't fixed plenum. Replaced ICV and pulled TB in process looked around plenum and didnt look super bad but am planning on doing it regardless. Girlfriends dad has shop in Romeo I can use so am thinking maybe sooner than spring. But just gotta get cash together so I can do water pump and timing etc while I there.

+ another one on the cat. Had this happen on a 2000 1500 with 5.2. Installed a magnaflow high flow cat and it sure woke up the engine.

Does anyone know off hand what the pipe size is when I remove cat? Wondering inlet and out let size?