depending on your mileage thats on your truck, and you say your oil is fine.. then you need to replace the cat. mine is a 1995 1500 with the 5.9 360. same set up at the cat to single 2 1/2'' cat back. mine had the same problem. wouldnt go past 3000 rpm. and heard a wind noise from the floor. the cat was clogged first i cut the pipe few inches back from the cat and slammed a crow bar in there few times blew the **** out and band clamped it back together. instantly went past 3000 rpm and had the 360ci power back.

few weeks later i replaced the cat with the y pipe from partstrain.com came out to be 200 bucks with shipping. any exhaust place wanted 400+ to get the cat and labor. just un did it from the manifolds and installed it myself. now with mine i only have a single o2 sensor thats forward in the cat. new one of those and then just put a straight exhaust pipe from the cat back to a turndown dump before the rear axle.

 

are u not runnin any rear o2 sensor? an im getting ready to cut my cat also an stick a muffler on it after the cat an dual it off of it or im thinkin of just cutting the cat off an just true dualing it

 

Nope mine only has a pre cat. O2 sensor. After that was when they started doing pre and post cat sensors.

 

It will also smell funny without a cat installed.. like a stinky smelly old truck. The cat really helps burn off the rich fuel smell as well as convert poison NOx gasses to something less toxic.

 

There's a right and wrong way to do things . If you want true duals , you need an Xpipe to replace the cat , and weld in a bung for the upstream 02 . The rear 02 is basically emissions , but , also with fine tuning . With it , you simply cheat it . I use a cheater very similar to this one except the ones I use have 3 different valves for the hole size on bottom .
http://www.ebay.com/itm/O2-Oxygen-ex...-/261686596307
Just run it on one side with the biggest valve , you should be fine

Done like this , No CEL's , no issues , and no harm to the great MPG's these engines can get . It's very true , your exhaust will not be so friendly to your lungs , nose or eyes . I wouldn't run it without rear exit

I guess I should mention regardless of your local emissions , it is a federal offense to remove a cat in the USA .

 

They work a treat.

 

I just got my O2 sensor today gettin ready to install it in the pipe of it don't help much then off with the cat an runnin true duals with no cats

 

17 years no cat true dual with 2 H's one for each 02. As of this summer two cats, no H's for 02's pre in one pipe post in the other. Having two cats has not changed the sound, mpg or power. It does smell better and idle better though. Just my .02c run cats.

This is what I went with, couple of Magnaflows.

 

What size pipe Merc?

 

It's all 2.25 supposed stainless, some is, some I beg to differ on. I used stainless band clamps as well. That was a test fit photo I gave the cats some room to move between them.

Edit to add: Stainless Magnaflow cats part #53005. And yes I have a muffler DI/DO Magnaflow stainless I think 18"