Carbon deposits left over from burning the ****ty gas they pump out these days.. summer and winter blends.. so on.. so forth.. I have a slightly clogged cat and it's not the plenum.. it got worse when I de-carbonized the chambers of its caked on carbon on top of the piston's...

Clogsthe Cat or its pre-screen right up.. in a hurry too..

 

My apologies for reviving this old thread, but I see that I never truly positively closed it out.

The cause of the problem was indeed the clogged catalytic converter, and a new converter got the truck running great again.

We had that 1996 Ram another 5 years after this and sold it in 2016 (it still only had <60k miles on it at that time). I've been running a 2005 Ram2500 Cummins for our truck needs since 2015.

 

That's cool, nicely done.
Most folks:
Post to multiple websites, looking for someone to 'tell them what to do'
Ignore the ideas/suggestions that require work, thought or learning
Immediately start replacing parts (because that's what forum parrots tell them to do) without any serious diagnosis (see above)
Never return with results, appreciation, or further help of their own