Alright I've got some serious acceleration issues, which happened about a month ago, and was resolved with a simple change of the air filter. But it started again yesterday and came with a check engine light. Took it to autozone and I'm getting P0420 and P0430. I've read through a few different forums and all pretty much say the same thing, replace your cats. From my understanding cats don't just go bad, something causes them to go bad. The printout i got lists 3 possibilities as to why the cats are failing.
Engine misfire
large vacuum leak
engine oil leakage into exhaust-valve guide seals, piston rings

I'm going to start looking through for cracks in the cats, weldpoints etc. I just don't know where to start to check for the oil leakage into the exhaust. If anyone can point me in that direction I'd appreciate it.

 

Cat efficiency codes can also be caused by running rich or exhaust leaks. Does the engine use oil? How many miles per quart?

 

I do go through oil, id say i add a quart every other week. Doing ~400 miles per week just to and from work. I'm not hearing a rattle in the exhaust like some of the videos posted. An exhaust leak would give me the bad performance, and explain the truck flat out telling me it'll accelerate at its own pace, right? I'm having to shift around 2-2.5k rpm for it to respond.

 

If it's going through that much oil and it's not obviously leaking, I think you may have found the cause. To fix that kind of oil burning, you are looking at an engine rebuild.

Which engine and how many miles?

If you don't have emissions testing, I'd drop the pipes off the cats, ream them out, and put it back together. They are probably starting to clog and the backpressure isn't helping.

 

The only thing that would cause the cats to go bad without setting a code is extreme oil use (burning). Vacuum leak or misfire you would get a check engine light and a code. Have you ever replaced the PCV? they can get stuck open and cause oil use. Generally not as much as your using though. I'd try replacing it anyway. Actually I would have replaced it right after buying the truck and every 75-100k miles after that but thats me..

 

The codes are for both banks.
It's unusual to get them for both bank at the same time.
Now, with acceleration issue, the only thing that causes that is the cats won't pass exhaust in the volumes the motor produces.
You will have to replace the cats no matter what the cause of the failure is and repair the cause as well "first".
The codes are the results of the rear Ox sensors detecting a lack of oxygen storage in the cats.
How this is done is the PCM monitors the switching ratio between the fronts and the rear sensors.
When the switch ratio exceeds a limit set in program the code is set.
This is how the PCM knows the cats are at end of life 'besides' being restricted.
.
To further prove the cats are restricted, a back pressure test can be done.
Drill a hole in the pipe just ahead of the front cat and thread fitting in that can be connected by metal pipe then rubber hose to a very low pressure gage such as a vacuum gage that will read a few lbs pressure. (Needle does past zero) in positive direction.
Start engine and rev to 2500. If you see more than about 2 psi at that low rpm the cats are restricted/melted.
Good luck.

 

4.2L @183k miles
full rebuild...

 

183k on a 97 4.2 and the front cover and lower intake gaskets aren't leaking coolant into the cylinders yet? You are lucky.

 

I've got a 97, v8, having the same issues. Odom stuck at 190+ during previous owner's possession. Fresh high mileage oil change every 4 months, always comes out clean, not burnt or too unusually dirty. Not showing any codes, but it is kinda loud from a real minor exhaust leak right before one of the cats and i think one of those are going too ( random spontaneous rough idle, random slow acceleration). would just cutting a hole open and cleaning them out solve this, or is there something else I'm being oblivious to? Although, it *was* worse last week after i hit a deep puddle during a major storm. Seemed to be slightly better after it dried and made a sound like someone slapped a steel pipe with a hammer.