Astonishing ... and in part is Emblematic of what can happen if P0300 or an accumulation of Engine Mis-Fires happens or the use of very Low Grade Fuel occurs again and again over time. One other artifact present and worth noticing is the result of having Bad Gel-Filled Bulb Style Motor Mounts.
The Engine can shake violently enough to release Flakes of Rust breaking off of the inner Cast Iron Exhaust Manifold segments and collecting as a durable blockage to the passing exhaust stream. In these images... you can see the external artifacts of this... but the same problem happens internally as well, remembering that a substantial portion of the by-products of combustion... is Water:
You may well ask,
"But... Bad Motor Mounts...?" Yes... If you look closely at the attached images below... Observe where the Cast Iron Metal is freshly exposed...Gray and Un-Rusted at the Junction where the Small Down Pipes meet at the collection point. You can see that the majority of the Rust Flaking happens in concentration.
This is because the Engine is being allowed too great a realm of motion as the Engine RPM Rises and Falls and the Engine Block moves Left and Right... from Engine Torque causing the motion of the Exhaust Manifold to fight against the Exhaust Flange and pipe leading down to the Catalytic Converter.
This constant bending motion along with changes in Temperature encourages the Rust Flakes to dislodge and move head long from Exhaust Gas Pressure up against the face of the Ceramic Honeycomb in the opening of the CAT S/S Can. What you can see happening on the Outside also happens ...On the Inside:
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There
IS a way to Test this Theory... Rust as FE2O3 is *Mildly Magnetic"... so if you can pick up pieces of that dislodged detritus scraped off the facing of the Honeycomb with a Neodymium Magnet...
All will be Revealed.
Hmmm... I wonder if this could also be accomplished by going through the Upstream O2 Port using THIS to first Inspect for the issue...and then Cleaning Off all of that Rusty Crap...Using Compressed Air through a Narrow Plastic Pipe ... Perhaps?
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One Last Suggestion...
You might request a Title Change from the present one that is a bit vague and perhaps confusing...to something like,
[SOLVED] A Clogged CAT Can Caused My Power Loss