StevieB

On a stock 98 exhaust as they age and more miles are on them, do they start to deteriorate in a way that they will become more or less or maintain their original restriction?

Thanks guys

Bill Curlee

IF,,,,,,,,,,,,,,,,,,,,, they don't over heat and the engine is not running really poorly, they usually just become less effective and then stop being a catalytic converter and more like a filter. Over time they can clog.

IF the engine runs excessively RICH, has a misfiring cylinder/s for a long period of time they can overheat and melt. Sometimes they clog and others they melt, break in to small pieces and the matrix ends up in the muffler

Vetteman Jack

Over time, they do tend to degrade in performance and eventually may require replacement.

Blow Torch

X2
I have seen Cats (for whatever reason) become so clogged that the cars would barely run or move when you try to accelerate.

Blue LS1-LT1

Having this exact problem now on my '99. Cats clogged. Headers, cold air intake, mild tune. Random Tech hi flo cats with ~ 40,000 miles clogged, destroyed. car would not rev past ~4000 rpm. sounded tired.
What causes this?

wrkdWS6

Same here, had it happen to me once. I had a 5 year old 50,000 mile completely stock Pontiac Grand Prix that developed a clogged cat and the thing would barely accelerate. I have no idea what caused it but you will definitely know if/when those suckers get clogged

Time to stick something in that mid-pipe and hollow those things out, assuming you don't have to pass the sniffer test.