I bought an aftermarket cat conv and it seems to be worse than the old (stock one). New/aftermarket is less than $125. New Toyota is above $700. Obviously the aftermarket one is a piece of junk. People have suggested running seafoam through the old cat to "renew" it. Is this the normal engine cleaner Seafoam (they other Seafoam for auto/marine and automatic trans). Which one do I buy? And by "running it through" the cat, do they mean taking it off and spraying this stuff through it (upright and in a bucket I would assume)?
Help, please.

Screw all that what usually kills them is as they plug up they start getting very hot and end up degrading inside. Just buy a high flow CAT, it'll end up costing less then a new Toytoa one.

When you say the new one is worse, what do you mean? Your MPGs are lower? Worse smell?
What I changed my cat over to the CatDaddy universal one, I had no problems at all - if anything, it was better - but thsi was also in conjuction w/ new muffler and 2.25" pipe.

Personally I do not see how using seafoam engine treatment would do much "good" for a cat, but I'm probably missing something.

A lot of carbon on the tailpipe. And the exhaust sounds raspier than the stock sound (this only happened after I changed the cat and muffler). It almost sounds like an exhaust leak except the sound is coming from the tailpipe, not either of the cat conv/muffler connections. Used all new gaskets. Broke the "nut-bridge between the cat and the exhaust manifold. Using nuts/bolts right now. Did buy the replacement and plan on putting it in this weekend (I don't feel this is that big a deal).
The old cat seems fine. All the material is inside but I can't see "clearly" through to the other side but, I dod see light. The new cat looks small compared to stock cat conv.

Seems like typical problems with cats are that they clog up. The exhaust prob is much less restricted now, so it should sound different, and since the cat is physical different in size, perhaps there is some resonant frequency created that makes it raspier than it was before. If you don't like the sound, you could change your muffler to a quieter one. As for the seafoam, I don't quite understand how it would clean the cat. Usually that stuff is either poured into your carb or an intake vacuum hose or poured in the gas tank. I believe I have read that some people may have gotten a clogged cat AFTER using seafoam b/c of all the gunk that it dislodged, burned, then passed into the exhaust. I haven't yet seen where it actually cleaned a cat.

Sean,

Run the seafoam through it, using 2 bottles (1 bottle exclusively for your intake cause the manifold seems to collect a lot of sludge) and let us know how its working after a complete tank of gas. Basicly your gonna do the seafoam in the intake 4 times over the course of the tank of gas, do it like every morning and then drive to work and back till the tank is gone.

You could just have a lot of crap bult up from the years of use. If that doesn't help then start looking deeper. Don't install a cheep restrictive CAT just take it back to the store and get a high flow one. Don't try to reuse the old one either cause you'll never get it back, you have to understand the extreme temps that build up in there when they clog, if you look at one it will be glowing red hot. While your at it get a cat-back system put on if you like with a 2.25" - 2.5" progressive set-up.

you might want to check that something that ruined your cat is still going to ruin the next one. If you have a cylinder not firing, your dumping unburned gas into you cat, for example.

I'm pretty sure all cylinders are firing but, that's easy enough to check.
The original Cat didn't fail and I still have it. I just replaced the cat and muffler because it was so old and I was trying to increase my milage.

Your cat wasnt bad and you replaced it? If you can see through it's little holes its gonna work fine as a new one.
I had one go bad and it was all melted inside and big chunkes fell out, that caused problems. Replacing a good factory one is probably just gona make things worse, though. two cents worth, maybe less.