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Ive got an 01 S.E. It was owned by an elderly lady before I got it. After I changed my UIM and LIM gaskets, I got on the car pretty good to get the car to running temp while bleeding the cooling system. I noticed the "rotten egg smell." I have ALWAYS related this to a bad catalytic converter.
Now I can get the smell to start by just getting on the car a little bit, like quickly accelerating.

Well, I took my car to my muffler guy. He asked if I had any check engine lights on. I told him no. He said then it prolly wasnt a bad cat converter. He said ethanol causes that smell. He said to run the gas in the tank low. Put about 30 bucks in non-ethanol gas in the tank, add some octane booster and drive it hard for the remainder of the tank. He said that should fix it. Then he said once you've blown it out then you can go back to regular gas if you want to, but the problem would likely return a little while down the road maybe months maybe a year. He said if it didnt fix the problem then come back and he would throw it on the rack and have a look at it.

This didnt make since to me. The whole ethanol thing but, I did have a bad cat converter on my 90 K1500 sitting outside and it never made a smell. So what do you guys think about all this?

Sounds like a bad cat

also, if i have to replace a cat are there any downsides to buying a universal fit cat. conv. opposed to buying a direct fit?
my local parts store has 2 different Magnaflow direct fits one for 169.99 and one for 398.99 and it says OE GRADE OBD II

Then the universal one is 87.99 and the other universal is 173.99 OE Grade OBDII

I've heard similar things about catalitic converters and have used a similar way to clean one out. I used 93 octane fuel @ quarter tank. Then poured two Lucas fuel system and injector cleaners in then just beat the piss out of it until tank is empty.(Idea is to get it as hot as you can and blow the crap out of it) I had a insufficient catalitic code in my camaro and this cleared the code. I used it for a year and no codes ever came back up. I have since changed the complete exhaust system so I cant say how long it would have lasted past that. I have had a couple converters go bad and never had one smell though. If you don't have any codes I really wouldn't worry about it much.

If you are just getting the smell, without any other symptoms, I wouldn't think that you have a problem with the cat. It could be as simple as the brand of gasoline you use. The higher the sulfur content in the gasoline, the more likely you are to have that smell. You could try switching gasoline brands.

Another possibility is an overly rich fuel mixture. A dirty MAF, or old O2 sensor could cause this problem.

walnut777 wrote:also, if i have to replace a cat are there any downsides to buying a universal fit cat. conv. opposed to buying a direct fit?
my local parts store has 2 different Magnaflow direct fits one for 169.99 and one for 398.99 and it says OE GRADE OBD II

Then the universal one is 87.99 and the other universal is 173.99 OE Grade OBDII

I bought a universal hi flow magnaflow cat from eBay around $60

If I go for a week or two without beating on Penny and I finally get on her say like 25 to 80 when I'm getting on the highway, next time I get out of the car she smells pretty bad. Same goes for my parents' GX.