Second cat melted need help diagnosing

[Doug]

I have a 98 bird with no mods and I need your opinions and insight on a problem I'm experiencing...

About a year ago my cat melted and I had it replaced. The mechanic didn't really diagnose what caused it and said, "You must have drove the car really hard". I knew this was crap because I rarely push my car at all.

After some investigation I found the car must be running to rich (fuel getting on cat and melting it). I replaced two oxygen sensors one in front and one behind the cat. The third sensor in front of the cat was impossible to get to but everyone assured me just replacing two should do it.

So now over the past year I have smelled the cat burning again, I have pieces of it rattling in my pipes again, and the car is getting sluggish again ARGH&%$!!! Exact same symptoms but the cat is out of warranty and I don't trust the original guy I took my car to.

Now I want to get my pipes redone (custom 3"), add a flowmaster, and of course replace the cat (with a high-flow cat). My question is what can be causing this problem so I don't burn up another one?

[John_D.]

<blockquote>quote:</font><hr>Originally posted by Doug:
About a year ago my cat melted and I had it replaced. After some investigation I found the car must be running too rich (fuel getting on cat and melting it). I replaced two oxygen sensors one in front and one behind the cat. The third sensor in front of the cat was impossible to get to but everyone assured me just replacing two should do it.

So now over the past year I have smelled the cat burning again, I have pieces of it rattling in my pipes again, and the car is getting sluggish again ARGH&%$!!!<hr></blockquote>

Excess fuel in the exhaust will burn up in the cat and ruin it. There are a couple of ways you can get excess fuel in there. Running rich is one way. Misfire is another. The pcm is supposed to be smart enough to cut the fuel to a misfiring cylinder. Maybe, maybe not.

Did you do plugs and wires somewhere along the way? My first guess would be a bad wire (burnt or loose), or a bad plug gap.

Running rich should not happen either. One way would be if you have put in major mods like bigger injectors or significant head/intake work. The other more typical way is if you have bad o2 sensors.

On the '98, there is a sensor in each exhaust manifold. They should still be good, but maybe not. If only one was replaced, then the other one probably needs to be replaced also. The one after the cat is only used to report back to the pcm that the cat is or is not doing its job, so it can set a light or not. The 2 in the exhaust manifolds are the ones that give the pcm the info needed to manage the air/fuel ratio.

[Brett Garret]

I've actually heard that if the timing chain is off you can do that too. Still sounds odd but take that to a dealership or skilled mechanic. Something like that sounds no bueno. Keep us posted. -B

[Doug]

I did replace all plugs and wires first thing (before first cat completely clogged shut).

Also when first cat was burning up I got OBD codes that I can't remember anymore. After the new cat was installed the SES light came on again. The mechanic said this was probably the new cat "burning off". Not sure if he knew what he was talking about but I'm convinced he disabled the SES light with his computer. I am having same problems only now ever since that day (a year ago) SES light has never come on.

Thanks for the replies, sounds like I need to find a reputable mechanic to really diagnose this one. I'll try some local message boards unless anyone from Austin reads this and has a good mechanic...

[12secondv6]

Actually you can also burn up a cat running too lean, I'm told.
Dealer, ask for service manager.
Maybe get a head tech involved