Hi all, I have a 1991 Chevrolet Camaro Z28 with 5.7 Liter tuned injected motor. It's a dual cat setup merging to a single 3" exhaust pipe going to a Flowmaster catback system. The issues I have been having is constant catalytic converter replacements. In 2001 or 2002, the original GM cats failed, they were producing a loud rattle sound every time I was accelerating or cruising at normal speed. I replaced both of them with the Car Sound brand, a year later the outer cat failed, started to rattle. In the meantime parts were thrown at it such as an oxygen sensor, new fuel injectors, spark plugs, idle air control, egr and egr solenoid. None of these solved the problem, by this time I had already replaced five catalytic converters all the Car Sound brand. In 2006, I took a local shop and they determined that the air recirculation box, located behind the air conditioning bracket on the passenger(photo provided), allowing more air than gas to mix creating a lean condition that was causing the cats to fail inside. This worked for a year and the outer converter failed again replaced it for the sixth time. Now, the inner one is rattling now, boy, this is getting really frustrating. Do you guys have any ideas as to why these cats are failing so easily? I'm to the point that I don't know what to do. I don't want to part with this car. I do live in California so I do have to be smog legal. So, once again, I would appreciate any responses related to cat. converter failure thanks.

 

I think that you need to verify that you don't have any miss. If you have a spark plug that misses even a small fraction of the time, that will destroy catalytic converters in a heart beat. If I were you, I would find a place that can measure your fuel:air ratio and go from there. I have never heard of a lean condition causing the cats to fail. However, I have heard of rich conditions causing them to fail. Raw fuel getting into the cats and igniting from the heat is what would cause the problem.
How does your fuel economy look? If you are getting too much air past the 02 sensor such as if you have an exhaust leak before the sensor, that would simulate a lean condition and cause the computer to dump more fuel into the engine. I think that you have tunning issues. Are you running the stock cam and stock computer chip?

 

For the most part seems to running smooth. I am running a stock cam and stock computer chip. My fuel economy is not so great though, it does burn a lot of gas. For example, going 20 miles I burn a quarter tank, it never used to do that when I bought when it had 19,000 miles on it. Right now, it has 118,000 miles on it. So you are saying that rich running conditions cause the cats to fail creating a rattle sound, right? Should I get my existing ecm calibrated from someone to get the right air/fuel ratio?

 

I don't think that you need to recalibrate the computer. Usually what happens is you have a faulty sensor or something of that nature, and that is throwing the computer off to think that it is running lean.
Do you have any stored trouble codes? If you do, I would start there. Find out what the codes are and that will give you an idea of where to start.
Also, if you have a bad fuel injector sometimes the other injectors will spray more fuel to compensate for the bad one.
Exhaust leaks at the exhaust manifolds or the piping before the O2 sensor will cause a similar situation. External air will be pulled passed the O2 sensor, and that can cause your computer to think that your car is running lean. Do you have any exhaust leaks that you can hear?
But yes, generally rich running condition over time will cause the catalytic converters to fail, or a spark plug that misses will do that much sooner.

 

The converters primary purpose is to consume any fuel your engine does not. If your engine is producing volumes of unburned fuel, the converters will melt themselves down. You MUST find and correct the source of the excess fuel or expect the same result.

 

Hi - I have a 1991 Pontiac GTA 5.7 having the same problem - the original cats failed about a year ago and my 3rd set have failed. I live in California and it passed smog last month - the car is running really good - I'm at a loss, too for a solution!

 

cats fail due to running to rich. You gotta find whats throwing too much fuel down your system.

 

Don't use premium.

 

What does premium do? That's what I've been using and I'm on my third set of cats that need to be replaced . . .

 

I bought a new set of dual cats from magnaflow, I ran premium for a few months, then my car started losing power going up hills. I found out I was running way too rich and messing up the cats, so I switched to 89 octane, problem solved. When I switched back to 93 octane, the problems started again after a few tank fills. I think the cats were slowly clogging with the premium fuel. I have an L98, and the owners manual recommends "unleaded fuel only". The corvette in my garage recommends "premium only". It might have something to do with running rich/lean and the cats.