Catalytic (sic?) Convertor Problems

[360RamGuy01]

On my Mom's BMW 1999 M3 the catalytic convertor went bad 5 months ago, and was replaced. She opted for the non BMW branded replacement because it was $500 less. 2 months later the catalytic convertor went bad again. She bought the BMW branded one this time. Now this one has gone bad and it has only been three months! The shop that does the work specializes in BMW's and said that it is impossible to have two defective catalytic convertors so there must be some other problem, possibly with the engine. The only code that the computer shows is for the catalytic convertor. Anyone have any experience with a problem like this or have any idea what is going wrong?

[kartoon]

That is really strange. I'm not really sure what could cause the cat to go bad. The only thing that sticks in my mind is the warnings to not overfill oil. Hope that you can find the root cause of this problem.

[Black240SX]

Some gasoline additives can foul the catalyst over long periods of time.

If the fuel mixture is too rich then the catalyst can get clogged up with carbon deposits. A bad oxygen sensor can cause that, but the engine would run poorly if the sensor was bad.

How do you know the catalytic converter is bad? Could it be an erroneous trouble code?

[bcwang]

If fuel gets dumped into the cats then it can go bad fast. This could happen if the spark plug occasionally doesn't fire, or doesn't fire at all from bad wires, plugs, or coils. Running into the rev limiter on some cars causes the computer to stop spark without pulling fuel thus dumping the excess down into the cats. Running too rich also causes the cats to get overworked.

Those are just some scenarios I can see for cats going bad, hope it helps.

[rjstaaf]

There is a TSB (180199) on Unnecessary Catalytic Converter replacement dated Sep-99 for the 1999 BMW M3 with the DOHC 3.2L

Go down to #45
http://www.alldata.com/TSB/06/99068537.html

Have your mechanic check it out.

[360RamGuy01]

Thanks for all the advice, it is interesting to know that there is a service bulletin on it. Now that I talk to my mom she said she knows the cat is bad because it rattles, like the insides of the cat has come loose. From what I read here http://www.bmwe34.net/E34main/Trouble/Catalyticconv.htm that means the convertor was hit by road debris, which doesn't make sense because that should not happen to three different convertors in 5 months.

[Lowejackson]

It is possible for 2 converters to fail but you would be unluckily. Firstly I would question if they really failed and it was not a faulty diagnosis.

If they both failed I would get a decent garage to check the emission carefully as it is likely that the engine system is failing in some way

[splattj]

Originally posted by Black240SX
If the fuel mixture is too rich then the catalyst can get clogged up with carbon deposits. A bad oxygen sensor can cause that, but the engine would run poorly if the sensor was bad.

Not always. Many cars will run fine if the O2 sensor goes bad they'll just get bad mileage. Most cars only run in closed loop when they're cruising. When in closed loop the car will adjust fuel trims for emissions and mileage. When in open loop the car will monitor the O2 but it will not trim the fuel as it is running off of the fuel tables and air meter/speed density input. (Amongst a host of other things such as coolant temp etc.)




Has your mom noticed any sulfur smell or major performance decrease? Usually a cat won't rattle unless you've melted it and there are chunks loose inside. The dealer should be able to hook up a scanner and see long-term fuel trims as well as actual O2 sensor readings. If he scans while cruising he should see the O2 readings oscillate. Anyhow, have her take it in and get some proper computer diagnostic work done. Don't settle for a code reading and guesswork.