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Cat failure?

1498 Views 4 Replies 3 Participants Last post by  BernardFarquart
My 2003 E320 has 102K miles on it, and has been making a "clicking/rattle" sound that I recognized as the catalyst. I have had cars make this noise for some time prior to failure, so although I was not pleased, I figured I had awhile to deal with it. after making some noise (usually at start up & taking off from a stop) It has had three occurences of hard stumbling/power loss, lights up the check engine light. The drivability issues will continue until I turn the car off, and restart, at which time it drives normally again. When I pull the codes, it is giving misfire on cyl. 4, 5, 6. but not a "catalyst flow insufficient " code, which I would have expected. :confused: I am using a standard code scanner, as I have not yet bit the bullet on purchasing a STAR knockoff. I guess my question is for anyone who has had a cat fail, and checked the codes. Is the misfire on one bank telling me what I think it is, that the pass. side cat is plugging? I am taking her into the dealer for their diagnosis and have pre-ordered the cat (can't believe they didn't have it in stock, actually) but I would love some confirmation prior to a 1600.00 :eek: repair.

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If I'm not mistaken, cylinders 4, 5, and 6 are the driver's side cylinder bank, not the passenger side, which would point to the driver's side converter, obviously. You say you recognized the rattle as originating from the catalyst--how did you do that? You know that one way to find out is to take a rubber mallet and hit the catalysts on both sides from below (pre-cat close to the engine and main cat farther to the back, as well as the pipe in between)? If you hear it rattle inside, that gives you a strong indication that there are broken pieces flying around and potentially blocking the flow of exhaust gases.

My experience might vindicate your hypothesis. I have a 1999 E320--a different car, but with the same engine. At some point (178k miles), I noticed a clunky rattle at idle and when decelerating, which I traced to the passenger catalytic converter. (The driver side converter had been replaced by the previous owner not long ago). A few weeks later an independent Mercedes specialist confirmed that the catalyst was internally broken.

Incidentally, two weeks later on the highway I accelerated pretty strongly (although I didn't put the accelerator to the floor), which I don't do often, and at around 4000 rpm in third gear the Check Engine light came on and the engine started to run roughly. When I drove off the highway, it became even clearer (and worrisome). I stopped the car and read the code, which said misfire on cylinder 1 (which is on the passenger side). I turned the car off, cleared the codes, and when I turned it back on, the misfire was gone and didn't come back.

While I didn't try to provoke this again, two weeks later I did replace the catalytic converter, which I had resolved to do anyway because I knew for sure it was broken (moreover, the rear 02 sensor readings that I checked using my scanner--one you can hook up to your laptop--also indicated that the front converter was not operating properly any more).

With the new converter installed (and new front oxygen sensors, although that might have been just an act of preventative maintenance, because it's quite possible that the old ones were still working well), no misfire or other driveability issues have ever come back, even under hard acceleration (although, again, I don't do that often...).
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