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Discussion Starter #1
Hi all,

Been reading around and am looking on advice for what to do for a rattling rear cat. Started about a week ago and while not affecting performance at all, is bugging the crap outta me with the obvious noise when idle or at low rpm. It is a metal on metal noise and from my searches sounds like something in one of the rear cats has failed with age and it causing my headache.
2001 slk 320

I put the car on ramps and was able to identify the noise was coming from one (or both?) of the two rear cats about half way towards the back of the car. If I smack it with a rubber mallet the sound goes away for 5-20 seconds then comes back. (Yes that's my technical test)

I see that new prices are about $350 for each pipe/cat side and am not looking to spend that much. From the old thread below I see that the rear cats don't have sensors so if they were removed and replaced with straight pipes I'd still be able to pass emissions and the sound isn't noticeably affected? Another option would be to cut the cats out, gut them, and put them back in? I could also drive the car for a bit and hope the sound sorts itself out? Or, the costly option, replace both rear cats and pipes.

Are my thoughts correct for this? Mainly the sensor question so if the cats were removed/gutted the scanner wouldn't find a fault?

What do you all think would be my best bet? Each option would require the help of a mechanic/friend with time as they are above my experience level.

I look forward to your responses.
Old thread in question below:

http://www.slkworld.com/slk-r170-general-discussion/16528-catilitic-converter-slk-320-a.html
 

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When my rear cat rattled about a year ago in my slk 200k. I asked the dealer how much it would cost to buy a new one, they quoted me around 2000 dollars for the part and that doesn't include the install. So I searched the interweb and found that the rear cat is a quick mod that some people do to get a better sound than the stock sound (deeper, throatier sound on higher rpm) also personally I think it adds horsepower and increases mileage because the part is really heavy. I think Removing the 2 heavy cat parts stuck underneath would do great on your mileage.

Also emissions weren't affected in my car, and the rotten egg smell that people say when your cat isn't working is not present.

I've been running my car without a rear cat for a year now, and no codes or problems have shown up
 

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Generally the rattling with no change in performance is cause by the catacomb breaking up. Unless you get lucky and it is just one piece that finally wedges somewhere, it will keep making noise. May even get worse as more pieces start moving around.

Mine did it right at 70K. I had mine replaced by the local muffler shop for $650. No sound difference at all with the aftermarket cat. Since I do not have to complete emissions where I live, do not know if it would impact the test.

Sorry no input on the gutting it, never would have crossed my mind back then not to get it fixed. Good info to know however when this cat fails.
 

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\I have the same problem with my 230k my local mechanic told me it was the cat rattling and apart from being a bit noisey nothing to worry about.
Dave
 

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My Sonoma did the same thing...I drove it like that for a good 6 months before it really started to drive me crazy (summer time with the windows down). Had an aftermarket cat installed because it's just an old GM truck.

Hoping my SLK cats hold together for while because we do have emissions here, so any replacement would have to get me through emission testing.
 

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Not cat? Do you have MOTs in the Philippines ???
Yes we have the annual emission testing every year before you get your annual registration. We also have checkpoints in random highways and streets doing the tests.

From the time I removed the cat, the car underwent 2 emission tests and it passed. So I guess it is okay to remove this part. You just have the watch out for the other cat failing on you. When I looked underneath my car, there are two cats underneath, I am not sure if the first one is the cat or the one I removed is the resonator, but some say these are two cats.

The cat I removed can be repaired, I opened it up, and it indeed looked like a honeycomb, the honeycomb was intact but it was loose, that's the reason for the rattling sound, I didn't install it again because it was too heavy, I liked the sound of the exhaust on high rpms, don't want to spend money having it installed again.

For those of you who are looking for a refurbished cat, I'm willing to sell mine, I'm not going to use it, but it is really heavy and the cost of shipping would be a pain in the wallet.
 

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Discussion Starter #10
Thanks for the input everyone. I need to setup an apt with my non-stealership mech and am planning on mentioning each option to him. I trust his judgement and he's got a great rep for providing honest feedback so depending on the response I'll probably just go with it. The car is just fine with the rattle.. but I can't stand it and to me it makes the car feel and sound broke.. so I guess I'll throw some money at her :/
 

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Discussion Starter #11
Hey all. I've been asking around by so far haven't found a shop that will agree to cut the rear cats out and replace them with straight pipes (my preferred option). Something about legal blah blah.. ha :/
Now I've only asked a few places so I still have plenty of hope but I'm hoping anyone with experience can shed some light on if I wanted to do this the diy route.
I know I'll need to measure the pipe before and after the cat's to get the right size Y pipe. After reading around a Magnaflow Y connector (about $60) is the top choice. Then get some extra piping to fill in any missing space.

But the big question.. will I need to drop the whole exhaust to do the swap? Or can I do it in place?
My dad would be helping me, who has a lot more experience, but doesn't have a clue how to get the thing off if we needed to.
I can see the connectors where it should come off but what is in it to do this if I need to drop it?
Would I need to weld the connections or is there a pipe clamp that would work better?

I know there are quite a few questions in there so answers to any of them would be greatly appreciated.
I can live with the rattle.. but I'd like to have a plan to eventually fix it, and not a plan that costs an arm and a leg.

Thanks in advance!
 

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Discussion Starter #13
Well finally got tired of the sound and after not finding a place to do it for me went ahead and gutted the rear cats myself.

Took about 3 hours total.

What you'll need:

2 x new seal rings for the exhaust joins. mb part: A 202 492 01 81 (About $7 a pop)
5 x M8x55 bolts and nuts. Could you reuse the old ones? Probably.. but not mine. had to cut 3 of them off.
13mm socket etc.
An air gun to help with the bolts is highly recommended

Taking off the exhaust took the most time due to some of the bolts not playing nice and needing to be cut. Remember to pop off the o2 sensor as well. Once off one of the cats is easily accessible and after about 10 minutes with a hammer and piece of metal got it completely clean.
The other side is where you need to cut it open and then gut it. Once done welded the one side back up and put her back on the car with new seal rings and bolts.
The bolts that connect the exhaust to the engine assembly were fine so I was able to reuse them no problem.

No more rattle and to the naked ear I don't notice any great difference in the sound =)
 
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