Replacing cats: DIY?
 

Hello All. '97 S420 / M119.981 here.

I've been following some recent threads about plugged cats, and noticing that my car was a bit down on power I decided to do some of the checks that were recommended. I have been noticing that the right side cat has been rattling for a while so I know it will need to be replaced eventually, but I think the time has finally come. Besides the rattle, I have noticed these other symptoms as well:

1. When the engine is at operating temp, I notice a diesel-like clatter/tapping coming from the lower right side of the engine bay. It's not really loud, but it's noticeable.

2. I took some temperature readings with an infrared thermometer and discovered that the right side is significantly different from the left:


Inlet-- Left:367 deg F Right:277 deg F
Outlet-- Left:467 def F Right:265 deg F

My best guess is that the right side cat is dead & partially plugged.

So, on to the questions. . .

1. Is my diagnosis correct?

2. Can the average, mortal, DIY mechanic replace a cat at home? Any "gotchas" to watch out for?

3. Can anyone recommend a good source for a replacement cat? I'd prefer OE if possible.


Thanks!

HI,

I can't answer #1 and I can only give you some general info since I don't have that car. First I have observed that there is a common problem getting the correct cat delivered. I had this problem and another guy here recently had that problem. So whatever you order make sure you are getting the right one.

As far as doing it yourself, one issue is some of these assemblies are quite large and difficult to remove without the car jacked up quite a bit. That was the issue on mine. I did manage it however. I found the Catco quality quite bad but others have said it was OK. If possible when you get the old one out, lay the new one beside it to make sure all the mating ends are the same and same angles etc. This is the problem I had with the Catco.

I went through this with my SL500. Whatever you do, don't go with a welded-in generic. That's what I replaced.

IIRC, the aftermarket was like $750 and the Mercedes part was $860 from an online source - see what Phil can do for you - so I went with the real deal. I did my own installation on ramps and it was easier than I expected. The bolts on the engine side were a little hard to reach, but not bad. I would not think it would be much different on the sedan.

Thanks for the reply! In addition to the cat itself, is there anything else I would need to order? I know there are a couple of sealing rings, but how about things like bolts or clamps? Is there an official M-B kit for these or is it just off the shelf hardware?

I took a look at the EPC and you will need your VIN to be sure you get the correct cat/pipe. I did not see a kit for what I believe is the setup for your car.

I re-used everything when I did mine, but it might not be a bad idea to have the sealing ring on hand. The one thing that I had to do was use longer bolts in the back to pull the flange in far enough so I could get the nuts started on the original bolts.

It's not a dirty, not terribly rewarding job, and your local Meineke guy would probably do it for 75 bucks.