Catalytic converter coming apart?
 

The car: 1989 260E with 250k miles, well maintained, CA emmissions
The problem: a knocking noise emminating from within the #4-#6 exhaust system. Most prominent when decelerating.

The noise is coming from within the exhaust pipe. There is a piece of loose metal in one of the "pregnant" parts of the front header pipe (about a foot after the exhaust manifold). On the attached photo it is the bulged part all the way to the left. When I hit it with a rubber mallet i can get the metal within to clang. Where did this metal come from?

A good MB tech friend of mine suggested that the cat is coming apart. The cat is very far downstream, but he claimed that during sudden deacceleration the engine could suck some debris in reverse of the exhaust flow! Amazing!

Should I just replace the cat or is further testing/inspection needed first? Could the loose metal have come from somewhere else?

If it is the cat i noticed that they go for about $1000 (perfect fit). the local parts stores (Kragen, autozone, etc) have them for about $100 (universal fit). Any comments? can't afford the perfect fitting ones from Fastlane, MB, etc.

the plot thickens
 

I hit the exhaust system with a rubber mallet. It sounds like there is a long (maybe 12"?) piece of metal resting in the header pipe. I get it to clang when I hammer along a foot long length of the header pipe between the cat and the enlarged (looks like a small football) portion of the header pipe. How could something big get in there?

I looked carefully at the cat. It looks like it was a replacement because each end of the cat has two circumferential welds for step down adapters. Furthermore the Cat shows no MB marks and has the following stamped into the steel shell:
CA/CE 94007
07/03 36398.

Would the 07/03 be the date? If so that is pretty new for the cat to be breaking up.

pre-cats
 

I believe the two pregnant peices you are referring to are the PRE-CATS (pre-catalytic converters.) My 380SL has pre-cats and look exactly as shown in your picture. They are built-into the pipes connecting to the exhaust manifold, one on each side. From what you describe, there is loose material inside one of the pre-cats. the pre-cats have material inside of them to perform as part of the emissions system/catalytic system. In that case, I believe what you are looking at is a replacement of that side of the pre-cat. Other possible explanations include

1) the exhaust piple going to the exhaust manifold is loose. Have you verified that both bolts are tight? Tried moving it with your hands and looking down from the top of the engine when the pipe is cold (for safety)?

2) I doubt this is the case, but I assume you have verified that the exhaust manifold cover is tight on that side of the car, that will also produce a rattle if it's loose enough but from what you describe I doubt it's the cause.

I tried moving the exhaust pipes and manifold evey which way. They are secure and don't touch anything including the heat shields, The noise is defenitely comoing from within as described above. I looked for the exhaust system in several online catalogs and print catalog. None show or mention a pre-cat. Is it sold with the main cat? If it is not sold seperatly it makes me wonder if it is indeed a wear item. Maybe these are only found in CA cars. Can debris get sucked back into the engine (disaster) if the loose metal is not removed? Can we keep driving the car with the loose innards? The car must me smogged periodically in CA so would cutting one out and replacing with straight pipe cause it to fail emmissions? They don't do visual checks, just tailpipe emmissions here in Los Angeles.

The left most part of this image also shows those so called precats (like little footballs just after the exhaust manifolds). I doubt they are only found on CA (50 state) cars. Maybe they only have innards for CA. I noticed that the online catalogs show much higher prices for the CA cats vs the 49 state cats,

A report a few months ago by someone who took the "pre cats" apart reported nothing but baffling inside - no catalyst bed and said that there was no indication that they every had a catalyst bed. I don't recall if it was a 49-state or CA car, but it could be that only the CA models actually contain a catalyst bed, or it could be that what we think is a "pre-cat" is just an acoustical device that is part of the muffler system.

There's probably a broken weld joint. You could remove the catalyst assembly and either reweld the loose piece if it's accessible or remove whatever is loose and install the assembly back on the car.

Duke

precat
 

I dont know what is inside the precat. I can tell you that on my car, one of them was blocked solid, apparently melted and thus I had it replaced. I looked inside it, confirmed.

For my car, a 380sl, Miller, a leading aftermarket cat supplier, also sells the pipes with the precats. So you can buy everything from them. I think fastlane sells miller cats. If they dont try www.autopartsauthority.com. It is usually presented as one of the parts that can be ordered with the main cat.

Duke may be right about what is inside. I believe that my car (an MB CA car originally sold in maryland) passed pretty stiff emissions tests with one of the precats replaced with a straight pipe. Then again, there were question marks on a relatively new cat just a few years later. If it were me, if I had the money and the sound bothered me I would replace that pipe precat and not do a straightthrough without precat. If I did not jave the money, i would leave it alone. If you try to remove the pipe and reuse it after trying to get whatever is in there out, note the age and rust on the pipes may prevent them from being reused.

For the 1988 260E
the ubiquitous online catalog used by Fastlane, Rusty, Auto part autority, etc the listed catalytic convertors is by Miller (no photos). The CA (50 state) one sells for about double the 49 state version. Is the main cat different on the CA model or is it the insides of the precats that are different?

Unfortunately have not found any hints besides price as to the difference. If the CA precats are not doing much for emmissions maybe replacing with a straight pipe would be OK. If so i'd take my chances with a muffler shop and have them cut out the precat and Mig in replacement straight pipe. If I had a high enough lift I'd probably try it with my 110V Mig wire welder. Existing pipes look good (no significant rust) and weldable, but of course i'm only looking at the outside.

The rattling noise is a bit disconcerting especially during idle) but my main concerns are:
1. More back pressure? It's not plugged but maybe there is a reduction performance and increase in gas consumption.
2. Damage the engine. I have been told by a friend who is a MB tech that during hard decel the engine can actually pull back debris from the exhaust; that would be real bad.
3. Damage the main cat with debris. maybe thats already happened and should just ignore the whole thing until smog time.

Thanks for your input

i spoke with a gentleman from a Cat manufacturer called DEC Catalytic Converter. He said that the precats do have platinum in them and do help reduce emmissions. The ones MB sells and manufactures have precats that help emmissions and are the same for both 49 and 50 state applications. The 49 state aftermarket ones typically have all the catalyzing material in the main cat and no precat catalyzing. Although the main cat could bring emmissions low enough to meet the CA regs, CA does not accept emmission redesigns. Even if an aftermarket emmissions device with a unique design is as good or superior to OE CA only allows OE design solutions for emmissions.

"Pre-cats" or "light off cats" as they are sometimes called typically reduce warmup emissions, but once the engine achieves operating temperature and closed loop operation, they generally have little or no effect, so field emission test results are not likely to be significantly affected with or without them.

That being said, it is illegal to modify the original emission control configuration. The FBI probably won't come to snoop around in your garage, but commercial shops have to be careful about removing or modifying catalysts.

Duke