I pulled the pipes loose on my '91 300SE, and thought I'd look
down into them to see if I could check the condition of what I thought were the little pre-cats. These are bulges in the pipes about the size of a short loaf of French bread, about 1 foot from the manifold. Corvettes have similar pre-cats, so I expected to see the typical converter brick, the honeycomb ceramic.

Well, to my surprise there was no converter brick in them at all!
All I saw was a funnel-shaped metal piece with several holes in the sides about 10mm in diameter.
I could not see all the way through due to the bend in the pipe, but I took a piece of half-inch hose, and stuck it right through the pipe. i was concerned at first that I had lost all the "brick", but a closer look convinced me these were just not shaped right to hold a cat brick.

So, these are not cats at all. They must be a type of resonance chamber or muffler, they kinda looked like the inner pipe of an old "smitty" glasspack, with all the holes.

now I know how they managed to put the O2 sensor downstream of these "cats" and still get proper readings.

Well, at least I feel good knowing I have only one cat instead of three.

Anyone else ever seen inside one of these?

Very interesting and news to me! I always thought they were precats with the typical honeycomb catalyst bed.

You described them as somewhat similar to a glasspack muffler. Could there be catalyst bed between the inner pipe and outer shell - where the fiberglass is located on a glasspack, or is this volume hollow?

Duke

Maybe someone gutted them? But then why would they install those cones?

I gutted the clogged pre-cata on my 190E, it was a hell of a job but it beat paying big $$$ for the replacements.

Did they look anything like these?:
Cones

Funny you should mention this now. Only last night I was reading an MB factory publication from back in the '80s (lent to me by another MB club member) that covered all the features of the then new 190E 2.6 (with M103). One item mentioned was the fitment of a small muffler in each of the two engine pipes ahead of the underfloor catalytic convertor. I too always assumed that they were pre-cats. Both my '90 190E-2.3 and '90 300TE have them but I have never investigated as to what they contain. I know that if they are pre-cats, I would prefer they were not there, especially after Gilly and others have mentioned that their failure can lead to engine damage, especially on the M102.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.

I find it interesting on another reason. It's odd because manufacturers have been recently going through great lengths to get the converter as close as possible to the exhaust. They're doing this for quicker 'light-off' to meet start-up emissions. Some manufacturers are even incorporating the cat with the exhaust manifold into one unit so the monolith is only inches away from the exhaust valves.

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95 E320 Cabriolet, 169K

Thanks for the comments, guys.
I know these are original cause I've had the car for 14 years.
I think they are little mufflers as you said; the outlet end of the
"funnel" is big enought to slide a 1" hose through, so even if there were some kind of converter material behind the holes in the funnel, the effect would be minimal. I couldn't see anything behind the holes, 'course after 15 years, if it was a packing material, it's probably moved downstream and clogged the main cat. (nah)
Speaking of cat light-off, the old car almost failed the sniffer test this year - the wife drove it in cold. So with the only cat being several feet below the mainfold, I can assume the secret to passing emissions is to drive that puppy hard and get that cat really warmed up before they test it.

Yes, a good, hard drive is necessary for maximizing you chance of passing emissions. Also, don't let them make you wait too long before testing, sitting in their lot. You can't let the engine cool down before testing.

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95 E320 Cabriolet, 169K

Mercs of our vintage, and particularly M103 engines are very sensitive to "conditioning", and many will fail marginally if the converter is not hot enough due to lack of warmup or excess idle time prior to stating the emission test.

Duke