OK, time to pay back all the good info you guys have provided.
Owning an MB would be a real hassle without being able to
find the answers to my maintenance questions here.

Here's the story: saw an add in my local craigslist for a 1989 300E, for 500 bucks...how good could it be? No pic of course. Ad said it had vacuum leak issues. But what the heck, it was nearby, so I went to look.
Beautiful car, 219,000 miles, perfect interior except some cracking of the
wood console. In the trunk, a complete intake assembly from throttle body to fuel distributor, two new headlights, new belts, lots of other stuff. Car has new brakes, rotors, master cylinder, 10 disc CD changer....yada yada.

On the test drive, fires right up, idles smooth, no smoke. Wont go over 3500 rpm. Otherwise drives wonderfully.
Sort of stutters like the ignition was cutting out, right at 3500. Everything else on the car functions perfectly, from the power antenna to the sunroof, windows, even the seats are perfect.
I haggle the guy down to $400 and buy it, since I'm fairly handy with a wrench, and if need be I could part it out on ebay for more than $400.

Every sensor had been replaced by the previous owner, along with Bosch plug wires, it had new H9DCO plugs. Pulled the air cleaner, and with the engine running I could hear a distinctive "crackle/pop" from the intake with the engine revved up. Vacuum gauge on the dash never goes near the left end of the scale, mostly stayed in the middle at idle, red area when revved, i.e. low vacuum.
Pulled the bolts from the header to tailpipe connection, and with a flashlight looked down at the surface of the pre-cats....completely plugged. Amazing that the car runs at all! Getting the tailpipe and main converter assembly off the car with only ramps and jacks is a REAL hassle, since it is about a third the length of the vehicle (I dug a trench in the ground under the car). I've attached photos of what plugged pre-cats look like, and my cure; cut an access hatch into the underside of the STAINLESS STEEL pipe, chop out the stainless honeycomb pre-cat material (tough!), and MIG weld the access closed. I also hollowed out the main converter, since it was probably clogged with whatever jammed the pre-cats.

The result? Car runs like a champ, gets 20.5 mpg in city driving at an adult pace, and passed Nevada smog without a problem, (although I cheat and use 50/50 mix of pure denatured alcohol/gasoline for the 10 minutes the test takes).
Vacuum gauge now stays pegged at the left, on the high vacuum side of the scale unless my foot is using some throttle. At a steady 35 on flat ground it is tight against the left end of the scale.

If you have pre-cats on an older MB, and if you have not looked at their surfaces lately, I urge you to do so, especially if you are down on power, have low gas mileage, and have low vacuum indications. That "crackle/pop" noise I heard was the exhaust valves opening and trying to vent through the easiest path...back out the intake!

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