i have a 1986 300e. i am a teenager going into mechanics and this is still my first car. it has about 200,000 miles on it and it has ran fine until now. it had a major vaccum leak a while back making the same symptoms. i fixed it and about 6000 miles later it is worse. it vibrates at all levels of the rev range now. it feels like a motorcycle engine right now. this just stumps me because i checked ignition and vaccum. all seems to be fine. my possible solutions are: loose valvetrain(ie. cylinder head retaining bolts), bad bearing somewhere near crank throwing off balance, bad compression in one of the cylinders, or just bad timing. has anybody ever had a problem like this on their e-class? i need help in troubleshooting this.

An easy one to check and a likely possibility would be your motor mounts. Engine will shake, etc. when they are worn.

Documentation for your car is at the following:

http://mb.braingears.com

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

thanx for the documents, they are another resource. unfortunately, i'm afraid it is not the motor mounts. i took the car for a drive yesterday and found that it is hard for it to hold idle. the "economy" gauge is up to the middle when it is in idle. the engine does not die, instead it feels the computer compensates keeping on the gas. it still rumbles and runs erratic. when i apply the gas, it seems that the harder i push the less power there is going to be. could it be the mass airflow sensor or something else related to the throttle?

I would double check vacuum. Check EVERYTHING. The economy gauge is actually just a glorified vacuum gauge. The fact that it is at 1/2 *could* indicate a vacuum leak. Of course it could be something else as well, but I'm inclined to think you have a vacuum leak somewhere. Bad bearings have nothing to do with what you're describing. As for the compression and timing, just check them to rule them out.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

what are the major parts i'm looking for in the vac system that could be causing the problem? i.e. idle control valve etc.

Have you checked the plugs, plug wires and caps, and distributor rotor and cap? Try removing the plug caps one at a time while the engine is running. If you remove a cap from a running cylinder the engine will vibrate worse and the revs will fall. If you remove a cap from a non-running cylinder it will make no difference. A 6 cylinder engine running on only 5 or fewer cylinders will vibrate quite badly, especially under load. If you find a bad cylinder, pull the plug and see if it's oil or carbon fouled. If it is, it could be caused by a bad valve guide seal letting excess oil into the combustion chamber. If you find coolant on the plug, you could have a blown head gasket.

Try the simple things first.

Just a thought