crackrx

I own a 87 E300, which I love. The only real problem I have with the car is that it rumbles violently when it idles for a period of time. If it has been idling like this for a while the acceleration
sometimes is very bogged down. I am not sure what to try to fix this. I also noticed that the spark plugs in cylinders 1,2, and 4 are covered in gunk. I believe this is due to oil burning in the
combustion chamber due to warpage in the cylinder walls. Let me know if I am wrong, and what I should due to fix these problems.

bamberger_1

To me this sounds like worn valve stem seals which allow oil to seep into the burning chamber, are well know problem for W124 '85 to '89. After that time the valve stem seals were made of silicon rubber and lasted pretty much for ever. If you need to see a mechanic to get this fix, dont let them talk you into an engine overhaul. This seals replacement shall not be more than a few bucks for the parts and maybe 3-5 hours max.
The engine does not have to come apart, just the valve cover and the rockers need to be taken off. It's worth a try before larger investments shall be made. Fouled spark plugs are the best sign for this problem.

mdp c230k

Bamberger is right especially if you are blowing blueish smoke after the engine starts rumblimg.