My mercedes starts and runs at a very rough idle missing and spudddering as soon as it warms up it smooths out with just an occasional miss. I changed the spark plugs and the car seemed to run good for a couple of days then got worse again. also, when the car is running I here a hissing sound that is coming from a round cylinder next to the gas tank.
Thanks john

I have the same problem and I have yet to diagnose and fix it. My park plugs on Cylinders 4,6 and 8 are fouling. I was told by the dealer I need a valve job as oil is getting into the combustion chamber. I have heard that it could also be a fuel distribution problem, or another oil delivery problem. I am going to get it checked out and hopefully fixed in the next month. I will let you know what turns up.

PS I have the same hissing sound from the rear passenger side wheel well. I posted on it a few months back and heard from several people that said they had it on their w126s too, but had no other running problems they could associate with it. Let us know what you find out about it.

John, ...I had experienced this same situation with my 88 420SEL. replacing the cap, rotors and plugs didn't correct the problem. One night, ..I open up the hood to work on the problem, ..guess what I saw//////Lots of spark shooting up in the air, ..coming from the plugs wires. I replace all of them the following day, ...the car runs smooth from then on. Hope that this will help.

The hissing at the round cylinder under the gas tank is the electic fuel up. If you don't here it it's not pumping any fuel