I have a 1995 e420 w/140k miles which I purchased in late Sept. On the 2 hour ride home the car ran flawlessly. It did not reach the optimal temp (80-85) on my test drive, (1 hour) or on the way home so I knew it needed a new t-stat.
I figured I would drive a few days and make a list of todo’s, more extensive than the original test drive list.
My first day commuting everything was smooth sailing until I hit bumper to bumper traffic. Upon creeping the engine temp remained around 60, and a misfire commenced anytime I was creeping. As I accelerated I smoothed out. It only occurred with the engine was warm.
I consulted the web and found that many people have experienced the same problem with many suggested solutions. I knew it was either spark or fuel delivery related.
I have the unfortunate advantage of recently totaling my 92 500E. There are many parts sitting in my garage, including the venerable m119.974 5 liter. I swapped many sensors (CPS, coolant temp), and distributor caps and rotors. Nothing resolved the problem, until I swapped the fuel pressure regulator (FPR) , Viola!.
As someone mentioned, it the best 2 minute fix ever completed. $40 bucks a ********AZ or ********.
Here are some DIY links part # included. Thanks for all the heavy lifting.
Photo DIY-Fuel Pressure Regulator R/R M119.981 Engine - Benzworld.org - Mercedes-Benz Discussion Forum
HOW-TO: Replacing M119 / M104 fuel pressure regulator
If the regulator is not your problem Good luck. Check the link below.
Everyday Diagnostics: Mercedes S-Class Misfire