We were having a debate about my 1991 300D running rough and all of you, as well as other suggested that it was the glow plugs. I did check the glow plugs, and they all were fine. It turns out that my multimeter had a near dead battery. I replaced the battery im the MM and found out that plug #4 was bad. I just changed it, and the car runs great.
Can you find it in your heart to forgive me for doubting you??