I sort of had the same problem with my 82 300sd , Had a rough idle and hard start while below 5 degrees C. I checked the first four plugs manually with jumper cables and they were all good. Because the plug closest to the widshield was so hard to get at I left it alone. I did some more research and decided it would be easier to go back in and pull the last plug to test it. Turned out it was the culprit and it's no wonder. My theory is that when people change their glow plugs on these models they tend to leave the 5th one alone because it is a bit of a pain to get to.


Unfortunatlly that only solved my problem last year.The snow is down again and so are the temps at adout minus 5 C. I have 5 fairly new plugs , the preglow lamp works as it should but if I don't plug the car in it won't start.