I have a 1991 300D 2.5 with the 602.962 motor. The glow plug light came on last week while driving and a few days later I have the rough idle and extra smoke for the first few minutes of a cold start. These are the typical tell tale signs of a bad GP. I tested the plug's resistance at the harness and the are all within range of each other. I did this test while the engine was cold and while the engine was hot and the resistance values did not change. Aside from pulling each plug, what else can I test.

Thanks!!