1982 300sd
very similar problem. It's in 20's here. My #2 glow plug was totally dead. Had to use block heater to get started. Changed front two glow plugs and she fires right up in 20degree weather without the block heater.

Just had my valves adjusted. It really makes a HUGE difference in cold weather and the engine idles better and runs smoother.

I took my multi meter, disconnected the relay for the plugs, put the ground to the chasis and started putting the red lead into the holes in the harness to the glow plug relay. Got the same ohm reading on every plug except #2. I got no reading. It was a dead one. Easy fix. But go to sears and buy a 8mm and a 12mm ratchet box end wrench first. 12mm for the plug and 8mm for the nut on the wire. Dont get an offset wrench, but a straight one. Then you can do the job without having to disconnect the injector lines and having to bleed the fuel system.