76 240D no glow

[76w115]

I took the car out last week for a ride. I hadn't really driven it much in the last month but it started up just fine.

After a few minutes, I noticed that between 35-45 mph, it seemed to have a little shudder, sort of like a misfire. It almost could have been nothing, and lots of folks wouldn't even notice it. From 50-62 or 63, it smoothed out and ran perfectly. No smoke to speak of and power was good.

Fast forward to later at night, after the car had sat for 3 hours. Pull out the knob, and wait, and wait, and wait. The element in the dash never lights up. No glow! I pull it out further to start, and after 10-12 seconds or so, and a little throttle, it starts up, belches some smoke and runs like normal.

Drive to the ice cream store, sit for an hour, still no glow. Cranks a little longer than normal but starts, and we go home.

Today, I pulled the glow plugs expecting to find a burnt one like last time this happened. No such luck. No carbon build up, as 3 out of 4 of them have less than 1000 miles on them. One burned out very soon after installation, so was replaced by one of the old ones. This arrangement has worked well, and it looks as good as the other 3.

Is it possible that the glow plugs can go bad without obvious visible damage? The wiring hasn't been disturbed and it has been working fine til now.