Shaking Idle repair

I had cured, or so I thought, the idle shaking on my 300SD. Used two cans or Diesel Purge. It ran as smooth as ever after the purge treatment.
Then this week it began to shake again. Not bad, but I had gotten used to a smoother ride.
I have an extra set of injectors from a core, so I decided to locate the bad injector and try just swapping it out. I loosened the injector lines, but it was obvious that wouldn't work well. This was a slight shake, and when I loosened a line the engine would shake hard. So bad I couldn't tell which cylinder was firing off in the first place.
So I asked a local tech. He said to use an engine stethoscope, put it on the inector. You can hear it clicking with each stroke. Then slow the engine by pushing in on the engine stop lever until the engine is running just above where it will die. Listen for the injector click to match the exhaust miss as you go down the line on the injectors.
Took about three minutes to isolate the cylinder that was causing the shake. Unscrewed the injector and install another.
Now she runs smooth again.

[whunter]

so simple. LOL