617 300D Injector Test with a Screwdriver?
 

So, a mechanic was working on a friends car and put a screw drive on the injector (of her 300sdl) and put the plastic end up to his ear.

She tells me that he was able to diagnose 3 "bad" injectors this way. My question is what was he hearing and does this trick actually work.

I would love to do this with my 300d to check and see if I should replace the injectors (without going to the trouble of popping them out and pop testing them).

Any thoughts on what to listen for?

This is the same as using an engine stethoscope; just listen for the sound of the injector popping open. If you have an injector that is not working properly you will be able to hear the difference; all the others will sound pretty well the same.

I used a cheap stethoscope from Harbor Freight to listen to my valves after an adjustment. At least I thought I was listening to the valves, I put the tip on each of the injectors.

I didn't really know what I was hoping to hear, but all cylinders sounded similar, so I thought that was good.

If you listen carefully you can hear the sound of the injectors separate from all the other clanking without any instruments.