I have a smoking issue with my 7.3 after running WMO (only 1 tank). After testing compression I went next to the injectors. The o-rings looked ok (Dipaco). One of the upper black o-rings was just starting to tear apart. I'm guessing it was leaking a little high pressure oil.
But since I had them out and had just rebuilt them 10k miles ago (worked fine until now) I decided to crack em open again. Everything looked ok except one plunger had the slightest score mark on it and also I found this:
Not sure if this would cause a problem.
Side note: I did a 'compression test' on the barrel/plunger assemblies on a few of them (including the slightly scored one) by putting a drop of oil on each plunger tip and plugging the hole on the back side of the barrel then pushing the plunger down using finger pressure and they all leaked a little and I could see bubbles coming out around the plunger. The worst one was the rebuilt Dipaco I bought when I redid these. I probably should have just kept the original for that injector but it had ever so light scoring and I figured why not replace it.