Here is a copy/paste from a thread that I had started years ago with the exact same symptoms and the solution.
An update on this, if anyone happens to be interested.
I needed to find the bad injector, but I did not want to remove them all and send them out to be tested. My thought was that I only wanted to remove the bad injector, not the other 5 that seem to be good. Plus I didn't want to spend more money than I needed to, and the cost of the tests would add to the bottom line.
I know that many folks recommend replacing all 6 injectors. And that is probably prudent. But I don't have the spare cash to replace 5 working injectors.
I borrowed a friend's Gensys scanner and a cylinder contribution test and a cylinder kill test was fruitless for me.
Then I thought that I would remove the exhaust manifold to see which exhaust port in the head was the wet one. Well, one bolt's head near the #5 cylinder and on the bottom tried to strip. So I abandoned that idea.
Then I thought I could just remove the wires from each injector, one at a time until I found the offensive one. I quicly abandoned this idea too because I realized that my problem was an injector that was stuck open. Whether the wires were removed from the injector or not, fuel was going to continue to flow through it.
So I ordered this fuel rail pressure test cap from Genos https://www.genosgarage.com/product/lt-testca/tools
. Then I capped one injector at a time at the fuel rail. I started with the easy ones, 1,3,4, and 5 first.
After capping #5 the smoke stopped and the knocking stopped and the engine ran quite well.
I had already bought a NEW injector from Engineered Diesel and had it onhand. I replaced the #5 injector and the crossfeed tube following instructions on this forum and the truck is running perfectly now.
I checked codes just for the heck of it, and it is showing P0201. I'm going to clear it and see if it comes back.
Thank you all for all of your help. It is greatly appreciated and explaining what I did and how I did it is my way of trying to help someone else who may have the same problem and finds this thread in a search.
END OF COPY/PASTE
The above post is from 17 APRIL 2015. Most guys on this forum predicted that the other five injectors would soon fail if I didn't replace them. Well, I am still driving them and the mpg has not changed and the truck still has as much power as it ever had. That was 4.5 years, and 55,000 miles ago.