If your van is starting and running ok try this.
My van has run on three cylinders on and off for years, puffing out smoke on cold start up's etc.
I did the usual, crack the injectors as per Haynes manual etc to no avail.
A month back after becoming used to the intermitent rough running she just started running smooth all by herself!
At the same time I notice she was leaking fuel, and after a brief investigation under the hood I found my fuel filter drain tap (on the bottom of the filter) had worked loose and was leaking a small amount of diesel!!
The Haynes manual tells you to check the injectors with the engine running, but it never says air can get trapped in the actual filter, which it clearly must have done with my old girl!
Try loosening off the drain tap a little (obviously put a container underneath) and then run the engine for a while to simulate driving and see if that clears it.
Might work, might not, but this has worked on my van even if she did it all by herself with no help from her mystified driver!!