[Tommygunner]

Maybe you just need to take it out for a nice long drive. Short trips are hard on a diesel engine. They like get fully warmed up and taken on a good haul.
Your injector nozzles may be carboned up and have bad spray pattern. If over 150K miles on the injectors. They are pretty much at the end of their service life. Should be rebuilt or replaced.
You should be using a diesel additive every fill up also. USLD is hard on older fuel systems.

 

[Tommygunner]

You really should use additive to your diesel fuel. It is not just to keep the combustion chamber clean. Altho it does help with that.
Diesel additives add lubricity. ULSD is a very dry fuel. Without additive the IP and injectors do not get enough lubrication from the fuel. This can cause wear and premature failure. Because the fuel system was designed to run on older higher sulfur diesel formulations that lubricated better.
I use Diesel Kleen in the grey bottle. It is economical, and provides satisfactory lubricity at recommended mixture.
It also adds more cetane which helps the fuel/air mix combust better.
That is a nice looking old van. Glad your problem is clearing up.

 

[Tommygunner]

After running Seafoam thru it with no better results. I think new Injectors and Glow plugs should fix your problem. Glow plugs wear out over time. as do injectors. As i said in previous post USLD is hard on injectors. After replacing yours. Be sure to run an additive to keep injectors and the injection pump lubed properly.
6.5s when running good, are a clean running engine. A little puff of grey smoke when first starting in the morning is normal. It should clear right up.
It should not smoke the way you describe. If my truck smoked like that. I would be on it fixing the problem.
With your business name on the side. A good clean running service van leaves a good impression with folks.

 

[Tommygunner]

A little knocking after opening up the fuel system is normal. Its just air working its way thru.
There is a small screen/ filter under the fuel filter on the bottom of the fuel manager. Did you check and clean that also? It is important that it is clean.