I would change both fuel filters,even fresh ones can be contaminated with bad fuel. I have...
I would change both fuel filters,even fresh ones can be contaminated with bad fuel. I have had project cars that worked OK quit in the driveway needing fresh fuel filters. A dirty air filter will cause smoke too and loss of power.
Do the easy things first. Next consider a synthetic oil of still not doing well. Mobil 1 will free sticking rings along with Italian tune ups.
Next if valves have not been adjusted within 15K do them. Time consuming the first time but gets to be easy and fast. Takes two 14mm wrenchs. Some of the cylinders can be awkward to do with out the special curved wrenchs. I bought mine for about $40 on ebay.
Some pals have bought cheap 14 mm wrenches, heated them with a propane torch and bent them in a vise.
No for the the injectors and fuel pump. If still misbehaving run Diesel Purge thru it. It doesn't go in the tank but use a hose to connect to a jar and the pump draws the powerful solvent into the IP pump and through the injectors. Injectors are pricey and can wear out to.
You may have a bad IP Pump, but they do tend to be quite hardy. Good fuel filters also protect the IP pump and injectors.
A seriously worn diesel can run quite well with just good oil, and filters and even have surprising pep.
Good luck.
John Gregg