Go to Walmart, or most auto parts stores, and you will find Diesel Kleen by Power Service in the grey bottle. Diesel Fuel Supplement, white bottle, is ok, but the grey bottle is like shock therapy for your diesel fuel system. Pour in a whole 32 oz bottle in a tank, drive it normally for maybe half a tank, take it out and run it like hell, but just be sure you have an extra fuel filter. Might work TOO well.

OTOH, you could just use 1-5 gallons of B100 Biodiesel. I had a friend who ran B100 in a TDI for the first time, at 60k. Fuel filter was clogged with dis-lodged gunk after 20 miles. Best detergent ever.