got a "89 B250 conversion van w/a 318 TBI.... wife ran the tank a lil low a month or more ago and it was running like dookie... I changed the  fuel filter, added "sea foam" injector cleaner before a fill-up then again after a few weeks.. figured maybe a basic tune-up might work (cap, rotor, plugs)... The coil was replaced about 2 years ago as was the pick-up... initially from stop to go its like I'm a first time driver w/a stick shift "bucking" then she's fine... I did notice fuel mileage has dropped significally... Did my wife just run her way too low and it needs a "professional fuel injector cleansing" or could it be something else?? Mind you, the cap, rotor, and coil were bought from a large chain discount autoparts store... "HELLLLLPPPPPPP"  

IMO 2 yrs warrants a tune up. So do this first then look in to changing the fuel filter again from all the crap that probably dislodged itself from the tank especially if you run it low a lot or stored it for any long period of time, then possibly fuel pump, pick up hose from tank to line and so on. Let us know.  

I think I would do a compression test to see were the cylinders are at. They should all be within 10% of each other.