I have a '90 D250 that's bone stock and has been progressively getting harder to start. A couple weeks ago, it wouldn't start at all and left my dad stranded in Warren. We've been able to get it to fire on starting fluid, then it idles, runs and drives perfect. No excessive smoke (white or black) while cranking or driving. The fuel filter has been replaced along with a new mechanical supply pump. I've had a "Cummins tech" look at it and he's telling me that the injection pump needs rebuilt. I'm having a hard time believing him because the truck runs perfect once it fires, again, with no smoke. I recently installed a clear in-line filter on the suction side of the supply pump. When the engine is running, you can see bubbles suck through the filter every now and then. Sooo...could air leaking into the supply lines prior to my supply pump be the culprit, or do I really need to tear into the injection pump?