i have an 85 f-250 that just recently lost power.  it starts fine but acts like its running on 3 cylinders. i just checked the glow plugs and needed to replace 3 of them also just replaced the fuel filter. when i replaced the glow plugs and ran it, it seemed to get some of the power back but only for a day and is still blowing a lot of blue smoke.  the truck has 73k miles and well maintained. do i have air in my injector lines or water in the fuel. any help would be greatly appreciated thanks  

Sounds like one or more of your injectors might be going out, or have already gone out. I don't know how you can individually test the injectors other than pulling them out & visually inspecting them. Anyone have any tips?  

It's your injection pump. When they go they will usually run OK till the pump heats up, then they loose power. The truck will blow huge blue smoke when under deaceleration...like backing off the throttle going down a hill. There not hard to change out if your mechanically inclined. When you get your new pump it's usually a good idea to do the injectors as well. I cannot stress this enough...USE A FUEL LUBICATOR!!! I used Howes in my idi's and would get 3 time's the service life out of my pumps.