Truck wont start

[femaff]

Hi im new to the site and posting this for my dads 2000 Powerstroke. His truck wont start unless it is plugged in, the other day it was 50 out and not plugged in so he had to use ether to start it. I told him it had to be a problem with the glow plugs because his gauge never drops when the WTS light is on it goes right up to around 14 and never moves. We checked the relay with a test light and it was kicking in i didnt however check the voltage on it. He took it to a shop and they said all the glow plugs are working and they checked the compression 7 cyl ranged from 320-390 and one was at 290 and they believe he has 3 injectors that arent working right not sure how they tested them though. He is going to have them test the voltage on the relay and let me know but does this sound like an injector problem or glow plugs are something else all together. The day it was 50 it acted like it wasnt firing but if its plugged in it fires right up. I posted this in the 911 section as well not sure where it belongs.

thanks for the help.