I was driving down the interstate and lost all power to the engine. The battery tested low but was good. I tested the spark plug wires and am getting good spark. Checked the fuel access valve located int he fuel line before it goes into the fuel injection, and I'm getting fuel there. I haven't had the alternater checked yet ? Back yard mechanics friend of mine said it sounds like a relay problem....possibly not ltting the fuel into the injectors ? Any ideas ?

You need full batery voltage.
Recharge and check battery.

On fuel pressure you need a fuel pressure gauge to check for full fuel pressure.
Like 60/66 lbs.
What is your reading?