So my F350 had some troubles starting, was informed batts and alt was bad, replaced all and truck ran great for a few days. Ran errands one day, sat for about 3 minutes, would not start. Roadside assistance started it with a quick spray of starting fluid in the air filter. Replaced fuel filter as well and had glow plugs tested/relay all fine. Mechanic basically said, I have no idea, guess you will just have to keep driving it till a code shows up since none popped up upon diagnostics. If it sits it will start with the fluid sprayed into the filter otherwise it just turns over and over. If I start it in the morning using the fluid it will continue to be fine and start for most of the day. Extremely tempermental and not related to plugging it in....not sure where to go next??

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check out this thread and see if you can figure anything out that might help Ford Powerstroke 99-03 7.3L [Archive] - Diesel Bombers

 

Sounds like a Glow plug issue...

First off, DO NOT ALLOW ANYONE TO SPRAY STARTER FLUID IN A GLOW PLUG EQUIPTED TRUCK... You can and will do damage to the motor.

Check the glow plug relay again and the entire glow plug system.

Need to find someone who can scan it as you crank, check ICP pressures and duty cycle.