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I have a 2003 f250 with the 6.0 in it wasn’t turning over very good one day acted like dead batteries so I put the trickle charger on it for a day still wouldn’t start batteries where old so I figured probably bad batteries bought two new batteries at Napa this morning and still doing the same thing turning over slowly but the batteries never actually go dead the truck just continues to turn over slow any other ideas?
Starter or wiring would be my guess as ford390gashog said. Fresh batteries should be cranking the engine whether it fires or not.
I'm not sure about the 6.0L, but mine has certain requirements to fire the HEUI injectors (voltage, RPM, and high pressure oil pressure minimums). Turning over slowly will not get me to the oil pressure or RPM necessary for the truck to start.
Replace the battery cables. I’ve cut open battery cable sheathing and seen tons of corrosion/acid/complete crud, and that stuff creates plenty of resistance, as well as overall drain on the battery (in some instances).
And clean up the ground point for the negative cable.