adi m, if you look at this truck's wiring shcematics and diagram it will give you the answer as to why the engine is spinning but not starting. When the starter is engaged a great deal of current is removed from the rest of the vehicle. This is shown by the voltage drop. Basic electrics it is the amount of current or flow of electrons that is needed to do work. To test my hypothesis you can spray a shop cloth with either and place it over the engine's air intake and allow the engine to just snif it while it is cranking and if the engine starts or just tries to start your problem is fuel related. However, if it doesn't attempt to start, the problem is electrically related.