* Air in the fuel system: This is the most common reason. When the fuel runs out, air enters the fuel lines and pump. Diesel fuel pumps rely on fuel to lubricate themselves; running dry can damage the pump. The air in the system prevents the fuel from reaching the injectors, thus preventing fuel injection and combustion. Getting the air out requires bleeding the fuel system, a procedure that varies depending on the engine.
* Damaged fuel pump: As mentioned above, running a diesel engine dry can damage the fuel pump due to lack of lubrication. This damage can be minor (requiring bleeding) or catastrophic (requiring replacement). A damaged pump won't deliver fuel effectively, even after the air is purged.
* Fuel filter blockage (though less likely immediately): While running out of fuel won't directly clog the filter, the process of getting air into the system might introduce debris or contaminants that could further exacerbate the problem.
* Low battery: The starting process requires significant power, and if the battery is already weak, several attempts to crank the engine (while bleeding the system) could drain it completely, leaving you unable to start.
* Injector issues (less likely immediately): While unlikely to be the *immediate* cause after simply running out of fuel, prolonged running dry could lead to injector issues due to lack of lubrication.
In short, the primary problem is usually air in the fuel system, but the lack of fuel itself can lead to other complications making it difficult or impossible to start. Always refuel before running completely out of fuel to prevent these issues.
