Have you seen the truck? Can you get a picture of the motor? Can you run the VIN through the above link? Can you call the guy and confirm the needed part? If that is what he says it needs and it is a PSD, I would tell him I will do you the favor of giving you $500 for the vehicle. b/c he obviously doesn't know what he has.

Normally the fuel (or lift) pump will leak like a siv or wont fill the filter bowl or won't create enough pressure.

Understanding how the injectors work on the PowerStroke engine can help in diagnosing a concern with this engine. Older diesels used a hydraulic injection system in which fuel pressurized by the injection pump would actuate the injector. The drawback to this system is that any air which enters the fuel lines will affect the operation of the injectors, or even prevent them from operating. Also, the amount of fuel injected is dependent on the mechanical operation of the injection pump governor, which adjusts volume based on engine load/RPM. Gasoline engines with electronic injection use a pressurized fuel system and the computer varies the actuation of the injector based on input from various sensors in order to control the amount of fuel to the cylinders. Since gasoline engines have an ignition system to ignite the air/fuel mixture in the cylinders, fuel pressure only needs to be sufficient to supply the injectors and provide an adequate spray pattern to ensure efficient combustion. But a diesel engine uses heat from compression to ignite the air fuel mixture, and this high compression requires high injection pressures. What has been done on the PowerStroke is both of these systems are used in conjuntion with each other. Fuel is supplied to the injectors through fuel rails inside the cylinder heads. Also supplied to the injectors is high pressure engine oil. As the computer determines that a cylinder should fire it signals the Injector Driver Module.

The IDM sends a 110 volt pulse-width modulated signal to the injector solenoid. When the injector solenoid is actuated, it opens a poppet valve which allows high pressure oil to flow into the intensifier piston. The intensifier piston is forced down, pressurizing the fuel inside the injector. When fuel pressure inside the injector reaches approximatly 2700 psi, it causes the injector pintle to rise off its seat and fuel is injected into the cylinder from the nozzel. As long as the poppet valve is open and oil is flowing into the injector, fuel will be injected. The computer controls how long the injector solenoid is energized (pulse-width, or time on in milliseconds), but it also determines the pressure of the fuel being injected by controlling the pressure of the oil (IPR duty-cycle, or the percentage of time on vs. off--AKA dwell) in the cylinder heads. The computer determines this based on engine load and driver demand by monitoring various sensors. Since the cavity at the top of the intensifier piston is seven times the size of the fuel cavity at the bottom, fuel is injected at a pressure seven times that of the computer-controlled oil pressure--oil pressure 3000 psi = injected fuel pressure 21000 psi. Due to the high oil system pressures, the spring which closes the poppet valve once the injector solenoid is deactivated has to be very strong--and because of this, the solenoid needs to be 110 volts. Once the poppet valve is closed, spring pressure returns the injector to its normal state and the oil is exhausted into the valve cover area to return to the sump. Because of the nature of how this system operates, air in the fuel is not as great of a concern as air in the oil. The PowerStroke requires a special anti-foaming agent in its oil to prevent this aeration. Oils with an API service rating of CF-4 or CG-4 already have this agent, but it becomes depleated as the oil breaks down, so regular oil changes (3000-5000 miles depending on vehicle use) are necessary. The anti-foaming agent can also be depleated by interaction with some silicone sealers. Split-Shot OperationSplit-shot injectors were originally installed on 1996 and 97 model/year trucks with California emissions, and are used in engines from 98.5 on. These injectors prolong the injection time to decrease emissions without reducing power. Fuel is delivered to the injector (green) past a check valve in the same manner as in the standard injectors. As the intensifier piston is forced down the fuel is pressurized (orange) and the check ball (blue) is lifted off its seat and fuel injection begins. Cut into the piston is a land (yellow) which receives fuel through bleed holes (red) as it is pressurized. As the piston travels down the land aligns with a port in the injector. When this happens, pressure drops below the piston and the check bal reseats and injection is suspended. As the piston travels further, the port in the injector is covered and fuel injection recommences.

You will be tested on this later!