Attention: TDR Forum Junkies 
I picked this up from another source - good reading from guys who should know...
HOW YOU CAN DIAGNOSE THE VP44 FUEL SYSTEM ACCURATELY
FROM BLUE CHIP DIESEL
If your truck "Bucks" under hard load or towing this is an indication the engine is probably starving for fuel. To test fuel delivery or pressure, install a fuel pressure gauge with a long hose on it after the fuel filter and before the injection pump. The long hose allows you to drive the truck and watch the gauge at the same time! You can install our “Low Fuel Pressure Warning Kit” and it will diagnose low fuel pressure too. To diagnose lift pump performance click the ignition key to the start function quickly, so the engine doesn't start, and let go, leaving the key in the run position; the lift pump should run 25 seconds. If you don't hear the lift pump, test for 12 volts going in to it and if it doesn't run with 12 volts going into it, replace it. If you do hear it run and it doesn't make at least 5 PSI replace the fuel filter. If, after changing the filter, it DOES make at least 5 PSI go DRIVE the truck UNDER LOAD. If it doesn't make at least 5 PSI after changing the filter, or if you have to pressurize the fuel tank to bleed the system to get the truck to run, then change the lift pump. Revving it up proves NOTHING. If DRIVING UNDER LOAD the pressure drops below 5 PSI, replace the fuel filter (if you haven't already done so) and if that doesn't fix it, you need a new lift pump if the fuel lines aren’t rusty and or sucking air. We proved on a dynomometer in 1998 that if you have 5 PSI, under load, you can make all the power available from a VP44. We do not recommend running more than 12 PSI or you will diminish fuel delivery to the rotor and make the truck run worse at high RPM and possibly overheat and damage the fuel bypass solenoid. This diagnosis is only for the lift pump, but is necessary for the successful diagnosis of injection pump issues. If you don’t have enough lift pump pressure it will cause low power or bucking under load. All other drivability issues usually are caused by the injection pump.
If you have an intermittent "Dead Pedal" this can be caused by either a faulty APPS (Accelerator Pedal Position Switch) or a faulty computer on the VP44 injection pump. Since the APPS is only $460. 00 from D/C and it is cheaper than a replacement injection pump, we recommend getting someone to "scan" the ECM (not the PCM) in your truck to check for any DTC codes pertaining to the APPS. If there are none pertaining to the APPS you DON'T need an APPS, as a bad APPS always displays a code. If you don’t have access to a scan tool, the difference between a bad APPS and an injection pump is that an APPS usually is just a flat spot at a certain throttle opening, usually 65-70 MPH and smacking the pedal a few times usually clears it up. This usually is worse during cold and or wet conditions. If it is an injection pump the “Dead Pedal” is dead at all throttle positions and may reset and play if you let the pedal go to idle for a brief time and reestablish “Idle Validation”. This one is most often when hot, but sometimes right after start up when cold.
If you see the code 216, or if there are injection pump codes, or even if you have NO codes for the injection pump AND you have ANY of the above listed drivability complaints, YOU NEED AN INJECTION PUMP! Code 1693 only means there are codes in the other computer, which have NOTHING to do with the fuel system.
There are basically only two other components to the fuel system in a VP44 fueled truck and they are the ECM (Engine Control Module) and the injectors. Neither of these give any trouble typically. In 13 years of doing ONLY Dodges every day I have never seen or even heard of a bad ECM and or injector. It is highly improbable that either component could cause any of the aforementioned drivability issues. Symptoms are different for these components.
The other situation that is pretty easy to diagnose is when the truck dies driving down the road, for no apparent reason, or when you let off the throttle, especially at high RPM. This is usually a seized rotor in the injection pump and most common on 1998 and 1999 trucks. The cause of this failure is a poorly "deburred" rotor which seems to have been mostly limited to those years of manufacture. If you run any year truck out of fuel at high RPM or heavy load you CAN seize the rotor also! If this happens to your truck and you want to diagnose it simply, positively and accurately, do the following. Loosen three injector lines at the valve cover. Crank the engine a few times for 30 seconds and if fuel only comes out one line or none of the lines, this indicates a seized rotor. I know this because we proved you can not put this pump in a hydraulic lock, so therefore fuel has to come out of an injection line even if the rotor is not turning. If you have fuel coming from one line only that is where the rotor is stuck. The feeble flow is due to having only lift pump pressure moving fuel. If you have no fuel from any line opened then it is stuck at one that is closed. If you get fuel from all three lines you must bleed the engine to get it to run again and then you have to figure out why it ran out of fuel.
What the common VP44 problems are….
The housings on the VP44 wear out due to low fuel pressure from weak lift pumps causing the diaphragm in the front of the VP44 pump to rupture. This causes the steel timing piston to vibrate in the aluminum bore of the housing and the result in a short time is the housing wears to the point that fuel bypasses the piston and full advance cannot be accomplished which causes the code 216. This makes perfect sense to me as it explains why as 24 valve trucks get older the fuel mileage goes down steadily, and when we replace the injection pump with one that has a new housing we get the mileage back! In a typical competitor's rebuild, if the case isn't worn out completely and the timing can be reached on the test stand then it passes the test and a partly worn out case gets to the customer, just to fail sooner. It should also be pointed out that the replacement housings we use, have an insert for the timing piston, which the original housings did not have. The code 216 only tells you that the housing is worn out, but does NOT cause any drivability issues, other than lost power and fuel mileage. As these symptoms come on slowly the driver isn't aware of the change until they drive one of our replacement units and get back the power and mileage.
The other component that causes almost all of the drivability issues is the computer on the top of the injection pump. The computer gets intermittent because of too many heat cycles. When you shut the truck off the latent heat in the engine heats up the computer and after many heat cycles the solder that holds the electrical components to the circuit board of the computer becomes crystalline and no longer makes a good electrical connection, causing intermittent drivability issues, such as "dead pedal", intermittent hard cold start, intermittent hard hot start and white smoke. These drivability issues usually get so bad that the customer finally takes the truck in for diagnosis, only to find none of these issues create a diagnostic code or a so called DTC. This makes it hard for the inexperienced mechanic to advise the customer honestly or accurately.
