Reading the streams on this and other forums, it seems that about the only two things we know for sure about the BOSCH VP44 is that they cost a bunch of money and, if you don’t run a FASS Titanium, they’re gonna burn up. For some folks, it seems that is enough; maybe it is. The VP44 sticky on this forum gives a lot of information about what the VP44 does, but precious little on how it does it. Several years ago, I decided there must be more to them than this.

VP30, VP44, for Cummins Diesel OEM Part number Bosch # Bosch VP30 Cummins Injection pump, 3.9 L ISB, 4.5 L QSB 3965404.

Here are my conclusions after studying the Bosch VP44 Service Manual (Rev 2000), the Bosch Distributor Type Diesel Fuel Injection Pumps publication (Issue 2003), the 2001 Dodge Ram Service Manual, and a VP44 Video CD that I bought on-line several years ago. I have also carefully read and filtered information from a number of forums.

Information taken from Mopar1973man’s actual tear down of a VP44 and his accompanying photos and discussions were very helpful. There is an outstanding, detailed description of VP44 internal operation on YouTube. Unfortunately, the audio is in Spanish, but it pretty well backs up my description of its operation. Although the VP44 is a precision device manufactured to extremely close tolerances, it’s not a magic box and the basic mechanics that allow it to accomplish its job are easily understood; it’s the control system that makes it more like rocket science. There is a lot more detail to the operation of VP44 than I cover here, but here are the basics: The Bosch VP44 is an electronically actuated, solenoid needle valve controlled radial piston Distributor Fuel-Injection pump.

Basically it consists of: 1. Low Pressure Stage: Internal feed pump (vane pump), Pressure Regulator, Accumulation Chamber, and Overflow Valve. High Pressure Stage: Radial Piston (plunger) High Pressure Pump, Distributor Shaft, Solenoid Valve, and Delivery Valve.

Timing Control: Angular encoder, Timing Ring, Timing Piston and Timing Control Valve (TCV) 4. Electronic Control Unit (also called the PCM (Pump Control Module)).

This unit controls the volume and timing of each fuel injection by the VP44 If treated right, it will last for a long time. I bought my truck new in the autumn of 2002 and it is going on 187000 miles and 13 years; others have gone a lot more than that.

Here we go: The High-Pressure Pump: The rotating parts of the VP44 are gear driven at camshaft speed (1/2 engine crankshaft speed). It is mechanically ‘fused’ so that anything that locks up the distribution head will not destroy the drive gears. The VP44 uses a 3 roller (cam followers)/6 lobe cam ring (one lobe for each cylinder) cam-driven radial piston high- pressure pump to compress fuel to a pressure sufficient to open a fuel injector. All three rollers (arranged 120 degrees apart radially) are cammed in at the same time, compressing fuel by use of cam followers and pistons (Plungers) in a central compression chamber (Plunger Chamber). This pump can raise pressure to as high as 1500 bar (21,750 PSI). The fuel is distributed to each cylinder in the desired sequence by the rotation of a shaft in the distributor head. This distributor shaft contains passages that determine where the fuel comes and goes in the distributer head and which cylinder will be fueled.

The sequence of the VP44 outlet is consecutive in a rotating manner (A-B-C-D-E-F); the sequence of the cylinders that the fuel goes to is determined by the piping arrangement from the VP44 output (1–5–3–6–2–4). One revolution of the distributor shaft delivers fuel to each of the six injectors (two revolutions of the engine crankshaft (four stroke engine)). The rollers run inside a movable ring containing the lobes and whose angular position is controlled by the timing plunger (piston). The ring is moved angularly to advance or retard the timing, depending on the desired engine operation (fully retarded for starting and advancing as needed to provide desired timing for proper engine operation). The angular position of the ring (and thus the timing) is also affected somewhat by the feed pressure to the VP because the feed pump (lift pump) pressure connects to one side of the timing plunger and timing solenoid that control the position of the cam ring. Because of the low RPM during cranking, it is desirable to have the timing fully retarded (thus low feed pressure).

This is why a properly controlled feed pump runs for a second or so to prime at key on, shuts off, and then runs at reduced (25 percent) pressure during cranking, finally going to full pressure following startup. It’s also why some folks with higher pressure relay-controlled aftermarket fuel pumps sometimes experience hard starting when the engine is hot.

I believe this unintended consequence occurs because the relay which controls the aftermarket pump cannot respond to the ECM method of command to reduce pump pressure and the pump stays fully energized, thereby keeping fuel pressure up and not allowing the timing to properly retard. By design, the ECM pulses the voltage to the fuel pump during cranking rather than supplying a solid 12 volts. I think the OEM pump can see this as a reduced voltage, but the relay associated with the aftermarket pump doesn’t. Some folks believe that, even with reduced pressure, the aftermarket pumps produce too much pressure during cranking. Take your pick (maybe it’s both).

But I digress. Low Pressure System: Now that we have seen how the high pressure pump works; all we have to do is get fuel to the high pressure compression chamber (Plunger Chamber) to be pressurized by the roller cam followers and plungers. Although the VP44 has an internal feed pump, in the case of our Dodge Ram/Cummins Diesel, fuel is supplied to the VP44 input by an external feed pump (lift pump). There will always be an argument on how much lift pump pressure you must have for proper VP44 operation.

The Bosch manual states “Cars, light/medium duty trucks and tractors, where the fuel tank is near the VP, may not need any more flow than the internal fuel pump supplies. Equipment where the fuel tank is a longer distance from the VP may need an external feed pump. Stationary equipment and tractors may have a head tank supplying pressure.” I really don’t want to get back into this argument; just stating what Bosch says (I’ve got to believe they know something about it).

My thoughts are that the Dodge Ram/Cummins, because of its sometimes unique uses and high horsepower requirements, may throw additional variables into the equation and thus require added input pressure to ensure the required volume is available. For instance, on a 45 degree incline, the internal Feed Pump would have to overcome about 4 PSI because of the location of the fuel tank and the weight of the fuel in the lines. I’m sure there are other variables also, hence the Ram Service Manual requirements for a minimum of 10 PSI lift pump pressure (minimum of 7 PSI while cranking (although one manual states maximum fuel pressure of 7 PSI while cranking—which, because of the effect on timing, really makes more sense)). Onward: The first thing the incoming fuel sees in the VP44 is a vane-type fuel pump (called by Bosch the ‘Internal Fuel Pump’) which raises the pressure to “8 – 22 bar (116-319 PSI) depending on the application and RPM” (direct quote from VP44 Service Manual and Bosch Fuel Injection Pump Manual). A pressure control valve at the output of the vane pump bypasses fuel back to its input to control the desired vane pump output fuel pressure and varies depending on RPM.

This is a pure spring-loaded mechanical devise; it only depends on vane pump pressure, it doesn’t depend on the VP input pressure nor any electrical signal. The amount the pressure control valve opens depends on how much pressure the vane pump is producing. The vane pump is a positive-displacement pump; therefor its output flow increases pressure as RPM increases. From here on, as long as there is a sufficient volume of fuel available at the VP44 input, it doesn’t matter how much pressure the external feed pump (lift pump) is producing, the VP44’s internal pressure will be controlled at design pressure by the vane pump and the pressure regulator. Since the vane pump output pressure increases with RPM, the amount of fuel bypassed varies to allow a desired, controlled pressure which is dependent on engine RPM (the higher the RPM, the higher the pressure). This higher pressure is necessary in a radial piston solenoid valve controlled injection pump (VP44) because the same cavity of the high pressure pump (the Plunger Chamber) must be filled to a design pressure for each injection, unlike the in-line injection pump which fills each of six cavities in sequence. The higher the RPM, the less time there is to fill the Plunger Chamber, therefore the higher the pressure must be to assure proper filling of it prior to the next injection.

The vane pump output is fed to an Accumulation Chamber which is isolated from the VP44 inlet pressure by a diaphragm. The purpose of the diaphragm is to absorb (or dampen) high pressure fuel spill pulses (we’ll talk about that later).

Some folks say this diaphragm doesn’t exist; I say it does; so does Bosch. Mopar1973man’s tear down of a VP44 doesn’t show a diaphragm looking exactly like we thought it should be, but does show a similar steel-backed plastic-like unit located in exactly the same position as Bosch drawings show the diaphragm.

I say this is the diaphragm; so does Bosch. It performs the same duty as the old style diaphragm (else, why the plastic-like material). I do say that, from the way it is now constructed, it isn’t likely to rupture (as the original designed diaphragms were prone to do). There has been a lot of confusion about this diaphragm. The VP44 has been around only since 1996; Dodge started using them two years later. It appears that, in the early VP44s, this diaphragm was under-engineered and prone to rupture and was blamed for many failures of the VP44 (mostly galling of the timing piston and liner by feeding pulsating pressure back to the piston chamber whose liner was also under-engineered (wrong material)). Later revisions changed its design, giving the diaphragm a solid steel backing which makes later VP44s much more dependable and has pretty much eliminated diaphragm failures.

The diaphragm material, however, still has to absorb the energy from the high pressure fuel spill pulses (again, we’ll cover these later). The pressure spike it must absorb is initially very high (injection pressure), but the volume is very small and the diaphragm has lots of surface area. I’m not saying that the steel backing will flex; I am saying that the diaphragm material attached to it will still dampen the high pressure pulses. And it’s gotta do it very fast to get ready for the next event (less than 0.02 seconds at 2000 engine RPM).

Later revisions to the VP44 also changed the lining material of the timing piston cylinder. These two major revisions plus the use of better solder in the ECU and closer attention to surface finishing are probably the main reasons newly re-manufactured VP44s are much more reliable than the early ones.

Let us continue: The fuel in the accumulation chamber supplies fuel to the distributor shaft of the VP44. Inside this rotating distributor shaft is a cavity and passage to the high pressure pump Plunger Chamber. Each time a fuel injection event to the engine is desired the Plunger Chamber must be filled from the accumulation chamber and then isolated from it to allow the fuel to be pressurized. Since I got cut off because I was too wordy, I'll add this as a comment: My thoughts on keeping the VP44 Happy? Good, clean fuel. Whenever you can, buy your fuel from a trusted dealer.

Locations that have a high turnover of fuel are best. Find one in your location and stick with him. Stay away from the low-volume stations. Also, when you fuel your truck, make damn sure you’re putting in diesel fuel. Brain fade at the fueling station will cause immediate severe heartburn. Assume that even the most conscientious dealers do not add any more lubricant than the code requires and that mistakes can be made. Knowing the extremely close clearance tolerances built into the VP44, I’m a lot more concerned about lubrication than I am about cooling.

If the engine is running normally, you’re getting cooling through the overflow valve and injector return. Add some lubricant of your own at each fill-up. My personal favorite is TCW3 two-cycle motor oil because it’s cheap and so am I; and it’s always available. There are others. Do a little research. Change the fuel filter on a fairly frequent time schedule. Time and your own experience will tell what that schedule should be.

Changing out a clean filter is a waste of money, but not changing a dirty one restricts fuel flow. Run on the top half of your tank, especially during hot weather. More volume in the tank makes a better heatsink for heated fuel returned from the injection pump, injectors, and most aftermarket fuel pumps (most high volume/high pressure aftermarket fuel pumps recirculate a lot of fuel to the fuel tank).

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More fuel tank volume in the winter means less volume of air for condensate to accumulate. Besides that, it does away with the one-quarter-tank/out-of-fuel phenomenon that confounds some of us (including me). Don’t let your truck run out of fuel. Getting it running again can be a real PITA and usually requires bleeding the fuel lines, and, while you’re attempting to start it, the VP is not getting proper lubrication.

That’s probably not a good thing. And, yes, get an in-cab pressure gauge, so you can watch and worry about your fuel pressure. If you truck stops running, the first thing you will want to know is if you have fuel flow, and the first thing the forum members will want to know is what the fuel pressure was.

Run your pressure above 15 PSI because everybody says to and it’s a good safe number and will give you a warm, fuzzy feeling. But if it gets down below that, don’t turn off your truck and walk home.

If the engine is running, you are still getting enough fuel for cooling and lubrication. It will get you home. Also, if you are going to run your fuel pressure above 20 PSI, remember that any fuel that leaks past the front seal of the VP is going indirectly right into the engine pan; diesel fuel is not meant to lubricate the engine. Don’t panic if your fuel pressure drops below the magical 15 PSI during a hard load. Pressure does not cool the VP44; flow does.

The pressure at the VP44 inlet drops during a hard load because flow increases. Any flow going through the VP44 cools and lubes it, whether it is going to the injectors or back through the overflow valve. Increased flow means increased cooling. Think about it. Buy a decent fuel pump. Sufficient volume is essential, as is enough pressure to give you that volume, but there are limits to what you really need.

Running a 1000 GPH lift pump in a daily driver doesn’t do anything but heat up fuel in the tank; it doesn’t provide any extra cooling to the VP44. Reliability is what you are after.

Contrary to popular belief, you don’t have to invest in a $650 fuel pump to get something that works; there are other options; again, do some research. If you are running a lift pump which has no pre-filter arrangement, install an in-line pre-filter to protect it. Fuel (lift) pumps are devices engineered and manufactured by man; even the best can fail, but I believe that any pump that is mounted on the hot, shaky engine block is going to fail sooner than one mounted near the fuel tank on the frame. Don’t immediately shut down your engine after a hard pull. Let the engine idle a few minutes to get the engine compartment temperature down and allow a little further cooling of the V44 and the turbocharger. An exhaust gas temperature gage is handy for this. I like to have my exhaust temperature below 350 degrees before shutting off the engine.

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