The main problem causing failure in those lift pumps mounted on the fuel filter is that they are pusher-type pumps, intended to be mounted back near the fuel tank, pushing the fuel up thru the filter - they don't last long in pull or 'draw' type service - it was a double fiasco with the VP44 injection pump because a steady supply of fuel was required for cooling to prevent the electronic driver module from self-destructing - as the lp failed, so did the VP-44

 

It's a fuel lubricated roller vane pump. The motor brushes are what fail.

As long as the pump has sufficient suction head to get the fuel to itself it should be just fine and they don't seem to have any problem getting the fuel to themselves. These pumps were designed to mount on the filter canister at the engine. The pump motor sees the same load pushing or pulling. All the pump does is create a pressure differential and that is the load the motor sees.

 

Yet the point stands - those pumps function better as a pusher rather than a puller... better to blow than suck.

In any event, they aren't relevant to a discussion of the CP3/in-tank LP system.

The CR CTDs are more likely to stop running from insufficient voltage supplied to drive the injectors than to quit from a disabled in-tank LP.

 

It may be capable of drawing fuel in certain circumtances but not in a Dodge truck. The CP3 does not stop pumping due to
lack of fuel, it still pumps irregardless. The engine will run and drive with as little as 2-3k of rail pressure. The COV
is responsible for routing fuel to rail, return, and lube. The lube fuel is a fixed orfice that circulates all the time
given adequate pressure. Since the rail orfice is much bigger it will take all the existing flow to feed the rail while
starving the lube circuit. Any available fuel will always be routed to rail first and the engine will continue to run. Run
it long enough under these conditions and CP3 eats itself up due to lack of lube and cooling. The minimum pressure needed at
the CP3 to supply adequate fuel for lube and injection for a stock Dodge implementation is roughly 4 psi. Ignoring the facts
doesn't change this. Lots of worngs in that statement for sure.


Oh, your one of THOSE that can't get the relationship between pressure and flow. Very simply put: no pressure equals no flow. Not
gonna change the physicsof that. We CANNOT adequately measure flow at the CP3 but we can measure pressure
to calculate flow. 4 psi equates to enough flow to supply the needed fuel for stock operations in a Dodge truck. Even the GM
implementation benefits from installing pusher pumps between tank and IP. 50 First Dates all over again.


The effects of air in the fuel are quite correct. However, how it gets there is off the mark. Dumping the fuel back to the tank is not going to
significantly increase areation nor will pressurizing the fuel. In a closed system that doesn't happen. As far the heat generated by the
pressurization, diesle fuel needs to be 80-110 degrees F to perform adequately. What little heat is added by a low pressure pump will
not significatly change the temp of the fuel. Even return fuel from the pump is not going significatly alter the temp in the tank under
most circumstances. If anything it helps raise it to temps that are better for the injection events. I would guess that the percentage
of uses that would drive that fuel temp beyond 110 degress is going to relatively small.

The aeration of the fuel is one point the FASS\Airdog have on all the other solutions that is worth considering. By reducing the aeration
and returning the fuel before it is heated by the engine compartment they provide a better quality of fuel constitution to the CP3. This will
also help reduce the cavitation that happens in the CP3 on intial pressurization. Better fuel equals better power from less volume.

 

no 6 oh no, in your statement that no pressure means that there is no flow is totally wrong. Not trying to make you mad, but that is totally not true. There can be all kinds of flow with no pressure, I deal with in my job everyday.

 

Maybe someone needs to hook up a in hg/ psi gauge and find out if the CP3 starts sucking fuel at 0psi.. I bet it does, but at a cost of performance because sucking thru the LP will add a fair amount of resistance.

 

i have a 2004.5 325/610 with 125K and the lift pump mounted to the filter housing under the hood. It is an oddball setup i know. The LP was never relocated to the intank.

I kind of like it knowing that there is a factory draw straw intank. What inline fuel pump setup can you guys recommend for my setup?

I want to keep the truck stock except for a whistler intake and my towing/mileage programmer.

thanks,

joel

 

Ok, thats a bit of an over simplification to make a point.

Answer me this, if you have 1 psi of pressure at the start of length of whatever you want to envision and there is 2 psi of restriction in said length, Will you have flow at the end of the length?

Point being pressure is a reflection of restriction. Unless the pressure overcomes the restriction flow does not happen. Speaking in just tems of flow does not take into account all the variables.

Technically, flow as a motion cannot happen without some type pressure or force driving it. Laws of physics. What does change is the ability to measure the differential that is driving the flow. It is entirely possible to have a flow with no psi, but, thats only because we cannot correctly measure the force that is driving it. Doesn't mean it does not exist, means we are either measuring the wrong differential or lack sensitive enough instruments to measure it. Taken in this prespective flow is really a function of pressure and the statment "no pressure equals no flow" is technically correct. Agreed?

Just saying flow is critcal factor is technically correct but functionally its a load of BS. A pump pulling fuid with 29 in of vacuum at the inlet will not flow the same as the same pump with 8 psi of pressure at the inlet. If demand to supply balances at 4 psi of inlet pressure, below that demand exceeds supply and something suffers. Can't change the physics of it no matter how hard we wish.

 

I agree with you that flow is a very important thing to consider, my only point is that you can have flow without pressure. Is it something I would want to have happen to my truck? No, not at all. I would say that flow would be a great thing to monitor if anyone has a guage to do such because you can have pressure and no flow such as if a valve were closed just like you can have flow with little or no pressure. Both of which could be a bad thing for the CP3.

 

Sure, there are gauges to measure flow but which type? Volumetric or mass? Which one is more applicable, and accurate, in an underhood environment? Then, which one will measure the relatively small flows we deal with and do it accurately enough to provide meaing?

Once you start into measuring flow and doing it accurately enough to provide enough data to base decisions on you start talking $$$$. Considering flow can be derived by other means, its just a lot easier to extrapolate the values from the knowns and build in a fudge factor. In the long run, its the observed functionality that matters the most.

 

Thimk, people, and make no misteak about this: internal valving in the CP3 supplys fuel to the internal mechanicals first, under all pumping circumstances - no fuel is ever bypassed around the internals to the rail, leaving the mechanicals to starve - if there is not enuff fuel to supply the mechanicals and the rail, the engine simply will not start - if there is enuff fuel to make 3500-5000psi idle fuelrate, the internals are being lubricated - rail pressure takes second chair to internal lubrication - and thassa fack, jack............

We live at the bottom of an ocean of air - all those molecules for miles above us press down upon us to the mean tune of 14.67psia - this means you, me, everybody, and everything on the surface of the planet is under Atmospheric, or Barometric Pressure - including the surface of the Diesel fuel in your fuel tank - submerge one end of a tube into the fuel and nothing happens, no flow - pressure on both ends of the tube are equal - however, place a device on the open end of that tube which can lower the pressure on that end, and the 14.7psia on the surface of the fuel will attempt to equalize, pushing the fuel thru the tube to the device - lower the pressure by 2psia, or 4"hg vacuum, to 12.7psia, and you get 2psia pushing the fuel thru the tube - if the device further reduces the pressure to 20"HG vacuum, you get 10psia pushing the fuel to the device - this is known as differential pressure, and is pretty much the foundation upon which our beloved infernal combustion engines breathe and function, amongst many other things we take for granted

Now, let's call this magic device that creates a Barometric pressure differential a lift pump, and let's install this device that reduces the pressure on that tube in the fuel, onto the back of our CP3 inj pump, then let's crank our CRD Cummins engine, and voila!: we do get fuel flow with quote\unquote no pressure - at least, it seems to be 'no pressure' to those of us who've forgotten the simple science and physics lessons that were (supposedly) learned in Jr. and Sr. High School Science Class, eh........

An auxilliary electric lift pump functions similarly, reducing pressure on the input to create flow, with increasing flow causing increasing pressure on the output: even the output port offers some resistance to flow, so there is always some pressure developed as the pumped fuel exits that small opening, trying to get out of the way of the incoming fuel

Resistance to flow starts in the fuel tank, with the pre-screen, then the plastic tubing, plus the plastic tubing connections, plus the inside diameter of the tubing, plus the length of the tubing from the pickup to the filter, then the fuel filter, plus the i.d. of the tubing fittings to and at the injection pump - it all adds up to large resistance to fuel flow from tank to CP3 input - plus the small diameter of the actual inlet fitting on the CP3 - this combined restriction is the reason the CP3 lift pump cannot supply hi-power tunings without auxilliary help - the inlet cannot be increased above a certain dimension without defeating the function of the gear-pump .

In pumping systems, there are three ways to increase flow:
- increase the cross-sectional area of the path thru the system - 'nuther words, increase the inside diameter throughout to decrease the restriction
- increase the displaced volume of the pump so the pumped flowrate increases - pressure also increases throughout the system - this can be done with larger pump, or higher-rpm pump
- add a second pump, in series with the original, which increases flowrate and pressure

With increased applied pressure, which causes system flowrate to increase, the CP3 can easily supply hi-power applications - for even higher-power applications, increased flowrate can be created with second CP3 in parallel with original

Diesel fuel foams in any system - the Dodge Ram fuel system is no exception, as it is not a 'closed' system, because the excess fuel is returned into an open-volume container, affectionately known as the fuel tank - therefore: foaming is a constant problem in a Diesel fueled system - the Fass and AirDog systems function to reduce the effect of this foaming on the engine - more in-depth info can be read on those sites - foaming is a problem in warm weather, and in systems where the fuel is over-pumped, as the impellers, propellers and compellers thrash the fuel when flowrates fall below the rated capacity of the pumping device - sometimes known as cavitation, Diesel fuel reacts with increased aeration - it is a fact known throughout industry, and one which can be observed by pouring ambiently warm Diesel fuel into a clear container, such that the foaming above the surface and the aeration below the surface is clearly visible (try that with gasoline and compare the results!) - remembering that Diesel fuel is constantly agitated in the tank as the vehicle moves, which means constant aeration, those two aftermarket systems may seem more of a solution to those planning hi-performance upgrades

BTW - by placing a small vacuum on the fuel tank, aeration is reduced - how? - reducing Barometric pressure on the surface allows the entrapped tiny air bubbles below the surface, each one a tiny microcosm of atmospheric pressure, to more easily coalesce and escape, joining their fellows in the foam above the surface - for this reason, your fuel cap is vented at 2"hg, or 1psia

Edit: In gasser (patooie!) fuel systems, where the lift\supply pump is of the in-tank sumbersible type, the common failure is burned brushes\commutator, where the system had been repeatedly\constantly run below 1/4 tank level - failure resulted from the pump starting when the fuel level was low, exposing the brush\commutator end - final result was the brushes "welded" to the commutator segments, pump locked up, no fuel being pumped = replacement required

Same scenario where the Dodge lift pump is attached to the Fleetguard fuel manager\filter, high up on the engine, above the fuel tank, above any in-tank fuel level - pumps are rated for head: how far and how high they can pump, or 'push' the liquid, and 'draw', how far they can pump\pull that liquid from\above the liquid source - some pumps require gravity-assisted priming, some can draw dry thru tightly-sealed piping - that Dodge pump is spec'ed for pusher service, where the pump is mounted on the frame-rail back near the fuel tank, where the 'draw' head is low: shorter lift from the bottom of the tank, less restriction resulted from the long fuel-line, less gravity-weighted lift and drain-back - plus, those plastic fuel line fittings are spec'ed for pressure service, not vacuum service, where-in atmospheric pressure has easier opportunity for in-leak, compared to the denser fuel mass being lifted from the bottom of the tank and dragged along the restrictive fuel-line length with it's several pressure-type push-to-connect fittings, up to the lift pump inlet, high above the fuel tank - result: same scenario = burned brushes, burned commutator resulting when lift pump is repeatedly started with brush\commutator end exposed above fuel level, rotor locks, no pumped fuel - that lift pump is a good one, just mis-applied - result: DCJ spec'ed an in-tank submersible lift pump, pressurizing the supply line from tank to the filter and CP3, with in-tank bypass-type pressure regulation, dumping excess volume directly into the surrounding fuel

IIRC, the owner's manual cautions against running fuel level below 1/4 tank, resulting in better driveability on several levels:
- the in-tank lift pump remains submerged at all times, increasing longevity
- low fuel levels result in greater sloshing, resulting in greater fuel aeration
- low fuel levels result in uncovering the fuel pickup in the longitudinal fuel tank position in the chassis, where the fuel sloshes end-to-end at acceleration\braking events, resulting in intermittent fuel supply
- low fuel levels result in increased bottom-tank junk being stirred up and drawn into the lift pump pickup

In short, and to wit: there is considerably more to this fuel supply system than meets the general eye, eh...................

 

I've read all this and all I can say is MY HEAD HURTS

I thank all of you that contributed to the thread and even some of you that didn't.

To me it sounds like the bottom line is Dodge screwed up and spec'd. a cheap bad setup and we need to spend extra money to make it right.

I'll just add that to the list.

Ball Joints because stockers are cheap and go away bring on DW
Steering box Brace to help with DW
Traction Bars because of wheel hop
Change light bulbs because stock head lights suck
Add foam to seats because ours are designed by Dr. Mengala
Clutch because the stocker is weak and hasn't held up
Lift Pump because the stocker sucks.

Is there something I'm missing? I'd like to get all the above done so I can do the things I want to do to my truck, oh nevermind I'm broke now.

 

Actually this is why I have created this thread, if your not loosing FP (and you have an in-tank pump) then no you really don't need another lift pump. Maybe after your factory one craps out, but not til then. I guess one of my questions is; if the factory (in-tank) LP is good enough for your needs then why not just replace it with an another one if it goes out. I have not seen any real proof that a aftermarket $$$$ pump is going to be as or/any more reliable then a factory one. Sure you might get a life time warrenty, but does that really mean that it's failure rate is any better??

 

6.0 and GMCCTD I am glad you guys are posting on here you are giving some great information and for the most part you agree about things, it scares me to think that more have not piped in about this subject. It makes me think that LP's (and thier use/need) is alot more mis-understood than I thought...

So I have a question for 6.0, why is 4 psi the majic number in your book? What happens at this point (other then power loss), I mean what causes it, because you still have + flow... Is it electronics does CP3 become a vacuum on the inside, etc?

If more psi is good then why not just bypass the regulator and pump full pressure to the CP3?


I definately have to agree with GMCCTD about the whole aerated fuel issue, even in just a circulation fluids can become more aerated if the surface it flows against is not perfectly smooth which our fuel lines, fittings, tanks are far from smooth.

I do know (from my old gas racer days) that those in line HO pumps can get hot enough not to want to touch and heat fuel after a while.

This whole thread makes me think that an "ideal" set-up for most would be to run a dual pump set-up having a small pump (like that factory in-tank) as the main pump, and then having another slightly larger in-line pump that was triggered by a low FP pressure sensor (so if the magic number is 4 psi have it come on at anything below 5 psi)... And then of course to make things more efficient by increasing the ID flow size throughout the system...

GREAT RESPONSES guys!!!!!, it is nice to see we are all keeping this a decent respectful discussion

Keep it up, I think you are informing alot of us here that really didn't have a clear understanding...

 

Rail pressure does NOT take second place, it can't due to the design of the COV. The internal lube\cooling is a FIXED orfice on the slide valve, the high pressure is a VARIABLE sized port. This is not a VP-44 where the fuel is pressurized into the internals first then shunted to the high pressure circuit. The design templates and Bosch descriptions call all this out in detail.

Your applying one condition across a wide range and attempting to make the description "fit". The engine will start, idle, and run down the road with inadequate fuel supply. As long as there is not a total bloackage in the line the CP3 will draw enough fuel to pressurize the rail for starting. What it won't do is draw enough fuel to adequately supply the rail and lube as the rpm's raise. You an be runnign down the road at 70 mph with 2k rail pressure and eating the pump up and it will do it. Again, the CP3 ALWAYS pumps fuel it is supplied and the rail will be #1 inline for the fuel.