Redesigned Low Pressure Fuel System

Last Updated: 23 September 2006

The low pressure fuel system on Dodge trucks with Cummins engines 1998.5 through at least 2004.5 is poorly designed.  By that, I mean it's highly prone to failure.  In my opinion, there are two issues to be addressed: the low pressure fuel pump (aka the lift pump), and the filtration quality.

The Pump

The factory lift pump, pictured here, is "weak" and has a tendency to fail.  This condition renders the truck inoperable.



Dodge owners have been complaining about these weak pumps since they appeared on VP-44 equipped engines in 1998.5.  In 2003 the change to the common rail CP-3 equipped trucks brought a major change to the factory lift pump and hopes from Dodge owners that the days of LP failures were over.  No such luck, although the new pumps are more reliable than the 1998.5-2003 pumps were.  Around 2004.5 Dodge redesigned the pumps again and moved them into the fuel tank.  So far, these in-tank pumps appear quite reliable.  The fact that Dodge completely redesigned the pump twice and finally moved it to a location common for gasoline-powered vehicles gives indication of the extent of the LP problem.


The Filter

It has been shown (by Taylor in The Internal Combustion Engine In Theory and Practice) that common rail injectors are more susceptible to damage from debris at the needle and seat than traditional jerk-pump injectors.  In a study I read some time ago, Caterpillar indicated that most fuel system damage occurs as a result of particles smaller than 5 micron.
The factory fuel filter for Dodge common-rail trucks is 10-micron at 99% efficiency.  The CP-3 injector pump, capable of pressures up to 1600 bar (23500 psi), employs remarkably tight tolerances which may be easily damaged by contamination in the fuel.

My Solution

In light of these facts, I set out in 2004 to completely redesign the low pressure fuel system on my truck.  In so doing, I set forth the following goals in priority order:

1. Improve fuel filtration 5 times over stock (to 2 micron)
2. Improve fuel delivery reliability and consistency (flow and pressure)
3. Improve serviceability

Along with these goals, I defined the following objectives (in no particular order):
-Use industry-standard components wherever possible
-Minimize fuel gelling or filter freezing potential
-Modularity
-Provisions for simple fuel transfers and testing
-Features for simple emergency service

With all this in mind, I set my first design out on paper in October 2004.  The fluid parts of the system were originally laid out as in the following diagram - a design which remains unchanged 2 years later (although some of the original components have changed).




Fuel lines:
A - SS braided -6AN
B - SS braided -6AN
D - SS braided -6AN
E - SS braided -6AN
F - Factory stainless 3/8 OD line

Connections:
1 - “Weber” fitting (M12x1.5 to -6AN)
2 - -6AN to 3/8 NPT fitting (Summit)
3 - -6AN to 3/8 NPT fitting (Summit)
4 - “Weber” fitting (M12x1.5 to -6AN)
5 - 1/2 NPT to -6AN fitting (Summit)
6 - -6AN to 3/8 NPT (Summit)
7 - -6AN to 3/8 NPT (Summit)
8 - 3/8 NPT to 3/8 OD tube (Swagelok)

Early designs used a Davco FuelPro 382 fuel filter ("supplemental filter" in the diagram above).  This filter is very popular on class 8 tractor trailers, and for good reason.  It offers a wide range of filters, a see-through filter housing (permits accurate indication of a need to change the filter), 1/2-NPT input and output, water drain ball valve, and optional 12v or 120v heat.  I bought one of these from Ebay for around $200 and built an adapter to fit it in my engine compartment.  After many hours of work, it became clear that the Davco unit is just too big.  So in early 2005 I changed the design to use a Stanadyne FM-100 filter.  The following picture demonstrates the size difference:



The Stanadyne is an acceptable replacement, as the filters are easily obtainable and the filtration quality is at least as good as the Davco.  The only bad part is the smaller filter necessitates more frequent element changes.

What I really like about Stanadyne is they seem to be the only company that openly provides efficiency and test data for their filters.  Here's the particle retention efficiency data they have on their website for this filter:






Notice my flow rate is more than double the rate at which these tests were performed.  It's not entirely clear to me what effect this has on retention efficiency, but its probably detrimental.

Originally the new fuel pump was an Aeromotive 11203, which is a carburator pump with peak pressure around 24psi.  My goal was pressure at the CP-3 of around 8-15 psi.  Rather than plumbing in a bypass regulator and associated return plumbing, I decided to build a pulse-width modulator (PWM) to regulate RPM at the pump.  This, in turn, provides a simple means to control pressure without over-complicating things with more plumbing for the regulator.  After doing some initial design work for the PWM, I discovered one in kit form at Carl's Kits, which was easily modified and enhanced to handle the pump power requirements.

I built a simple rig to test the arrangement, shown here:



Note the bypass regulator on the downstream side of the pump.  This was done only on the test rig to simulate the back pressure created by the CP-3.  I ran this rig for several hours (endurance testing) pumping ordinary food-grade soybean oil.  I used soybean oil because I knew it would be messy and wanted something that wouldn't be toxic like diesel fuel, but that could be burned in the engine so the system didn't need to be completely flushed clear after testing.  I came up with the following relationship between pump current and flow rate for the soybean oil (which, by similitude, is directly related to the flow rate of diesel fuel).



This relationship is almost linear, which is very convenient.  Minimum required flow to the CP-3 for normal engine performance is 38 GPH, so I was shooting for anything between 60 and 120 GPH.  Although I didn't bother running the analysis (finding good numbers for the viscosity of soybean oil is not simple), it's apparent based on the advertised flow rate for the Aeromotive pump that flow with soybean oil is roughly 1/2 that for diesel fuel.  So this pump would provide sufficient flow to keep the engine running even on the lowest setting.

Notice all hoses are braided stainless.  These are from Summit, and have a double stainless braid - one on the outside and one embedded in the rubber hose.  Unfortunately the rubber is nitrile, which is not compatible with biodiesel.  This may be a future problem, but I've been running 2% to 10% biodiesel fuel for over a year now with no problems yet.

Hose connections are Aeroquip -6AN.  I do have 1 Summit -6AN 90° fitting at the fuel filter, but the quality is nowhere near Aeroquip's (which is why I only have 1).

I installed the system on the truck in the early summer of 2005.  The following picture shows the pump installed in its mount (which I made from aluminum and some steel parts) connected to the vehicle fuel tank via the factory 3/8" stainless line.



And here's a couple pictures of the new filter and mounts, which I made from general purpose (6061-T6) square-bar aluminum.  The mount bolts to a couple spare M10x1.5 tapped holes on the intake manifold (where the throttle position sensor mounts on early 2003 trucks).





After installation I ran a temperature test on the mount.  It gets too hot to touch after the engine has warmed up, so I wanted to be sure there's no danger of a fire.  As you can see below, lack of airflow after shutdown causes a dramatic rise in temperature, but nowhere near anything dangerous.




It performed flawlessly through the summer until around October, when I experienced my first failure.  During a maintenance inspection of the system I discovered the Aeromotive pump leaking heavily from the junction between the electric motor and the pump body.  This is exceptionally dangerous, since fuel vapors inside the motor case can easily ignite and cause a nasty truck-eating fire.
I immediately removed the pump and replaced it with a spare (an operation made very easy with the use of AN fittings and the excellent serviceability of this system).  I dismantled the pump to find the main seal between the motor shaft and the pump input disintegrating.  I figured this degradation occurred as a direct result of using biodiesel fuel, which I had switched to in a 2% mix with petroleum diesel just 2 months prior to the pump leak development.  I discussed this at length with Aeromotive, but they deny any incompatibility with biodiesel.  They refused to provide me with the supplier of their seals, so to this day I have been unable to determine whether a replacement could be ordered from a tougher material (such as Viton).  Biodiesel is a strong solvent, so I continue to operate under the opinion that biodiesel failed this pump.

After this incident, I decided I wanted to simplify my system a little.  The single most likely point of failure (other than a leaking pump) was the PWM circuitry.  Although I went to great efforts to properly heat-sink the MOSFET unit, I continued to fear it might fail during extended use in the under hood environment.  So I decided to change pumps (since the Aeromotives apparently won't work with biodiesel) to one whose full-speed pressure and flow output are more consistent with what I'm looking for sans regulator.

I ended up with a common pump - the Holley Blue, (P/N 8-802-1) which offers an excellent match to the pressure and flow I want without a regulator.  This pump has been used with some success by other diesel owners, although some consider it somewhat unreliable.  But I figured with pump changes being so simple on this system, it could be swapped for a spare in just a few minutes alongside the road if necessary.

The bottom line is I replaced the Aeromotive with the Holley and eliminated the PWM circuit.  Now the pump runs using simple relays.  Here's a wiring diagram for the main underhood junction box:



This looks complex, but it isn't.  On a basic level, it's just independent relays connecting to the lift pump.  One of them is wired into the engine computer, the other is wired into a switch in the cab.  The cab switch permits engagement of the lift pump independent of a pump signal from the ECM.  The relays (bottom right) are automotive-style plug-in power relays that are roadside replaceable.

There are a total of 8 connectors on the junction box, with functions as indicated in the drawing.  The big 17-pin connector links the underhood box to the "brain" in the cab.  There are provisions for LEDs to indicate power being applied to the lift pump, a signal from the ECM, two grid heater indicators, and a water in fuel light.  Connector 7, labeled here as "fuel heater" is in place but left unwired for a future link with the factory fuel heater.

Each connector is an Amphenol MIL-SPEC environmentally sealed circular unit.  These are not your average sissy connectors you find all over modern automobiles.  I hate plastic automotive electrical connectors, because they break 2/3 of the time on disconnect.  Amphenol connectors have aluminum alloy bodies and screw in place for a positive connection.  They're also at least 20 times more expensive than a standard plastic piece of crap connector, but you get what you pay for.  This goes along with my requirement to use "industry standard" components.

All the wiring has an SXL cross-linked polyethylene jacket which withstands temperatures up to 257°F, diesel fuel, oil, gasoline, and most acids.

Here's a picture of the implementation of that wiring diagram:



All connections are soldered and coated with either liquid electrical tape or dual-wall polyolefin heat-shrink tubing.  The enclosure is an aluminum NEMA 4X box which is watertight.  Here's a look at the outside of the box with the lid on:



Note the silicone sealant around each connector.  Here's what this box looks like installed on the back side (firewall side) of the passenger-side battery tray:



It's sitting in an empty cavity which would be occupied by the PCM on a truck equipped with an automatic transmission.  The mounting is a simple 1.5" x 1.5" solid aluminum bar with 2 vertical bolts that secure the box.  This, in turn, is bolted to the side of the plastic battery tray as shown with an aluminum bar to distribute the stress evenly and prevent cracking the battery mount.  Note the "fancy" aluminum foil heat shield covering the 17-pin connector cabling.  The exhaust manifold is located just a few inches from that bend in the cable, so I figured a little aluminum foil would offer some protection against radiant heat.  Hasn't melted anything yet!

Here's a picture of the cab controls implemented:



The WIF light is for the water-in-fuel sensor on the Stanadyne fuel filter.  That sensor kit originally came with a full-size (2-1/16") "gauge" for the light (pictured below), but I rewired it to work with a standard LED light similar to the LEDs I have for the grid heaters.  I thought the one Stanadyne provides is really ugly and over-the-top.






Here's a final bill of parts that are actually in use:

Description Part No. Source Price Quantity Net
SS Female Branch Tee.  3/8 tube X 3/8 tube X 3/8 FNPT SS-600-3-6TTF Swagelok $28.40 1 $28.40
SS Plug for unused end of branch tee SS-600-P Swagelok $4.80 1 $4.80
3/8 NPT to -6AN AER-FBM2005 Summit (Aeroquip) $4.95 8 $39.60
Fuel Pump 12-802-1 Summit (Holley) $110.00 1 $110.00
Scotty lift pump eliminator fitting ? Scotty Air Systems $60.00 1 $60.00
Weber adaptor AER-FBM2116 Summit (Aeroquip) $9.50 2 $19.00
90 degree -6AN Reuseable hose end AER-FBM4032 Summit (Aeroquip) $16.88 2 $33.76
3/8 MNPT Pipe plugs SS-6-P Swagelok $5.30 2 $10.60
SS Street Elbow (3/8 MNPT - 3/8 FNPT) SS-6-SE Swagelok $21.60 1 $21.60
3/8 NPT Street Tee.  FNPT, FNPT, FNPT SS-6-T Swagelok $36.70 1 $36.70
Total length of hose - 12 ft.  Need to buy minimum 15 ft. of Summit hose SUM-230615 Summit $55.95 1 $55.95
Stanadyne Fuel Manager 100 33642 Scheid $57.57 1 $57.57
Stanadyne Fuel Filter Element - 2 Micron 35614 Scheid $19.01 1 $19.01 Total Plumbing 496.99
Dakota Digital FP Gauge ODYR-10-01 Dakota $79.95 1 $79.95
Dakota Digital FP Sending Unit (30 psi) SEN-10-1 Dakota $23.37 1 $23.37
Toggle Switch - Pump Turn-on 44251 Waytek $2.62 1 $2.62
Toggle Switch Guard 44218 Waytek $3.27 1 $3.27
NEMA Enclosure for underhood 49H5778 Newark $25.43 1 $25.43
2-pin box receptacles MS3102E10SL-4P Newark $5.20 7 $36.40
2-pin cable mount straight plugs MS3106F10SL-4S Mouser $14.07 7 $98.49
17-pin cable mount straight plugs MS3106F10-29S Newark $27.06 2 $54.12
17-pin box receptacles MS3102R20-29P Newark $11.68 2 $23.36
Pump relays (Bosch standard) 75102 Waytek $4.62 2 $9.24
LEDs - Red 559-5101-007 Newark $2.25 2 $4.50
LEDs - Green 559-5201-007 Newark $2.25 2 $4.50
Stanadyne Clear Water Collection Bowl 29899 Scheid $42.00 1 $42.00
Stanadyne WIF sensor kit 129268 Scheid $96.00 1 $96.00 Total Electrical 365.25
Total 862.24

That spreadsheet contains most of the parts.  There are a few minor items missing, such as resistors for the LEDs (a few cents each at your local electronics surplus shop).

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