Common Rail


A-Primary Filter  
B-Final Filter   
C-Diagnostic Port  
D-Transfer Pump
E-High Press. Pump 
F-Overflow Valve 
G-Common Rail 
H-Relief Valve 
I-Fuel Return Line 
J-Electronic Injector 
L-Flow Limiter 
M-Control Valves 
N-Check Valves 
O-Hand Primer Pump 
P-Transfer Pump Out.
Q-Transfer Pump In. 

(Note: This is a very basic introduction to the Deere common rail fuel system. See the John Deere engine repair and troubleshooting summary in the John Deere Technical Manual.

Common Rail Fuel System Operation
The common rail fuel system enables engines to produce power efficiently while still meeting ever more stringent emission regulations. The common rail system works because it makes the extremely high fuel pressure that is needed to more finely atomize the fuel droplets. Smaller droplet size permits better combustion in the cylinder.

Here’s A Simplified Explanation For How It Works: Fuel is drawn from the vented fuel tank and through the boat’s fuel/water separator filter, and then through the primary fuel filter (A) by the fuel transfer pump (D). A pressure relief valve on the final filter allows fuel to bypass the final filter if the filter becomes plugged. Bypass fuel is directed back to the tank to prevent both filter rupture and unfiltered fuel from entering the high pressure fuel system. Check valves (N) are used to prevent fuel from draining out of the fuel filter and the high pressure fuel pump when the engine is not running.

The fuel exits the final filter and flows to the high pressure fuel pump (E). The high pressure fuel pump begins raising the pressure of the fuel to prepare for injection. Pump control valves  (M) control when fuel enters the pump. These valves are controlled by the engines’ computer (ECU).  When the necessary volume of fuel is in the pumping chamber to maintain the correct fuel pressure in the high pressure common rail (HPCR)  (G), the ECU will shut the valves.

When the fuel pressure in the pump exceeds the pump’s delivery valve opening pressure, the high pressure fuel is allowed into the (HPCR) which evenly distributes fuel to all of the electronic injectors (J). The HPCR uses flow limiters (L) to maintain a constant pressure to the injectors.

The ECU sends a signal to the two-way valve, inside the injector body to control the volume of fuel, the timing of delivery, and the rate of delivery for each injector. Excess fuel from the nozzles travels through the fuel rail return line.

A pressure relief valve (H) will allow excess fuel in the HPCR to flow into the low-pressure fuel rail return line (I). An overflow valve (F) on the high pressure fuel pump will also release excess fuel into the fuel rail return line and back to the tank.  The fuel transfer pump is mounted on the high pressure fuel pump and is driven by the high pressure fuel pump camshaft. The transfer pump draws fuel from the fuel tank, through the primary filter, and into the transfer pump inlet (Q).  Fuel is then pressurized, exits the transfer pump (P), and travels to the final filter. The hand primer (O) is provided for bleeding air from the fuel system.

The final filter is a 2-micron filter. Fuel enters the final filter at the fuel inlet, and flows through the filter element, and exits through the outlet to the  high pressure fuel pump.

The John Deere 6081 high pressure common rail engine uses the Denso ECD-U2 high pressure fuel pump. The main components of the ECD-U2  pump are the driveshaft, two 3-lobed cams, timing wheel, two pumping plungers, pump control valves, delivery valves, and the pump position sensor.

Filtered fuel from the primary filter fills the high pressure fuel pump at the fuel inlet. Both cam lobes and the timing wheel are attached to the drive shaft.  As the driveshaft rotates, both cam lobes operate their respective plungers to increase the pressure of the fuel. The timing wheel is used to keep the high pressure fuel pump and the engine timing in sync with each other.

The timing wheel has 6 equally spaced notches plus 1 additional notch. The ECU uses the pump position sensor to detect each notch on the gear as it rotates past the sensor. The ECU uses the additional seventh notch to determine when cylinder #1 is approaching Top-Dead-Center (TDC).

The transfer pump and the high pressure fuel pump come as an assembled unit. The entire pump must be replaced if a failure occurs.

 In The Common Rail: High pressure fuel is delivered to the high pressure common rail (HPCR) at the fuel inlets from the high pressure fuel pump. The HPCR distributes high pressure fuel to the Electronic Injectors (EIs).
The fuel rail pressure sensor, flow limiter, and pressure relief valve work together to regulate fuel distribution. The fuel rail pressure sensor detects the fuel pressure inside the rail. The engine control unit (ECU) uses this sensor to monitor the fuel pressure to determine the timing of the pump control valves on the high pressure fuel pump. The flow limiters (L) use a piston and ball valve to reduce pressure pulsations. This ensures a steady pressure in the fuel lines to the electronic injectors.

The flow limiters are also used to limit the maximum fuel flow to the injectors to prevent engine damage due to a failed injector or a high pressure leak by shutting off fuel to that particular injector. This is done by moving the ball valve until it seats closing the valve.

If a fuel pressure of 200 MPa (2000 bar) (29,000 psi) is generated within the HPCR, the pressure relief valve (H) will release the excess pressure and drain fuel back to the tank.

 Electroninc Injector Operation: The electronic injectors (EIs) are located inside the engine’s cylinder head and are electronically controlled by the ECU. The amount of fuel delivered to the cylinder is in direct proportion with the length of time current is supplied to the two-way electromagnetic valve on each injector.

20 responses

24 02 2009
Khiem Nguyen

What are the common causes for the transfer/high-pressure pump failures?

25 02 2009

We haven’t seen a Deere or Isuzu common rail pump fail yet. However, we know from experience on past injection pump failures, that when it happens it will be due to water or contamination in the fuel.
The higher the pressure, the greater the need for fuel conditioner, as a preventive.
Thankyou, Ben

12 04 2009

How to determine if electronic injectors are malfunctioning?

14 04 2009

From the engine user’s perspective, your first clue that an electronic
injector is faulty, is that you will hear the engine missing. Then, if you
find water in the primary fuel filter, you have found a reason that an
injector would fail.

The actual testing of electronically controlled injectors is done through
the ECU (computer that controls the engine), and it will take a call to
your local engine dealer to get a technician to hook up the software
that does such tests.

For more on this, please contact our Service Manager, Herb Knight:, he can also be reached at 206-286-1817.

5 06 2009

Hi, I have a questions like, when we doing the condition monitoring to the fuel injection system, like conventional system, we have to monitor several important parameters, like maximum pressure, the opening pressure, the opening angle, the duration of injection, etc. I just want to ask, what are the parameters we should look into if we want to do the condition monitoring to this common rail sysgtem?

6 06 2009

Hello Tan,
On older, mechanically fuel injected, engines we would remove the injectors
when the engine was running poorly or smoking black. We would then put the
injectors on the “pop stand” and watch the stream of fuel for good atomization.
We would also observe the “pop” pressure and monitor the rate of pressure

Engines equipped with Common Rail fuel systems have an electronic control
system (computer) that monitors engine operation. When a problem occurs
with a Common Rail engine, the computer generates a touble code and alerts
the operator with a warning light. If the problem is severe, the computer also
reduces power.

Very few repair shops now have the equipment to test and rebuild the common
rail injector, because the pressure is often over 20,000 psi. When the
technician decides to change injectors, they are now replaced with new
ones, or units that have been reconditioned at the factory.

So, now it is the injector rebuilder that monitors all injection conditions when
the testing is done. Meanwhile there is much more electrical troubleshooting
done by diesel engine technicians than ever before.

Thankyou for commenting and please keep in touch.
Ben @ MER

28 10 2009
kadiri o chato

how can i get training on commonrail fuel system on line?i am a kenyan,thanks.

29 10 2009

I would like to receive info on vehicle mechanics

3 11 2009

Thankyou for your interest. Depending on where you live,
your John Deere Dealer may be one of the best resources.
Otherwise, try Googling: “Mechanics”, “Diesel Mechanics”,
or “Marine Mechanics”.
Ben @MER

18 12 2009

i have a mitsubishi triton with a 3.2 litre common rail i would like to fit a cav filter with a water trap in before the primary fuel filter can i do this .

19 12 2009

We often use a Racor or Dahl fuel/water separator filter.
If the specifications on the CAV filter are right for your
application, it will work.
Sincerely, Ben@MER

19 12 2009

The only common rail fuel system training we’ve seen is through the
engine manufacturers. Please visit with your local John Deere or Isuzu
dealer and see what they offer.

8 04 2010

You have told Kadira the only training you have seen is at the dealer. I was wondering if you have seen any software sources out there to program/ diagnose John Deere’s ECU’s.
Thanks Reuben

17 04 2010
adel elbadry

Please , send to me how the common rail system work ?

28 04 2010

Hello Reuben,
JD now has an end user software package that lets you monitor the system.
Please check with your local Deere distributor.
Thank you,

28 04 2010

Hello Adel,
Please call your local John Deere if you wish to purchase the Common Rail manual for your Deere engine.
For example: The Deere 6081 engine, which is an 8.1 liter engine has a service manual for the fuel
system, titled, “Component Technical Manual 255”, or “CTM 255”. This manual can be purchased from
Deere. Inside the manual, in section 03, the theory of operation is well explained.
Thank you,

24 07 2010


4 08 2010

Hello Kadiri,
At this time we have no dedicated training focused
on the Common Rail fuel system. However, John
Deere publishes their manuals on the subject in
several languages.

For example, their manual #CTM255 (in English)
covers the 8.1 Liter diesel with the High Pressure
Common Rail (HPRC) fuel system.

This manual is available through John
or your local Deere dealer, and is a great place to
start learning the fuel system.
Take Care,
Ben @ MER Equipment

26 07 2011

I was talking to someone about my hpo common rail pump on my loader and he told me that I had to time that pump to my engine. I was just interested in knowing if that is true, because all that other common rail pump that I have installed did not have to be timed


9 08 2011

Hello Josh,
We don’t recognize the “HPO” pump you mentioned, and would need to know the maker of your engine and the engine model to do justice to your question. However, it is safe to point you to the engine manual or your local dealer, for your specific engine. Thank you for visiting the MER Blog.

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