Cummins Common Rail Fuel System

6.7L, 5.9L Common Rail Problems

The Cummins common rail fuel system is a good system, in fact the number one cause of fuel system failure isn’t the system, it is contaminated fuel. Many people do not even get to 50k miles before they have a problem caused by fuel contamination. A single tank of contaminated fuel can and often does cause catastrophic and expensive fuel system failure! This is compounded by the fact that the filtration system on these trucks is sadly inadequate. The new trucks have 3 micron filters but the older trucks have 10 micron fuel filters and the injectors see abrasion at 4 micron. We suggest replacing your fuel filter each and every oil change and we also suggest fueling up at Truck Stops where fuel is turning fast and not getting a chance to pick up water from condensation in the tanks.

These systems operate at extremely high pressure (up to 26,000 psi) and have extremely fine tolerances so any contamination at all, even microscopic will have an abrasion effect, leading to more contamination and premature wear if not seize up completely. Both the CP3 pump and the injectors can be affected.

Failure to completely clean and flush contamination from the fuel supply system before replacing parts WILL result in repeat failures. Before installing any new fuel system components we recommend disconnecting the fuel supply line from the CP3 pump and activating the electric fuel pump by turning on the ignition while collecting the fuel in a clean glass sample container. Ideally this fuel should be sent to a lab for analysis. At a minimum, set the sample container aside to settle and then check for any water separation or particulate matter under a bright light. ANY signs of contamination require complete fuel system cleaning.

To properly clean the fuel system the tank must be removed, emptied, and flushed with clean fuel. Do not use water to flush the tank. Fuel lines must be flushed thoroughly, then inspected. If rusty inside, they must be replaced. The fuel filter must be replaced regardless of whether or not contamination is found in the fuel system.

Long Crank/ Slow Start?

  1. No or low fuel supply, should be 10-15 PSI at idle, to the high pressure injection pump(CP3). Most starting problems due to low pressure are caused by bad (eroded check ball seat) injectors. You can unplug the fuel control actuator and the pressure should default to maximum (26,107 PSI), however if there is a leak in the injection system then the pump will not build enough pressure. If there has been a major contamination issue with dirt and or water then it is very likely that the high pressure pump will need to be replaced. The injectors are typically damaged first, but any contamination that got into the injectors also went through the CP3 pump.
  2. Monitor rail pressure and see if you have over 4000 PSI during cranking, if not one or more injectors can cause a hard start, see below for further diagnostics. No smoke from the tailpipe after about 10 seconds of cranking means no fuel is getting into the cylinders.
  3. Injector high pressure connector tube (feed tube) not seated in injector, bad tube or improper torque (final 37 ft lbs) on nut.
  4. Leaking high pressure limit valve, should not leak at idle or during cranking.

One of the most common Long Crank failures occurs when an injector body becomes cracked. When the body is cracked, the engine will not necessarily produce a miss but will take an extended period of time to crank. In addition, the customer may notice some fuel dilution in the oil by seeing that the oil level is rising on the dipstick.

When the engine is shut down, the crack in the injector’s body will often cause fuel to drain back from the fuel lines and rails back to the tank. When the leak down occurs, the engine will have to spin over for an excessive period of time in order to re-prime the injection system. A normal crank time in a common-rail injection system is usually around 3 to 5 seconds. This is how long it will take the common-rail pump to build fuel pressure to the “threshold.” The threshold for cranking is when the fuel rail pressure reaches around 5,000 psi. Normal common-rail systems will operate at 5,000 psi at idle and can reach up to 30,000 psi at wide open throttle.

In a Cummins engine, the injectors will not be actuated by the controller until the fuel rail pressure reaches the threshold. So when an injector becomes cracked and the fuel has leaked down in the injection system, crank times will become almost tripled in order for the fuel system to re-prime and the desired threshold reached in order to fire the engine.

So how do you determine which injector may have a possible crack? This can be a lengthy process to determine exactly which injector is the problem. Cummins recommends a simple visual test to start. First remove the valve cover, then crank the engine and let it idle. With a light, study the injector body of each cylinder. Sometimes, if the injector body is cracked externally, you may be able to notice a small wisp of smoke from the injector. The wisp of smoke that can sometimes be seen is actually the atomization of fuel being released from the crack. If the injector is cracked externally and producing a smoke wisp, you will be able to smell the hint of diesel fuel in the air.

This type of diagnosis can be very useful in trying to identify which injector may have an external crack. But what if you still can’t determine which one is the problem child? Then you’ll have to dig a little deeper and isolate each cylinder. The only way that you can isolate an individual cylinder is to cut off the supply of fuel – in order to do this in a common-rail system you’ll have to cap it off.

For the Cummins engine, I start with the first cylinder and remove the hard line between the fuel rail and injector. Next, I place the cap on the fuel rail where the fuel line was. (A word of caution here: this “cap” is a special tool made by Cummins specifically for this test. This cap is made to withstand the high pressures associated with a common-rail system. Do not use anything else or you may suffer injury or death from the high pressure fuel.) Next, I crank the engine and see if the crank time is reduced. If not, I proceed to the next cylinder until I can determine which one is responsible for the long crank time.

On some occasions, I have worked on trucks in which the Cummins engine would not run at all. This usually happens when the injector is cracked so badly that the fuel system can never reach the threshold. The oil will also be heavily diluted with diesel fuel. By installing the cap on each cylinder one at a time, the bad injector can be isolated – you’ll know you’ve found it when the engine fires normal and fast.

Other causes for extended crank time can include Heater Grid (on cold days), Sensor Inputs, Actuator, Supply Volume CP3 Pump, and more… Other issues include battery voltage, slow crank, ECM goes “brain dead” where you have to disconnect both batteries and allow the ECM to power down. Fully charge both batteries and before reconnecting them touch the positive and negative terminals together and you may even need to reflash the ECM to the latest calibration.

Soot and EGR Cooler

the 6.7 engine is plagued with soot issues plugging the EGR cooler. THIS CONDITION SETS A p0401 CODE. Be aware that this is likely a fouled EGR cooler not a plugged EGR valve. If the coolant level is low the cooler is also probably cracked.



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