Diesel injectors fail because of two main reasons. The first is the mechanical soundness of the injector, and the second and most common failure of injectors is quality of the fuel running through the injector.
Like anything else, injectors can get tired and become weak over time. Even though they are electronic, sometimes the mechanical components inside the injector may also become worn, cease to function properly and even fail. In cases such as these, a scan tool generally will pinpoint the cylinder with the contribution problem.
One of the most common mechanical soundness failures occurs when an injector body becomes cracked. When the body is cracked, the engine will not necessarily produce a miss but will cause other problems, which can be even more difficult to pinpoint.
Although the injector body can be cracked, the engine may still run fine but just take an extended period of time to crank. In addition, you 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 has 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 three to five 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 (WOT).
In a Cummins engine, the injectors are not 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. Determining exactly which injector is cracked can be a lengthy process.
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. But this wisp should not be confused with blowby, which will be seen also.
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.
What if you still can’t determine which one is causing problems?
Then you’ll have to 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, start with the first cylinder and remove the hard line between the fuel rail and injector.
Next, place the cap on the fuel rail where the fuel line was. (((Warning: 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, crank the engine and see if the crank time is reduced. If not, proceed to the next cylinder until it is possible to determine which one is responsible for the long crank time. If the Cummins engine won’t run at all, then the injector is usually 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.
Some things still may have to be done the old-fashioned way in order to properly diagnose engine complaints.