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Cummins Injector Leak Diagnosis May Be Easier, But Can Still Be Tricky

A phrase that I often hear from customers is, “This is a diesel engine; the problem should be easy to diagnose.” The right part about that statement is that the customer realizes that he has a diesel engine; the wrong thing is the assumption that it is easy to diagnose.

By Bob McDonald

Certainly, there’s some common repairs needed on specific engines that are easy to do but doesn’t mean that it makes everything else easy to diagnose. Owners often think that because diesel engines are now electronic the technician should be able to hook up a scan tool and immediately see what is going on.

The good thing about electronic diesels is that the technician can link up with a scan tool to analyze data in order to try to pinpoint problems. But take the diagnosis of an injector issue. It may be tougher than your customer – or you – think.

The most common injector issue is that the engine produces a cylinder misfire. The misfire is generally associated with a loss of power and no unusual smoke. 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, the scan tool generally will pinpoint the cylinder with the contribution problem.

However, injectors can fail in different ways, other than becoming just worn out or tired. One of the most common 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 run fine but 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. But this wisp should not be confused with blow-by, 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. 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.  

While you may realize it, you may have to remind your customers that  there have been great technological advances in diesel design, but it doesn’t mean that it has become simple. I think sometimes with newer diesel engines, owners may think that there are easier ways in determining failed parts, but it can be still just as aggravating as older diesel engines. Some things still may have to be done the old fashioned way in order to properly diagnose engine complaints.

One other thing that hasn’t changed: if you find an injector issue such as this, be sure to tell the owner about the additional labor that is likely to be involved in the bill for your expert diagnosis.

Robert McDonald is owner of Atlantic Engines in Granite Falls, NC, and specializes in high performance diesel and gasoline engines and cylinder heads for street, marine, dirt and drag racing. Robert can be reached at rmcdonald@enginebuildermag.com.