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Diesel misfire diagnosis

Locating misfires on a diesel isn’t difficult if you understand how they work.
Monday, January 14, 2013 - 12:59
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In today’s world of diesel diagnostics, most every engine manufacturer gives some sort of cylinder contribution data on the scan tool. Some manufacturers only provide “at idle” date, while others also make it available at higher engine rpm. This data is an indication of the fuel injector control that is needed to keep the engine running smoothly. For you gas guys, you might think of this as fuel injector fuel trims. This data is valuable not only when looking for a misfire, but when looking for rough running engines or poor power complaints. With a little practice it is possible to use your scan tool to test the fuel injectors while they are still in the engine. Stop and think how valuable this can be since it will save you a lot of time in testing without the need to remove all the injectors and send them off for testing. One other benefit of this type of testing is the injectors are tested dynamically in their real world working conditions. This is something that no injector test bench can do.

The power of the scan tool bidirectional controls is awesome. All engine manufacturers allow several bidirectional and scan test options. When working on common rail engines, things like control of rail fuel pressure, turning off individual fuel injectors, relative compression testing and much more are available. The HUEI systems allow for injector electrical tests (injector buzz test), high-pressure control and cylinder contribution tests. Learning how to use your scan tool and how to quickly move from test to test will make your testing procedures quicker and more accurate. There are also the diagnostic trouble codes available to get a direction for diagnosis, but I caution you about using codes alone as a basis to replace parts, since this can lead you down some long dark damp alleys of trial and error.

Labscopes and diesels
Many times a labscope is needed to enhance the diagnostic capabilities of the scan tool. One instance is a relative compression test. As engines get more complicated, it is a must to have quicker and easier diagnostic procedures. Because compression is one of the essential ingredients for combustion, relative compression is a great way to find that low compression cylinder without having to remove engine parts or covers.

If a relative compression test is not available on your scan tool, then you can use a labscope and a high amp current probe to do the testing. Having to pull valve covers, heat shields and glow plugs can be very time consuming to do a mechanical compression test. One thing to understand about relative compression testing is that while the actual compression pressure cannot be determined, the test can provide a quick comparison between cylinders.

If you are looking for the cause of a misfire on one cylinder and you have five other cylinders that are working, you have a way to compare the misfiring cylinder. In a case like this, your relative compression test will save you a lot of time and will give you great results. Hook your high amp current probe around a battery cable that powers the starter, trigger from a cam position sensor (CMP) so you have a reference to your waveform and give the engine a short crank.  

While we are talking about using a lab scope on diesels, I think it would be good to consider how the injection system/computer controls work. If you’re comfortable with gasoline engines and how the ignition and fuel injection systems are controlled, you’re well on your way to being able to diagnose modern diesels. The electronic diesel injection control is about the same since they also use a crankshaft position sensor (CKP) and CMP sensor to report the crankshaft and camshaft positions to the engine control module (ECM). If these signals are not correct or corrupted, misfires can occur. In some cases a labscope will be required in the diagnostic process to scope these signals. With the electronic injector control, it is possible to have one cylinder fire out of time or not fire at all if the CKP does not report properly.

A poor running 2006 F250
Let’s look a a real world example I had of a misfiring diesel to illustrate the process. The vehicle is a 2006 F250 6.0L engine, manual transmission with 245,000 miles on the odometer. Please keep in mind that this vehicle is used as an example and whether you are working on a Dodge, Chevrolet or a Ford, the diagnostic processes will be about the same.

In any diesel engine, all we need is the proper cylinder heat, a fuel that is capable of combustion and injected into the combustion chamber at the proper time and in the proper amount. The complaint of this vehicle is lack of power and a rough idle. Any time I get a problem vehicle like this, I always wonder what people really expect out of their vehicles. When I opened the driver’s door I immediately knew how this truck was being used. A company that does pack trips with horses and mules owns the truck, which was evident by the smell that came rolling out of the cab and the brown stuff that was on the floor of the driver’s compartment.

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