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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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Being raised a farm boy, this didn’t bother me at all. I just took it in stride.  I came armed with my IDS scan tool and all the while, the same thoughts kept running through my mind. Do some faulty injectors or low compression cause this misfire, or is the problem electrical in nature? Any and all of these problems could be the cause of the misfire on this engine, so let’s use the power of the scan tool to see if we can quickly get to the bottom of this problem. The vehicle has been sitting overnight, so before the engine is started, I want to check for any diagnostic trouble codes that might be stored in memory. In the case of this vehicle, there are several codes set.

P0272 = Cylinder No. 4 contribution

P0275 = Cylinder No. 5 contribution

P0281 = Cylinder No. 7 contribution

P0299 = Turbocharger under boost

P0478 = Exhaust pressure control valve high input

Any time I get this many codes on a diagnostic problem, I try to break them down into two or three different categories. That way I can divide and conquer without getting confused. The three cylinder contribution codes are performance codes, which are set because there was no power or low power produced in these cylinders. The P0299 and P0478 codes are set because of lack of boost and because the EBP actual pressure does not agree with the calibrated limits. Both of these codes can easily be the product of the three misfiring cylinders, so I will put them on the back burner for a while and concentrate on the three cylinder contribution codes.

From what we discussed earlier about the requirements for combustion in a diesel, we need to check for only three things: compression, fuel injector operation and a combustible fuel. Since the engine is cold and has not been started, the first thing to do is use the scan tool to do the KOEO (Key On, Engine Off) electrical injector test. This test is going to operate the electric solenoids in the injectors and test the electrical circuits of the injectors. It will test the electrical circuits from the FICM (Fuel Injection Control Module) all the way through the fuel injector coils.

When doing the test, get your head up under the hood somewhere near the center of the engine so you can hear the test being performed. The scan tool will first activate all of the fuel injector electrical solenoids then each solenoid is activated using the cylinder numbering sequence. The scan tool will run this test three times. If any of the injector solenoid valves are stuck they will not click and you will be able to detect this with your ears. If the electrical circuits are shorted or open, diagnostic trouble codes will be set.

As the injectors are being clicked on and off, you can hear the missing clicks of the stuck solenoids as they are cycled. In this truck’s case, the injectors on cylinders 4, 5 and 7 did not click. The next test I want to do is to verify if the problem is caused by low compression on the misfiring cylinders. The quickest way to get this data on this particular vehicle is with the scan tool. This is easily accomplished by selecting the relative compression test and following the on screen directions.

When the test is completed, the scan tool will display the relative compression data. This data is taken from the CKP sensor speed during cranking. If you do not have this function on your scan tool, you can use a lab scope and high amp current probe to get your data. Be sure and trigger from the CMP sensor when doing this test so the low compression cylinder can be identified using the engine’s firing order.

Now its time to start the engine and select the cylinder contribution test. Here you can see which cylinders are not contributing and are able to kill each injector while watching the graph of engine rpm. My testing found there was no problem with compression. The lack of clicks in the earlier injector test led to the cause of the misfires. The hydraulic solenoids were stuck in the three silent injectors. Three new injectors fixed the power and rough running complaints and returned the engine to its original condition. 

By using these three simple tests, the cause of the misfire was easily found, and all by through the use of scan tool bidirectional controls and data graphs. Once the misfiring cylinder(s) is identified, you may have to use pinpoint tests to narrow down the fault’s cause. A few examples of pinpoint tests are an injector leakage test, a balloon test (to see if compression is being blown back through an injector and collecting in the fuel rail), or a voltage drop test of the electrical circuits to verify the integrity of the circuit. You will find that starting your testing procedure with the basics, though, will get you to the finish line the quickest. 


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