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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

Any time I work on a diesel engine, I always think of the simplicity of the engine. Maybe this is because of my background in diesel, which started back in the 1960s. Back then, the diesel engines were pretty basic and I actually found them easier to diagnose problems on than their gasoline counterparts.


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For the moment let’s take a step back to the basics of how the diesel engine works, which would get us all started out on the same page. The diesel engine has no ignition system and relies on high compression and heat in the combustion chamber to ignite the diesel fuel that is sprayed (injected) into the hot combustion chamber at the correct time and in the correct amount. This boils misfire diagnosis down to only three things: fuel quality, compression and fuel injection.

A closer look
Diesel fuel has more things to do in the engine than simply burn in the combustion chamber. It has to also lubricate the injection system, seal the injection pump and injectors and cool the injection system.

Combustion chamber heat comes from two things: compression pressure and glow plug heat. Without the proper compression, the combustion chamber gas temperature will not be hot enough for ignition to take place. For most diesel engines, the combustion temperature has to be at least 450° F before the engine will start. To get this to happen, most late model engines will use some sort of engine starting aid. This might be a glow plug tip protruding into the combustion chamber, or an electric heater inside of the intake manifold. Sometimes these electric heaters are used for starting and sometimes they are used for emission control to keep the exhaust from blowing a cloud of smoke when the engine has started, but no matter what is used, the starting aids are used only for starting and once the engine is running the starting aids will not be the cause of a misfire. It’s this misunderstanding of how a glow plug works that often leads gas guys astray. They are not spark plugs!

The fuel injection system is probably the most problematic part of the combustion process. Basically, there are two distinct types of fuel injection systems. One type is a unit injection system where there is no traditional fuel injection pump (common rail and Hydraulic Unit Electric Injector (HUEI) for example). The other type is known as the Pump Line Nozzle (PLN) system where an injection pump is mounted to the engine and high-pressure lines carry the fuel from the injection pump to the fuel injectors, fitted into the combustion chamber. Each of these fuel systems will require a different diagnostic approach.

The basics of misfire diagnosis will always be the same. The key is to find the combustion process element that is lacking. In the case of a diesel, find out whether the problem is caused by lack of combustion chamber heat or lack of fuel injection. Notice I did not say anything about low compression or glow plug operation, because either of these can cause insufficient combustion chamber heat and must be considered in the phase of the diagnostic process.

Down and dirty misfire detection
Misfire detection has changed a lot in the last few years. Back in the day, it was quite easy to loosen a fuel injector line while the engine idled and listen for the idle speed to change, or to hear the engine start loping as we moved the fuel injection charge outside of the combustion chamber. Those methods have no place in modern diagnostic, and can even be unsafe when tried on some systems.

Instead, we are going to use a lab scope and a scan tool to find these misfires. Many times, all of the work can be done from the front seat of the vehicle using just a scan tool. With the advent of OBDII, scan tool capabilities only seem to get better every year.

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