diesel fuel system problem diagnosis

Hello, everyone. Today I will talk a little about diesel engine fuel system problem diagnosis. This will be just a quick overview of what to look for and not specific to any brand. Many of the page views on the Toolbox are related to fuel system problems, so I know many people are up against these issues.
Before a fuel problem can be solved, we must know how a fuel system works. I don’t want to outline every type of system out there, but the following panels show the more common ones. Remember that these illustrations only show the mechanical components. The electrical sensors and circuits that control the system are just as important and can cause problems, too. After the panels I will outline each system and talk about some common problems.
One word of note here; engines with modern computer control, in other words those build in the last 10 – 15 years, are impossible to diagnose and repair without a diagnostic program to interface with the truck. Just because the check engine light is off doesn’t mean there isn’t a sensor or circuit problem, and a code is only a starting point on which to base further efforts.  These programs also do much more than just show codes. They also give you datastream readings showing what the sensors are telling the control module and what the module is sending back to the control systems. You can do a cylinder balance test, which is the only way to find an injector with a mechanical failure. Unfortunately, OEM programs are usually the only ones that provide all these capabilities, and they are expensive. Generic aftermarket programs offer code display and some datastream, but probably not bidirectional tests like cylinder balance.
Actually, if you know a specific system very well, you can repair some problems just from flash codes off the dashboard display, but this makes things harder than they need to be, adds time to the repair, and puts limits on how far you can go on an unusual problem. Throwing parts at a vehicle until you replace the right one is very expensive, time consuming, and doesn’t always work.
Shown below are three of the common systems being used today. Keep in mind that there are variations on each of these. When you look at these panels, remember that they are simplified in order to be viewable and do not show everything actually in the system. For instance, transfer pumps have check valves at both the inlet and the outlet, and debris can stick them open and cause low fuel pressure. The fuel regulators are also check valves, holding pressure in the system when the engine isn’t running for quick starting.

schematic diagram of the high pressure common rail fuel injection system

    The high pressure common rail system is used in several new engines, including the 6.6 liter Duramax and the Navistar Maxforce. As the name implies, it uses high pressure fuel in a rail that feeds all the injectors, and when they say high pressure, they mean it. Rail pressures in the Duramax can reach at least 27,000 psi! That is enough pressure to inject fuel through your skin, so you do not search for fuel leaks with your fingers, and you don’t loosen fittings or lines without relieving system pressure.

    The rail pressure is manipulated by the ECM to inject more or less fuel depending on engine load and operating conditions, and the injectors are opened and closed electrically for precise fuel metering. This makes it easier for manufacturers to meet present and future emissions and drivability requirements. It also results in a simpler system that is potentially easier to diagnose. The computer can use cam and crank sensor data to measure each cylinder’s power contribution so that each injector can meter the exact amount of fuel the cylinder requires.

schematic diagram of HEUI (hydraulic electric unit injection) system

    The HEUI, or hydraulic electric unit injection system uses high pressure engine oil to provide to force needed to inject the fuel. The system was developed in a joint effort by Caterpillar and Navistar when mechanical injection could no longer keep up with emissions and drivability requirements. It is used in the 7.3 and 6.0 liter engines Navistar sells to Ford, Navistar’s DT line of engines, and the Caterpillar 3116 and 3126.

     Unit injection means that the fuel is pressurized to injection pressure inside the injector itself, instead of in an external pump and fed to the injector nozzles. The advantage is more precise fuel metering and less mechanical complexity, as compared to mechanical injection. The system can also be cheaper and easier to repair. One failed injector can be replaced instead of the whole pump when it wears out or something breaks. Unit injector development was also accelerated by low sulfer diesel fuel, as sulfer was a lubricant needed for the close-fitting precision parts in a mechanical pump. 

    The ECM varies the pressure of the high pressure oil system to provide more or less fuel to the cylinders. The injectors are opened and closed by high voltage AC provided by an injector driver and controlled by the control module. This design is being phased out in favor of high pressure common rail being simpler, as no system is needed to pressurize engine oil. HEUI also is somewhat prone to oil leaks, and it is for this reason that Navistar put the high pressure oil rails under the valve covers. Then if they leak the oil just goes back into the oil pan instead of on the ground. The computer can easily compensate for minor leaks from the system.

     Despite being seemingly complicated, the engines using this system tend to be robust, mechanically simple, and fairly easy to repair. Injectors can be sensitive to contaminated fuel and engine oil, and the high pressure oil pump can fail if the engine is run low on oil.

schematic diagram of diesel unit injection system

Traditional unit injection develops injection pressure inside the injector, but the injection force is provided by an additional rocker arm running off the engine camshaft. Injection timing and metering is manipulated within limits by an electrical solenoid built into the injector. Injection pressure is also a function of engine speed, as the faster the rocker arm moves, the more pressure the injector develops. The injectors are fed low pressure fuel usually via passages in the head. Fuel pump pressures are usually in the neighborhood of 30 – 60 psi. This system is still used in most heavy road diesels on the road today. The system is durable, simple, and relatively easy to repair. 

Thanks for checking this out, and check back for the next part of the discussion.