Diesel Engine Diagnostics
In the United States, diesel engines for many decades were typically seen in medium and heavy truck applications. Noisy, smelly and dirty, light trucks (including SUVs) were no place for such an engine. That has changed over the last 10 or 15 years. With today’s diesel engines being cleaner, more powerful and lighter than their predecessors, they’re finding their way into more light trucks.
Chrysler Group LLC uses several different diesel engines in a variety of vehicles. For quite some time, the 5.9L and 6.7L Cummins® diesel engines have been used in many of the Ram pickup models (these engines are very similar in design and function). Since its inception in 2003, the Dodge Sprinter vans have been powered by two different turbo diesel engines: 2.7L (2003–06) and 3.0L (2007–09). In SUVs, the 2.8L 4-cylinder common rail, direct injection diesel has been fitted into the 2005–06 Jeep® Liberty. And recently, the 3.0L turbo diesel has been used in the Jeep Grand Cherokee.
Obviously, there are basic differences between gasoline and diesel engines. While mechanically the diesel engine is a bit simpler (no ignition system, per se) than its gas counterpart and with a reputation for being more durable, performance problems still can occur. Due to its popularity and length of use, we will focus on some basic troubleshooting and diagnostics for the big Cummins engines used in Ram pickup trucks.
SMOKE PROBLEMS
When you think of diesel, smoke is one of the first things that comes to mind. While smoke is not as visible as it was in the past on diesel engines, when you see smoke, that means there is a problem. With a diesel, the smoke can be black, white or blue.
Black smoke is often associated with air intake problems. A dirty or plugged air filter, or a restriction elsewhere in the air intake system limits the amount of air fed into the combustion chamber. This results in incomplete combustion and the black smoke.
White smoke will be seen when coolant leaks into the combustion chamber. Also, very cold temperatures will cause white smoke. If the engine is equipped with an engine block heater and the smoke is still white in frigid conditions, the heater is probably malfunctioning.
Blue smoke is often caused by a turbocharger issue. These include a restricted turbo intake duct, an air leak in the boost system between the turbo compressor outlet and intake manifold, and obstructions in the exhaust. Problems in the valvetrain can also cause blue smoke. Worn and brittle valve seals and worn valve stems and guides are leading culprits.
FUEL PROBLEMS
Fuel and fuel delivery problems can cause starting, driveability and acceleration issues in diesel engines. There are several tests, though, that can be performed to diagnose and test fuel system components. Let’s review several of the tests that are commonly peformed.
The high-pressure fuel lines deliver fuel under pressure (17,000+ psi) from the injection pump to the fuel injectors. These lines expand and contract from the high pressure fuel pulses generated during the injection process. Leaks can cause starting problems and poor engine performance. Checking for fuel line leaks is pretty straightforward.
Start the engine. To find a leak, use a sheet of cardboard, as shown in Figure 1. Move the cardboard over the fuel lines. A fuel line leak will spray fuel onto the cardboard. If the connection is leaking, bleed the system and tighten the connection.
Note: Although this test is simple, be very careful when performing it. The fuel is under very high pressure and contact with the skin can cause personal injury. Always wear safety goggles and protective clothing.
Restrictions in the high-pressure fuel lines also contribute to starting problems and poor engine performance. Also, engine misfires can occur along with white smoke in the exhaust. Examine all high-pressure fuel lines for any type of damage. Each radius must be smooth and free of any bends or kinks.
Check for damage (bends and kinks) in the low-pressure fuel lines, too. Restrictions and blockages within the lines can also cause fuel delivery problems. Flush or replace the suspect lines, as necessary.
A leaking fuel injector can cause fuel knock, poor performance, black smoke, poor fuel economy and rough engine idle. If the injector needle valve does not operate properly, the engine can misfire and lose power.
Determining which fuel injector is malfunctioning can be done a couple of different ways. First, run the engine and isolate each cylinder using the DRBIII® scan tool, or equivalent. Note the RPM drop for each cylinder. If you don’t have a scan tool handy, loosen the high-pressure fuel line fitting at the fuel injector connector tube (Figure 2). Listen for a change in engine speed (be sure to tighten the line fitting to the proper torque spec).
If the engine speed drops, the injector is operating correctly. If the engine speed remains the same when the fitting is loosened, the injector might be malfunctioning. Test all six fuel injectors, one at a time. Remove the suspect injector(s) from the engine and test it.
The fuel transfer pump supplies (transfers) a low-pressure fuel source (1) from the fuel tank, (2) through the fuel filter/water separator and (3) to the fuel injection pump. The pump is self priming and always provides more fuel than required by the fuel injection pump (excess fuel is returned to the fuel tank through the return fuel line). A fuel transfer pump that is not operating properly can cause a loss of engine power, excessive white smoke and/or hard starting. If that is the case, you can check for correct operating pressure and restrictions in the system (before performing these tests, inspect the fuel supply and return lines for restrictions, kinks and leaks).
Because the transfer pump operates at two different pressures, two different pressure tests must be performed (engine cranking and engine running). Remove the protective cap on the fuel pressure test port fitting (Figure 3) and attach the special fuel pressure test gauge (#6828). Next, remove the fuel system relay from the Power Distribution Center (PDC) to prevent the engine from starting. Now, crank the engine and observe the pressure gauge. The pressure reading should be between 5 and 7 psi. Re-install the fuel system relay and start the engine. The reading on the pressure gauge should be at least 10 psi at idle.
The fuel supply restriction test can only be performed if the transfer pump pressure is good. To perform this test, you will need a DRBIII scan tool with the Peripheral Expansion Port (PEP) and the 0-15 psi transducer. Disconnect the fuel supply line quick-connect fitting at the left rear of the engine. Remove the plastic clip from the metal fuel line and snap it onto the fuel supply hose. Install the Adapter Hose Tool (#6631) into the ends of the disconnected fuel supply line. Next, install the transducer from the PEP module to the brass T fitting on the hose tool. Then, hook up the DRBIII scan tool to the transducer.
Before performing this test, be sure you are not standing in line with the cooling fan to prevent personal injury. Start the engine and record the vacuum at high idle (engine speed at 100 percent throttle and no load). The test must be done at high idle. The vacuum reading should be less than 6 in/Hg. If the reading is higher than 6 in/Hg, a restriction exists in the fuel supply line or fuel tank module. Check the fuel supply line for damage, dents or kinks. If the line is okay, remove the module for blockage and the fuel pump inlet filter for obstructions.
