Light Duty Diesel Engine Tips: An Overview of Common Issues

Oct. 1, 2020

Diesel engines are the powerplant of choice for those who tow and require plenty of pulling torque. GM, Ford and Chrysler all vie for the business, and the need for shops to keep abreast of current diesel engine expertise is growing. Courtesy of GMC

Shops that don’t necessarily specialize in diesel engine repair and maintenance need to do their homework to stay abreast of both previous and current diesel engine control systems in order to service their light truck/towing customers. Here we’ve included a selection of various tips and tricks to help you get your feet wet in the diesel light truck market.


Today’s light duty diesel engines, featured in ½-ton to 1-ton trucks and their SUV and passenger car counterparts, are wonderful machines capable of great fuel mileage and producing plenty of torque...when they run properly, that is.

One of the common problems regarding diesels is the issue of hard-starts or no-starts, especially in cold ambient temperatures. As temperatures drop, the engine oil begins to thicken. While engine oil in a gasoline engine is intended to only lubricate the internals (main bearings, rod bearings, oil rings, cylinder walls, valvetrain etc.), in various diesel engine designs, the oil is also used to run a high pressure fuel pump that provides fuel pressure to the injectors.

When the oil viscosity is too thick to run through the small orifices in the high pressure pump, the injectors may not operate properly. When a customer’s diesel experiences a hard-start, the first item to check is the engine oil -- not only oil level, but the condition of the oil. Oil that is too thick also causes a drag at the bearings, resulting in the starting system to battle against the extra draw demand.

Regardless of what someone’s uncle or cousin recommends as the proper engine oil, it’s best to simply follow the engine maker’s recommendation, based on current weather conditions. For example, in cold temperatures, the manufacturer might recommend a CG-4 rated full synthetic oil.

If the oil is OK but the crank speed is low, obviously check the battery state of charge/condition and output, as well as all cable connections. A slow crank can make it impossible for the engine to turn over fast enough to generate adequate cylinder pressure.

Most diesel engines feature a glow plug system. The glow plugs are powered by a dedicated relay and assist in engine starting. They provide elevated heat in each cylinder to work along with cylinder compression to ignite the low-ignition point of the diesel fuel. The glow plug relay operates with a timer that allows voltage to activate the glow plugs for a specified number of seconds. If the relay sticks, the glow plugs can over-run and eventually burn out. Glow plugs can be checked by testing for resistance/continuity and comparing to specifications.

Excess resistance or no continuity is a definite sign that the glow plug(s) are not operating properly. Glow plug modules provide power to the plugs. If the module is bad or connections are poor, no or inadequate power will be available to the plugs. Problem engines of note include GM’s 6.2L and 6.5L engines where excess engine heat can kill the module.

Another example is Ford’s 7.3L Navistar engine where the module can cut off if two or more glow plugs are bad. Any of these problems (oil type/condition, glow plugs, glow plug relays and glow plug control modules) will be exaggerated with low ambient temperatures.

On Ford Navistar 7.3 engines, the fuel bowl features a yellow drain cock. Adjacent to this, note a white connector. This is a connection that provides current to the fuel bowl heater. It’s not uncommon for this connector to experience a short, which will kill power to the ECM. Unless the vehicle is operated in extreme cold conditions, disconnect this to prevent unexpected engine shut-off.

Of course, it’s common knowledge that diesel fuel is very susceptible to water contamination, requiring regular replacement of the fuel filter/water separator and draining of the fuel bowl. Filters should be changed at least once each year. If water is in the fuel (not actually mixed but present), in cold weather this can cause the fuel to gel and block fuel flow.

It’s imperative to maintain fuel cleanliness and to take advantage of adding approved fuel conditioner additives to reduce the effects of condensation in the tank and lines. In some cases, the fuel pump might be blamed for hard starting or engine dying after starting. Instead of assuming that the pump is bad, check for air in the fuel system. Air may be entering the system through a fault in lines or line connections. Check by installing a clear hose to the fuel return line. Crank the engine and inspect the hose. If you see air bubbles, that’s a telltale sign that air is entering the inlet side of the pump.

A bad injector solenoid can be the cause of inadequate fuel delivery to the injectors. When the ignition is turned to the on position, you should be able to hear a click noise at the solenoid, If not, suspect the solenoid. If it clicks but there’s no delivery to the injectors, suspect a bad pump, assuming the lines and filter(s) are free of obstructions.

Diesel fuel injectors run at high pressures. This may be 300 psi or higher, depending on engine design. If the injectors are gummed up/dirty, this will result in a lean mixture which should create a rough idle and white smoke at the exhaust outlet. Faulty glow plugs and/or a bad glow plug module can also result in white smoke, which may clear up once the engine reaches normal operating temperature. Black smoke indicates a rich condition. Keep in mind that black smoke can also be caused by a restriction in the air inlet, so check the inlet and air filter.

Despite the current engines that feature highly sophisticated management systems, the venerable 7.3L single-turbo Navistar engine utilized in the Ford Powerstroke line was in production 1994-early 2003 and is still very popular due to its reputation for reliability. Courtesy of Ford Motor Co.


The now-mandated diesel exhaust “after-treatment” is designed to reduce and remove harmful emissions particulates.

 DOC: Diesel Oxidation Catalyst. Similar to a catalytic converter in a gasoline engine system, a DOC is designed to oxidize carbon monoxide, hydrocarbons and diesel particulate matter (DPM) to CO2 and H2O. When exhaust temperatures reach approximately 392 degrees Fahrenheit, the catalyst “lights off,” initiating the conversion. The DOC is the first component in the after-treatment process. It serves as a flow-through “filter” that begins the oxidation process (regeneration) of cleaning up hydrocarbons and CO, as well as any unburnt fuel (and any oil carried by the fuel).

DPF: Diesel Particulate Filter. After the DOC, exhaust enters the DPF, which is intended to grab any exhaust soot that the DOC didn’t oxidize. Any soot trapped in the DPF is oxidized when exhaust temperature reaches about 400 degrees F. This is referred to as passive regeneration that takes place automatically as exhaust temperature rises.

Be aware that the DPF may require maintenance. As a result of the oxidation process, ash begins to slowly build up inside the DPF, and can eventually cause an increase in exhaust backpressure. If ash is allowed to build up, enough can be trapped to harden and plug the DPF. A clue to DPF contamination is shorter regeneration intervals. The DPF can be removed and cleaned (unless clogged to the point of requiring replacement). If the DPF is due for cleaning, it makes sense to also clean the DOC at the same time.

DEF: Diesel Exhaust Fluid DEF consists of a mixture of high-purity urea and deionized water. Controlled by a “dosing” module, DEF is injected into the diesel engine’s exhaust stream. DEF consists of a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water.

DEF dosing is controlled by the engine’s ECU. When heated, DEF splits into ammonia and carbon dioxide, which is then atomized and vaporized. Once DEF enters the exhaust, the water in the DEF vaporizes, leaving ammonia molecules to travel to the catalytic converter where it neutralizes NOx molecules. This reaction converts NOx to harmless nitrogen and water, substantially cleaning up diesel emissions.

All diesel-equipped vehicles that feature an SCR (Selective Catalytic Reduction) system utilize DEF injection.

SCR: Selective Catalyst Reduction. The use of SCR systems reportedly provides an additional benefit of a 3% to 5% increase in diesel fuel economy. This is a result of enhanced combustion as exhaust temperature rises, especially in the heavy-duty applications. SCR is the final component in the after-treatment system. DEF is dosed into the SCR catalyst, serving to further reduce NOX as oxides of nitrogen pass through the SCR filter.


The color of the vehicle’s observed exhaust can also provide helpful telltales in determining a drivability or engine problem. Following are potential clues based on exhaust smoke color.


Black...............              Dirty air filter/Clean/replace filter

Black...............              Air intake blockage or leak/Check system/Repair

Black...............              Fuel contamination/Check Fuel Quality

Black................              Fuel injection system malfunction /Check fuel system

Black................              Turbocharger malfunction/Check /repair

Black................              Exhaust system/malfunction/Check and repair DOC/DPF/SCR

Blue.................              Dirty air filter or restricted intake/Check and repair/replace

Blue.................              Plugged turbo oil drain tube/Repair

Blue.................              Crankcase ventilation system/Repair/replace

Blue.................              Valve seals/guides/Repair/replace

Blue.................              Rings or pistons or cylinders/Repair/replace

Blue.................              Exhaust system malfunction/Check and repair DOC/DPF/SCR

White (steam)..              Coolant leaking in combustion chamber/Repair

White..............              Internal engine damage/Repair

White...............              Fuel issues/Check fuel and injection system

White...............              Valves out of adjustment/Check adjustment


Following are selected examples regarding a few specific diesel engines and common issues.


Cummins 6.7L Turbochargers: Carbon buildup inside the turbo can prevent proper movement of the nozzle and will often set a P2262 insufficient boost code.

If no DTC is found but the vehicle fails state smog or emission test due to incomplete readiness monitors, check and see if the customer has disconnected battery power prior to the emission test. If so, ask how many miles have been driven since. Check with the vehicle’s service manual to see exactly how the vehicle needs to be driven to complete the readiness monitors.

The following criteria need to be satisfied to make certain the vehicle can complete the testing procedure, these include.

•              Fuel level at ¼ tank or more.

•              Battery voltage 12 to 16 volts.

•              Coolant temp over 140º F

The DEF fill point for GMC and Chevy light trucks is located underhood adjacent to the passenger-side firewall. Especially for trucks equipped with 4-wheel-drive that sit high, accessing this fill point can require a bit of an arm stretch.


If DTC P2463 is stored and the customer complains that check engine light is on and that the truck has reduced power, check with a scan tool and observe DFP regeneration status. Also check to see the last time DPF was regenerated. If the vehicle is equipped with a driver information center display, check to see if a DPF warning message is displayed.

If the DPF status shows that regeneration is needed, or the driver information center displays a DPF message. then manually command a regeneration as the system will not be able to automatically complete a regeneration.

Talk to your customer and ask them about their driving routine. Do they drive short trips? Do they routinely drive at low speeds? Are they ignoring the DPF message?

Explain to the customer that in order for the filter to clean itself sufficiently, the vehicle needs to be driven over 35 miles an hour until the message clears itself. The system is set up to monitor certain conditions and determine when the filter needs to regenerate. If the filter is not allowed to regenerate, then it will be damaged and require replacement.

Remind diesel owners that a blue fill cap on any late model diesel powered truck indicates the DEF fill. This is a good point to mention if the DEF fill is located next to the fuel fill, as found on many Ford applications. 


On 6.0L Fuel Injectors: Prior to installing the injector, it is critical to remove any oil or debris that may have fallen into the threaded injector clamp hole in the cylinder head. Failure to do so can cause the injector not to be fully seated, which can lead to compression leakage in the fuel system and catastrophic damage.

6.0L Fuel Injection Control Modules: Fuel injection control modules and glows plugs can both cause hard start or rough running on a cold engine. Here’s one way to tell the difference: if there’s white smoke coming out of the exhaust after a long crank, it indicates unburned fuel and points to the glow plugs as the problem. If there’s no smoke, the FICM is the problem.

6.4L Piezo Fuel Injectors: If one of the injectors leaks, the fuel will enter the crankcase and raise the oil level. To check for excess oil, you can “short stick” the oil dipstick. Simply insert the dipstick part way until the bottom edge of the handle contacts the top of the dipstick tube.

6.4L High Pressure Fuel Pumps: If you must remove or install the high-pressure lines from the pump to the cylinder heads, take special care to hold the pump fitting in place with a wrench while using another wrench on the line. Failure to do so can result in catastrophic damage to the pump.

6.4L Horizontal Fuel Conditioning Modules: The PCM cannot monitor the flow of fuel volume supplied from the HFCM to the high-pressure pump. Restricted fuel filters (or anything else that may limit the flow of low-pressure fuel to the high-pressure pump) can cause catastrophic pump failure, which may result in debris in the fuel rails and injectors.

Note that the selective catalyst reduction (SCR) tank runs through a heated feed line, using an injector to provide a dose of DEF into the exhaust prior to the SCR catalyst on the diesel particulate filter (SCRF).

The 6.4L engine used in Ford Powerstroke applications is known for common EGR and DPF issues. The Exhaust Gas Recirculation (EGR) system consists of a pair of EGR coolers and an EGR valve. Exhaust gas enters the lower (horizontal) EGR cooler from the driver’s side exhaust manifold. Exhaust gas then passes through the vertical EGR cooler. This cooler connects to the EGR valve that controls exhaust that enters the intake manifold. The DPF, located near the front of the exhaust system connects to the catalytic converter.

Signaled by a series of sensors, the EGR and DPF systems work together to help the engine meet emission standards. Sensors monitor the exhaust gas temperature prior to the DPF, inside the DPF and after exhaust leaves the DPF. In addition, an exhaust backpressure sensor is located on the DPF. When the sensors determine that the DPF has reached a predetermined threshold of soot contamination, the regen process begins, alerting the driver that regen has started.

When regen begins, the EGR valve opens to allow hot exhaust into the engine in order to increase exhaust gas temperature, which in turn raises the temperature of the DPF. At a predetermined trigger point, the engine’s fuel injectors release excess fuel during the exhaust stroke, sending fuel into the exhaust system, which burns soot out of the DPF. Once the filter clean-out trigger point is reached, the regen process stops. Naturally, while regen takes place, fuel economy will be reduced, since extra fuel is being used to clean the filter.

Because fuel is being sprayed into the cylinders during the exhaust stroke, fuel wash down and oil dilution occurs. Even with a properly functioning EGR and DPF, a gallon (or more) of fuel can be introduced into the crankcase, revealed by an overfill on the oil dipstick. For this reason, it’s vital for your customers to understand that the engine oil and oil filter should be changed at least every 5,000 miles. Ignore Ford’s spec of the 10,000 mile oil change interval.

The late model Cummins 6.7L engine found in heavy-duty Ram trucks outputs 1,000 ft.-lbs. of torque and as much as 400 hp. The high pressure fuel pump produced a whopping 29,000 psi of pressure.


Neglecting recommended fuel filter replacement is a known problem for the 2008-2010 Ford Powerstroke 6.4L engine. Failing to follow the filter change per the service manual can damage the high pressure fuel pumps. The dreaded DTC P0088 (fuel rail/system pressure too high) indicates that the fuel pressure control valve in the pump is sticking. The pressure control valve cannot be repaired or replaced on its own, as it’s integral to the pump assembly. If DTC P0088 is stored, this indicates the need to replace the high pressure fuel pump. Replacement pumps can be costly, at $1,000-$1,600. Ford techs are seeing high pressure pump issues due to metal debris contamination. One likely cause is the failure to perform proper maintenance.

The low pressure fuel pump/water separator/filter unit is mounted near the tank, on the frame. A water drain valve petcock is located on this unit, which allows draining water from the fuel. Ford recommends draining water every 30 days. However, due to the unit’s location and the owners’ negligence in not routinely opening this drain, water builds up in the fuel, contaminating and diluting lubrication for the high pressure pump. In the end, the metal debris contaminates the entire fuel system, often requiring replacement of the entire fuel system, at a parts and labor cost of about $6,000. The need to perform routine maintenance of the fuel system cannot be overemphasized.


Some 2011-2012 F-Super Duty vehicles equipped with the 6.7L diesel engine may exhibit DTC P1291 and/or P1292 due to an internally shorted fuel injector. An internal short in an injector may be caused by fuel being contaminated with DEF (diesel exhaust fluid) or by fuel gelling.

Remove the fuel conditioning module mounted filter. Allow the filter and filter bowl to dry for at least two hours. If the filter turned white, this indicates that the fuel is contaminated by DEF. In this case, the complete high pressure fuel system and diesel fuel control module (DFCM) needs to be replaced and the system flushed.

If the filter did not turn white, inspect the wiring harness for chafing near the EGR cooler. Repair the harness as needed. If no chafing is found, disconnect each fuel injector electrical connector for injectors 1, 4, 6 and 7 (for P1291). Disconnect injectors 2, 3, 5 and 8 for P1292. Check for continuity between the injector electrical pins and the injector body. If continuity is present, replace the injector(s) and the injector(s) return hose.

PART                                        P/N

Injector return hose……….……BC3Z-9A564-A

Fuel injector (cyl 1, 2, 7, 8)…….BC3Z-9H529-A

Fuel injector (cyl 3, 4, 5, 6)…….BC3Z-9H529-B

About the Author

Mike Mavrigian | Editor

Mike received a BA degree from Youngstown State University in English Literature with a minor in Journalism in 1975.