Understanding Diesel Particulate Filter service and repair

Oct. 1, 2017
Proper servicing of Diesel Particulate Filters will prolong service life and help your customers avoid problems

The primary products of combustion produced by automobiles and light trucks, whether powered by either gasoline or diesel engines, are carbon dioxide, water and nitrogen. As we all know, it’s the other gases that create a problem. Unlike gasoline-fueled engines, a diesel engine’s power is controlled by the fuel supply instead of throttling the amount of air entering the engine. This causes diesel engines to produce a different array of pollutants than spark-ignited, gasoline-fueled engines. One example is carbon monoxide (CO) emissions. Because diesel engines burn their fuel in excess oxygen, even at full load, their CO production is considerably lower than gasoline engines.

This DPF filter from a Ford 6.4L Powerstroke engine needs to be replaced around 80,000 miles. Once the filter becomes 90% plugged it can’t be regenerated by driving or undergoing a forced regeneration using a scan tool. Replacement cost is around $1,700.

However the lean-burning nature of diesel engines comes at a price, and the high combustion temperatures result in significant production of nitrogen oxides (NOx). In fact, NOx from gasoline engines have decreased 96 percent through the use of catalytic converters, but diesel engines still produce 20 times more NOx if left untreated. Some OEMs reduce their diesel NOx emissions through a method called Selective Catalytic Reduction, or SCR. This process involves injecting urea into the exhaust gases. It reacts with the hot gases and releases ammonia that helps the catalytic converter turn the NOx into nitrogen and water.

The other major exhaust component that separates diesel from gas engines are the particles of soot or particulate matter. The black-colored soot from a diesel engine contains carbon compounds that are formed because of fuel that is not fully atomized, combined with low temperatures within the combustion process. Cylinder walls and large droplets of fuel are relatively cool when compared to the rest of the combustion chamber and carbon deposits (soot) are formed in these areas because of the rich air/fuel mixture and lack of oxygen. To keep particulate matter from being blown out of the tail pipe, modern diesel-powered cars and light trucks use a diesel particulate filter (DPF) to capture and intermittently burn them off. Trapping soot particles is important because of their small size, which can be inhaled and cause serious medical problems for humans.

DPF filtration

Diesel particulate filers can capture around 90 percent of particulate matter and because of the environment they operate in have to be mechanically and thermally durable. Because there is limited space to locate a DPF in a vehicle, their physical size can only be so large and depending on engine operating conditions they can quickly accumulate a considerable volume of soot.

If a DPF collects too much soot, it restricts exhaust flow drastically, affecting engine performance. DPF systems have to provide a way of removing the particulates from the filter to restore its capacity and lower exhaust backpressure. This process is known as filter regeneration and usually takes place continuously as the vehicle is driven. Typically regeneration is triggered when the backpressure in the DPF reaches a predetermined level and the vehicle is operated for a time period long enough to burn off the particulates. Initiated by the vehicle’s PCM, continuous regeneration should be “invisible” to the driver.

DPFs use thermal regeneration where the collected soot particulates are oxidized or burned. The DPF is heated and oxygen/nitrogen (air) is added, causing the soot to burn and transforming it to carbon dioxide or CO2. The DPF must reach a high enough temperature (1100 Degrees Fahrenheit) to oxidize the particles and on some systems the source of heat is the exhaust gas. This type of filter is known as a passive filter and regenerates continuously during normal operation of the engine. Passive DPFs use a catalyst, which lowers the soot oxidation temperature to a level that can be reached by using the engine’s exhaust gas alone. Passive systems may also use fuel additives that allow the soot to be burned off at lower exhaust temperatures.

This diagrams shows how a passive DPF filter operates. Heat from the engine’s exhaust combined with air burns off the particulate matter collected in the filter by transforming it to CO2

Another method for burning off soot is called active regeneration. The “active” process heats the DPF using additional fuel. During the process, exhaust temperatures can be increased by late cycle injection of increased fuel quantities or injection and combustion of fuel in the exhaust system. Exhaust gas combustion consists of fuel burned in a fuel burner or burned in an oxidation catalyst. Another method is the use of electric heating by placing a heating element upstream of the filter substrate or using electrically conductive filter media that acts as both heater and filter. Whatever method is used, regeneration is performed periodically as determined by the vehicle’s on-board computer.

In addition to passive and active filter regeneration, there is a third type called passive-active or quasi-active. These systems use a catalyst that allows regeneration at lower temperatures and/or shortens the regeneration time period. For example, a passenger car would regenerate passively during sustained freeway driving but use active regeneration during city driving. Having the ability to perform DPF regeneration under both sets of driving conditions results in fewer incomplete regeneration attempts and presumably less flashing DPF dash lights warning the driver of impending DPF failure.

Unfortunately, if a car or truck is operated only for short driving periods, the automatic regeneration process does not complete. The PCM will turn on the DPF warning light, letting the driver know that a regeneration is required. The vehicle owner’s manual provides the steps that need to be taken to regenerate the filter. Here is a generic guide to get the vehicle’s PCM to go through a regeneration cycle. Warm the engine to operating temperature and drive it for 15 minutes at a constant speed on the freeway, keeping the engine RPM above 2000. These conditions should initiate a regeneration cycle. Adding DPF cleaning additives may help in the regeneration process. These additives include a fuel born catalyst (FBC) that doesn’t raise the temperature of the exhaust, but instead lowers the temperature at which the soot blocking the DPF will burn off. True DPF cleaners are more expensive than traditional fuel additive injection cleaners so read the container label to ensure that it contains an FBC. If in doubt about how to initiate a DPF drive cycle, or adding non-factory chemicals to the fuel tank, consult the owner’s manual or service information.

There are three types of DPF systems: Passive that operates all the time as the vehicle is driven; active regeneration that is initiated by the vehicle’s PCM under the right conditions and passive/active that uses a combination of both systems to burn off soot.
Hot exhaust gases enter the DPF and are allowed to pass through the filter. The filter media collects the soot, or particulates and traps them. Periodically the filter has to be regenerated to burn off the soot and clean the filter media.

Forced regeneration

Manual, or forced DPF regeneration, may be required when a vehicle is not driven at a high enough speed to allow automatic active or passive regeneration, especially if the driver ignores the DPF light. When the DPF differential pressure sensor detects a significant blockage in the DPF and automatic regeneration is not initiated, a manual regeneration is required. There are two modes of manual regeneration that can be initiated by a scan tool: static and dynamic. Static regeneration takes place with the vehicle stationary, and dynamic regeneration requires the vehicle to be driven. Some auto manufacturers disable static regeneration due to the high temperatures created at the DPF. The combination of a do-it-yourselfer at the controls of a scan tool, and the high exhaust temperatures present during regeneration have undoubtedly resulted in OEMs being burned on warranty issues with “cooked” DPFs and melted engine components.

As long as the DPF is not severely blocked (over 90 percent soot capacity), a scan tool can initiate a regeneration. Scan tool data could include DPF pressure sensor voltage, soot accumulation as a percentage, distance since last regeneration and distance since last DPF replacement. In addition, a successful DPF regeneration can only be achieved if all the exhaust and engine management system components are in good working order. If this is not the case, the PCM will disable manual regeneration and set a “failed or unsuccessful regeneration” message on the scan tool display. The reasons for this message can include: DPF over 90 percent of capacity (requires DPF replacement); fuel level under 25 percent (usually a minimum of 20 liters in the tank is required); oil qualify degraded/diluted (on vehicles with oil quality monitors); any DTCs set (glow plug circuit, DPFE sensor, exhaust temp sensors, O2 sensors) and DPF service life reached (DPF needs to be replaced). Replacement intervals can be as low as 75,000 miles on some vehicles.

The DPF differential pressure sensor measures the difference in pressure between the inlet and outlet of the filter. If it becomes too high the PCM may flash the DPF warning light and initiate a regeneration process.

P2463, Diesel Particulate Filter Restriction — soot

With the DPF warning light turned on, the vehicle is probably suffering lack of power symptoms and may have a DPF-related trouble code. If the PCM detects exhaust back pressure levels over a programmed limit, a P2463 could be stored in memory and the check engine lamp may be illuminated. Because a DPF restriction could lead to engine or fuel system damage, this code should be considered severe. According to SCM Hotline Diagnostics (www.autohotlineusa.com – 800-847-9454) symptoms of a P2463 code may include: other DPF-related codes, excessive black smoke from the exhaust, difficulty in maintaining desired RPM levels, lack of power and increased engine temperature or overheated DPF components. Other possible causes can include a bad DPF pressure sensor, low urea level (tank is empty), problems with the DPF injection system and faulty exhaust pressure sensor.

DPF service

Diesel cars and light trucks with DPF systems were introduced in 2009 and the first ones are now eight years old and have a corresponding number of miles on them. On some of these vehicles, DPF filters that have in excess of 80,000 miles can become clogged with ash. Ash is a by-product of the diesel combustion process with its main ingredients being calcium from engine oil and sulfur from diesel fuel. However, unlike soot, the ash plugging up filters cannot be burned off by a regeneration process or by adding chemical cleaners to the fuel tank.

When a vehicle’s DPF warning light comes on, it’s generally about 45 percent blocked and the normal regeneration (passive or active) should burn off the particulates and turn off the DPF light. If the vehicle owner ignores the DPF light, a forced regeneration using a scan tool will work up to around 60 percent blockage depending on the specific vehicle. Because of ash accumulation, a high-mileage DPF that is blocked by 90 percent can only be repaired using two options: cleaning with special equipment or replacement.

DPF cleaning requires the filter to be removed from the vehicle and placed in a cleaning oven where it is subjected to intense heat, pressure and chemicals to remove the blockage (ash and soot) from the filter. DPF filter cleaning is common for the heavy truck market and off-road construction vehicles, but almost non-existent for the automotive industry. The incentive for cleaning these heavy-duty filters is that it’s less expensive to clean a plugged, high-mileage DPF than it is to spend $3,000 to replace it. The problem with cleaning DPF filters used on cars is that they are oftentimes a welded component of the exhaust system and do not fit the standard adapters used for large truck DPF filters. This may change in the future as more and more diesel cars and light trucks need replacement DPFs and cleaning may be an attractive option. DPF replacement is the only option that dealerships offer because they don’t have cleaning equipment nor are they willing to offer a warranty on a DPF cleaning/repair.

This DPF filter fits a Volkswagen 2.5L TDI turbo diesel. Owners’ should be aware that when the DPF light comes on that they should drive their VW in such a manner as to allow the PCM to regenerate the filter. Ignore the light and it could cost upwards of $1500 to replace the DPF.

Some DIYers have found an option to replacing a plugged DPF filter — cheat! With 2009 and later diesel cars and light trucks coming on the used market, owners of these vehicles are often faced with a $1,500-plus repair for a replacement DPF filter. Because many states only require a visual inspection for diesel vehicles, and a check to see if any emission-related OBDII codes are set, some owners entice their technician to cut a hole in the top of their DPF filter, remove the guts and weld it back shut. The DPF will look normal during a visual inspection and because there is nothing inside it to become clogged up, the DPF pressure differential sensor will never cause the PCM to set a code. Other more blatant DPF replacement components are offered on Amazon or eBay under names like, DPF Delete Pipe, DPF Test Pipe and Off-road Competition DPF Pipe. These DPF replacements have literally nothing inside them to filter anything and are considerably less money than new DPF filters. This is such a problem in Europe that many countries are starting to add particulate detection to their emissions testing equipment and not just rely on OBDII DPF-related fault codes. Can the U.S. be far behind with this technology? Keep an eye toward California to lead the way in emission testing that detects if a diesel car or light truck really has a working DPF filter.

About the Author

Tracy Martin

Tracy Martin writes for Motor Age, Aftermarket Aviation Defense, Contact! and Turbo Diesel Register magazines. As an ASE certified master technician and consultant, he has worked in the automotive aftermarket for over 25 years and has provided consulting services for Snap-On Tools, Sun Electric, Chevron Oil and Nissan of North America. He has developed automotive/powersports curriculum and taught courses including: engine performance, automotive electronics, fuel injection, ABS braking systems and advanced motorcycle riding techniques. He has also provided consulting services as an expert witness for automotive and powersports-related court actions.

He is the author of five books: How to Diagnose and Repair Automotive Electrical Systems, Motorcycle Electrical Systems–Troubleshooting and Repair (two editions), How to Use Automotive Diagnostic Scanners (two editions) and How to Modify Motorcycle Fuel Injection, all published by Motorbooks International. 

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