A comprehensive guide to diesel particulate filters

Materials Used in DPF Construction

Today’s light duty DPFs consist of ceramic materials, which function as the soot filter. There are metal DPFs, but they are used primarily in heavy-duty and industrial applications. The DPF that we are familiar with is a highly porous ceramic structure, which consists of tightly packed, close-ended (plugged), square or honeycomb channels, which provides thousands of tiny passages for exhaust flow and soot collection. The filter media looks like a catalytic convertor monolith, but unlike the catalytic convertor, you cannot see through the monolith because each DPF passage is blocked.

As the exhaust flows through these channels, the porous walls act as a fine mesh, capturing the PM and preventing it from being released into the atmosphere. DPFs are built from components meant to resist high temperatures and pressure. Two materials used in DPF construction most often are:

1. Cordierite: This is reasonably cheap ceramic material with good filtering qualities. But its low melting point (2,600 degrees below zero) can lead to problems during regeneration, and too much soot build up. Many manufacturers such as Ford, Volkswagen, BMW, and Mercedes-Benz have used cordierite-based DPFs. But if too much soot builds up, its low melting point can cause problems during the regeneration process since it may generate thermal stress or even damage (melt) the ceramic filter media. The cordierite DPF is also more susceptible to thermal cracking due to sudden changes in exhaust temperature.
Silicon Carbide (SiC): SiC possesses numerous technical advantages over cordierite, establishing it as the superior option for a dependable and efficient DPF. It has superior filtration efficiency (exceeding 99%) compared to cordierite and possesses a more uniform structure. This enables more even soot capture, which can enhance DPF regeneration. The SiC material has a higher melting point of 4,900 degrees below zero and has better mechanical strength, superior chemical stability, increased thermal shock resistance, and elevated thermal conductivity, compared to cordierite. These features make the SiC DPF the preferred choice by manufacturers even with its higher initial cost.

How Does a Diesel Particulate Filter Function?

Over time, the soot trapped in the DPF is burned off through a regeneration process, restoring the DPFs efficiency. As captured soot accumulates, the DPF becomes restricted. To maintain peak performance, the DPF needs to go through a cleaning process called "regeneration," which happens in three forms, to preserve best performance:

1. Passive Regeneration: This process occurs naturally when exhaust temperatures are high enough, typically between 550 degrees and 1,000 Fahrenheit, to burn off accumulated soot. This happens during sustained highway driving, where the heat from normal engine operation oxidizes the soot into ash and carbon dioxide (CO₂). Passive regeneration often relies on specialized catalytic coatings, applied inside the DPF to promote soot oxidation at lower temperatures. Many modern DPFs contain precious metals such as platinum (Pt), palladium (Pd), and occasionally rhodium (Rh). These metals are applied as a coating to the filter’s ceramic structure and not only help oxidize soot, but they also reduce harmful gases like carbon monoxide (CO) and hydrocarbons (HC). While not as heavily coated as a Diesel Oxidation Catalyst (DOC), these metals enhance the DPF’s efficiency and longevity. The passive regeneration can take between 30 minutes to several hours and many miles of continuous driving distance to complete.
2. Active Regeneration: When the DPF reaches a certain level of soot accumulation, the Powertrain Control Module (PCM) senses this and starts an active regeneration. Extra fuel is injected into the exhaust system to raise the exhaust temperature to burn off the accumulated soot. The increased temperature helps to effectively oxidize and remove the soot, restoring the filter’s efficiency. The temperatures can reach 1,000 degrees to 1,300 degrees Fahrenheit during this process, which can happen at idle or while driving.
Forced Regeneration: A forced regeneration (also known as manual, service, or forced active regeneration) will be started by a technician and a scan tool. The PCM will intentionally accelerate the regeneration process, often by injecting additional fuel and modifying other engine parameters to increase exhaust temperatures. The temperature during forced regeneration can rise to between 1,100 degrees and 1,500 degrees Fahrenheit, depending on the vehicle. This higher temperature ensures that the accumulated soot is burned off when the DPF is heavily clogged and normal regeneration methods have failed or are insufficient.

DPF System Sensors and Components

The DPF system uses multiple sensors to monitor its operation and provide data to the PCM, allowing it to adjust engine settings for optimal regeneration and performance.

• The Differential Pressure Sensor (DPS) measures the pressure difference between the DPF inlet and outlet. A clogged filter will be indicated by a larger pressure differential across the DPF. This signal is used by the PCM for decisions on when to perform an active DPF regeneration.
• Exhaust Gas Temperature (EGT) sensors monitor the temperature of the exhaust gases. The typical DPF system will have an upstream (pre) and downstream (post) EGT sensor reporting to the PCM. The EGT sensors play an essential role in checking the operation of the DPF and are monitored closely by the PCM during regeneration events. The PCM will monitor the exhaust temperatures to determine if the temperature is high enough for a passive regeneration to occur efficiently and remove the deposited soot or if an active regeneration is needed. The EGTs are also used to prevent the DPF from being overheated and causing possible damage during a regeneration event. EGTs can either be thermocouple or resistance-based sensors.
• A Particulate Matter (PM) sensor may be used to measure the soot levels in the exhaust gases to monitor the amount of soot buildup in the DPF and help determine when regeneration is needed. They are typically mounted upstream of the DPF, but many times they are located upstream and downstream. These sensors provide real-time data to the PCM on DPF operation and trigger a regeneration event when soot levels reach a predefined threshold. The PM sensors function by measuring the electrical conductivity between two special electrodes in the sensor assembly. As the soot is deposited on these electrodes the resistance will decrease, indicating a higher soot load. The vehicle's PCM will monitor the soot accumulation on the PM sensor and will periodically command the PM sensor to perform their own burn-off of the accumulated soot and ash from its sensing surfaces, using an integral heating element. These cleaning events often occur after a DPF regeneration event. The Mercedes Benz BlueTec system and GM Duramax use the PM sensor to monitor the effectiveness of the DPF system on their vehicles.
During a DPF regeneration event, fuel injection is strategically controlled to increase exhaust temperatures and burn off accumulated soot. This process varies based on the type of regeneration (active, passive, or forced) and the design of the DPF system. There are two main variations: in-cylinder post injection (late injection) or exhaust fuel injection (hydrocarbon dosing). GM Duramax, Ford Power Stroke and Ram Cummins engines use the late injection method where extra fuel is injected late in the combustion cycle, after the main fueling event. This extra fuel will enter the exhaust and increase the exhaust stream gas temperature to burn off the accumulated soot in the DPF. Many heavy-duty systems will incorporate a dedicated fuel injector to inject extra fuel directly into the exhaust stream, increasing exhaust temperatures to burn off the soot in the DPF.

Typical Reasons DPF Failure Occurs

While DPFs are intended to be durable, operational problems or inadequate maintenance can lead to premature failure. Common reasons of DPF failure are as follows:

• Driving habits: Clogging can result from inadequate or incomplete DPF regeneration. Vehicles driven mostly in urban areas with stop-and-go driving, short trips, excessive idling, low loads, and low speed operation might not develop the exhaust temperatures needed to undergo passive regeneration sufficiently. Soot deposits will accumulate over time and can exceed the capacity of the DPF, causing clogging.
• Contaminants: Malfunctioning engine components, leaking fuel injectors, internal coolant leaks, engines burning oil or other failed components may introduce unburned fuel, engine oil, antifreeze, or other contaminants into the DPF. These contaminants can damage and clog the DPF and do not burn off during regeneration.
• Malfunctioning sensor: Sensor failures or sensor contamination, such as DPS, PM sensor or EGT sensors, may prevent accurate DPF monitoring and regeneration, allowing excessive soot accumulation.
Excessive ash accumulation: Fuel and oil additives cause ash to accumulate in the DPF over time. Unlike soot, ash cannot be burned off during regeneration. Its accumulation progressively limits exhaust flow, eventually requiring DPF replacement or cleaning. The ash accumulation value is typically displayed on the scan tool in grams and each manufacturer will have its own service limits. Older (2011-2016) GM Duramax would typically set a P2463 when the ash levels reached 200-250 grams.

A Diesel Particulate Filter's Lifespan

The normal lifespan of a typical light duty DPF varies between manufacturers, but the range is often 120,000 to 250,000 miles, before either replacement or cleaning is performed. But this mileage can be greatly affected by such things as vehicle maintenance, driving behavior, the type of fuel used, and oil quality. Ultra-low sulfur fuel, DPF-safe fuel additives and low-ash engine oils that meet specifications are necessary for long DPF life. Regular fully completed regeneration cycles and adequate highway speed driving will also help to extend the life of the DPF. Frequent incomplete or partial regenerations, stop and go city driving, extended periods of idling, poor quality fuel, or oil qualities and various engine mechanical issues can, however, dramatically shorten the DPFs lifetime.

Quickly Identifying GM DPF Problems

This quick and easy test can help find whether the DPF is cracked or failing internally for GM vehicles that show elevated smoke/soot or diagnostic trouble codes (i.e. P2002, P226D, P244A).

1. Rev the engine eight times to the rpm limit and then return it to idle.
2. Cover the tailpipe with cheesecloth or a like material.
Check whether the cloth gathers too much soot. Should the cloth get quite dirty, this indicates that the DPF is failing to efficiently capture soot and might call for a replacement of the filter.

Diagnosing the DPF

Diagnosing a light-duty DPF system requires a professional scan tool capable of reading live data and retrieving trouble codes (DTCs). Key PIDs to monitor will include DPF soot load (%), exhaust gas temperatures (EGT1-4), differential pressure sensor readings, ash levels, and regeneration status. A high differential pressure or excessive soot load (%) may show a clogged DPF. If regeneration is not occurring, check for faulty EGT sensors, a malfunctioning DPF pressure sensor, or injector post-injection issues. Always inspect for exhaust leaks, EGR malfunctions, or turbo issues, as these can increase soot accumulation, and these issues need to be addressed before replacing a DPF. Also ensure that there are no internal coolant leaks (EGR cooler failures) as antifreeze will quickly damage a DPF.

Common DPF Problems — By Vehicle Make and Model

Ford Diesel Engines (Power Stroke)

• Typical problems, particularly with the 6.4L and 6.7L Power Stroke diesel engines, are that they are prone to incomplete regeneration and too much soot accumulation. These vehicles sometimes have clogging problems, particularly when driven in urban settings for short trips.
• Typical repairs include DPF replacement or cleaning, EGT or differential pressure sensor replacement and repairing fuel injector issues. Additionally, there are PCM software updates meant to enhance regeneration control.

Ram Cummins Diesel Engines

• Common problems with the 6.7L Cummins engine in Ram trucks are DPF soot clogging, usually brought on by insufficient passive regeneration, and extended low-speed driving. There have also been issues with EGT sensors, leaking EGR systems and fuel injectors on these engines.
• Usually, repairs consist of replacing or cleaning the DPF after diagnosing the root cause of the excessive soot. Major engine repairs for injector issues and EGR issues must be addressed first. But often addressing the operator to ensure better driving habits and less extended idling can avert these issues.

Duramax Diesel Engines Chevrolet/GMC

• Common DPF problems affecting Chevrolet and GMC trucks with the Duramax engines are excessive short trips and stop-and-go driving that don’t allow passive regeneration to happen. Defective fuel injectors, stuck VGT turbocharger vanes, EGR failures and overuse of forced regenerations (causing DPF overheating or cracking) are also common DPF issues on these trucks.
• Often repairs involve a manual regeneration but sometimes the DPF may need to be replaced or cleaned. All base engine operations must be repaired first; faulty sensors including the differential pressure sensor could also need to be replaced.

Volkswagen/Audi Diesel Engines (TDI)

• The TDI engine family, especially the 2.0L and 3.0L engines, have experienced DPF problems, particularly following emissions-related software updates. But poor driving habits (low speed/stop-and-go) and low-grade fuel aggravate soot clogging problems.
• VW/Audi DPF replacement or cleaning is typical. Updates in ECM software might better support the DPF regeneration process.

 Mercedes-Benz BlueTEC Diesel Engines

• Mercedes-Benz BlueTEC engines experience ash accumulation, often from the use of incorrect engine oil and over time, suffer from regeneration failures brought on by a malfunctioning AdBlue system.
• Typical repairs involve replacing or cleaning the DPF, replacing malfunctioning sensors, and fixing AdBlue system issues to ensure correct DPF regeneration.

BMW Diesel Engines

• Common BMW DPF problems occur from insufficient regeneration that can cause clogging and high soot generation, particularly in vehicles driven mostly for city use.
Often the clearing of the soot requires manual regeneration. In severe cases, DPF replacement could be needed because of too much ash accumulation (using the improper engine oil or non-DPF safe fuel additives) which regeneration cannot clear.

Summary

As you can see, the most typical DPF issue is soot loading or plugging, and the most frequent cause is excessive vehicle idling, city-use or stop-and-go driving that doesn’t allow the engine management system to properly regenerate the DPF. There are other common issues that can cause the DPF to become plugged, and faulty sensors, fuel injector failures, EGR system issues, failing turbochargers and PCM software are all other common concerns that can adversely affect the functionality and durability of the DPF filter.

A DPF that has excessive ash buildup can be cleaned and restored if the physical filter monolith is not damaged, loose, or cracked. Professional cleaning methods (off vehicle) involve thermal baking, compressed air cleaning, ultrasonic or liquid flushing, or a combination of methods.

Reducing harmful diesel particulate emissions depends heavily on the DPF assembly. Extending the DPF's lifetime and avoiding expensive repairs depends on regular maintenance, proper fuel, engine oil, and fuel additives, as well as proper DPF regeneration. Often simply explaining to a customer that they need to adjust their driving habits can help them avoid many DPF issues. Understanding the DPF system, its components, sensors, and common issues across different vehicle models allows owners and technicians to diagnose and resolve problems effectively.