Exhausts, cats & accessories 42 Investigating the root causes of repeat DPF failures While a DPF problem can often be resolved through effective cleaning, it is essential to identify the root cause to prevent recurring DPF failures. David Eszenyi, Commercial Director at the remanufacturer Ivor Searle, explains. If a diesel particulate filter (DPF) keeps failing, the filter itself is typically not the real problem. In most instances, an issue upstream in the engine or emissions system is causing excess soot, which prevents the filter from cleaning and regenerating itself properly. If the DPF is cleaned or replaced without diagnosing and repairing the cause, it is likely to fail again, sometimes within just a few days. The common causes of DPF failure are generally related to engine problems, faulty sensors and oil and additive issues. Engine related issues Engine problems include boost leaks and split hoses, where small splits in intercooler or intake hoses can let air escape after it has been measured by the mass air flow (MAF) sensor. The ECU may then add excess fuel, which creates extra soot and blocks the DPF faster. Contaminated EGR valves, which have become clogged with carbon and stick open, also create excess soot, which is sent back through the engine and into the DPF. Leaking, worn, or blocked fuel injectors also lead to poor combustion, unburnt fuel, and heavier soot build-up. While turbocharger seal failures also cause exhaust contamination, as worn seals allow oil to enter the exhaust system. Oil can burn into a thick residue in the exhaust, coating the front of the DPF and making it harder for the filter to regenerate and clean itself. Today’s diesel vehicles typically utilise glow plugs to raise exhaust gas temperatures while driving and not just when the engine is started from cold. If a glow plug or control module has failed, the ECU is likely to stop the automatic DPF cleaning and regeneration process from taking place. Incorrect DPF pressure readings can also cause problems. The sensor checks how much soot is in the DPF by measuring pressure before and after the filter. If the sensor is inaccurate, or its pipes are split or blocked, the vehicle may not start a cleaning cycle when required. Even stored fault codes can be problematic. Unrelated faults, such as a service warning or thermostat issue, can cause the ECU to stop DPF regeneration as a precaution. Additional causes Use of the incorrect engine oil can also cause ash deposits in the DPF. It is important to use the correct specification, vehicle-specific oil as some standard engine oils create ash when they burn. Unlike soot, this ash cannot be removed by normal DPF regeneration and can permanently block the filter channels. A low or empty DPF additive tank can also cause excess soot. Some vehicles use particle additive tank (PAT) fluid, also known as Eloys, to help burn soot at lower temperatures. If the additive tank is empty, or the vehicle computer has not been reset after a refill, the DPF may not clean itself properly. Interrupted DPF cleaning cycles also cause soot to build up, as the DPF needs sufficient heat and time to finish cleaning itself. If the vehicle is switched off during an active regeneration or is mainly used for short trips, the soot may only partially burn off and continue to build up in the filter. To prevent the DPF problem from recurring, it is recommended to perform a full diagnostic evaluation during a drive cycle, rather than just scanning the vehicle in the workshop for fault codes. Technicians can gain a wealth of information from live engine data, including exhaust temperatures, EGR operation, and boost pressure, to identify the root cause of the blocked DPF and prevent repeat failures. www.ivorsearle.co.uk
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