Why Do Diesel Particulate Filters Fail?

Most DPF filter failures occur as a result of DPF regeneration process either not initiating, or running to completion, regardless of whether the regeneration process is active or passive. As a practical matter, an active regeneration process happens when the engine management system injects raw fuel directly into the DPF where it combusts, thus burning off the accumulated load of diesel soot.

By way of contrast, a passive regeneration process occurs when the engine management system injects fuel into the exhaust manifold (via the fuel injectors) where the fuel combusts, thus raising the temperature of the exhaust gas to the point where the accumulated load of diesel soot can be burned off. Note that diesel vehicles have either passive or active DPF regeneration- not both at the same time.

Nonetheless, regardless of which system is in use on any given vehicle, the causes of excessive diesel soot buildup are largely the same on all diesel vehicles, although the rate at which excessive soot loads accumulate can vary between vehicles for a variety of reasons. Let us look at the most common of these reasons in some detail-

Frequent short trips

In this context, “short” trips are those that are not long enough to allow the engine to reach its optimal operating temperature, which means that if the vehicle has passive regeneration, the exhaust gas never reaches the temperature at which diesel soot can be burned off. If however, a vehicle has active regeneration, the vehicle might never, or seldom, reach the road speeds at which the engine management will initiate an active regeneration process.

In both cases, the DPF might accumulate excessive soot loads relatively quickly, but since the conditions that regeneration processes require to initiate are never (or very rarely) met or satisfied, the DPF is never, or seldom, regenerated to restore its functionality.

Note that while a forced regeneration performed with a scan tool might be successful in regenerating a severely clogged DPF, there are no guarantees that forced regenerations will always initiate or be successful.

Excessive engine wear

Since severely worn piston rings cause excessive amounts of oil to seep into the combustion chambers, the exhaust gas will contain high concentrations of oil additives and partially combusted oil. As a practical matter, the partially combusted oil and oil additives will condense onto the core of the DPF and while a regeneration event might burn off some of the oil, much of the partially combusted oil will be carbonized in the DPF. This will eventually form a hard crust of carbonized oil on the surfaces of the DPF core, thus preventing or impeding the free flow of exhaust gas through the core.

So with every generation event that does not remove all of the carbonized oil, the accumulation of carbon increases, further impeding the flow of exhaust gas through the core, which further reduces the DPF's ability to trap and contain diesel soot particles. If this condition is allowed to persist for extended periods, the DPF will become clogged to the point where exhaust gas can no longer pass through it freely, which causes progressively higher exhaust back pressures to develop as the DPF becomes progressively more clogged up.

Note that forced regenerations are typically not able to remove or burn off carbonized accumulation of engine oil. The only reliable remedy for this condition is the replacement of the DPF with an OEM or OEM-equivalent DPF.

DPF pressure sensor failures

 Since an engine management system can monitor neither the condition nor the efficiency of a DPF directly, DPF devices are typically fitted with two exhaust pressure sensors- one before the DPF and one after the DPF.

The engine control module monitors the pressure(s) reported by both sensors; if there is no pressure differential between the sensors, the engine control module will conclude that exhaust gas is flowing freely through the DPF. If, however, a pressure differential develops between the pressure sensors, the engine control module will conclude that the DPF has accumulated a significant volume of diesel soot. As a practical matter, though, since the engine control module can also not measure the degree of soot accumulation, it will reference pre-programmed look-up tables to calculate an efficiency value for the DPF that is based directly on the magnitude of the pressure differential between the two pressure sensors.

Thus, when the calculated DPF efficiency value drops below about 95 per cent (assuming that both pressure sensors are working as designed) and all required enabling conditions are met, the engine control module will initiate a regeneration event to restore the DPFs functionality.

While DPF pressure sensor failures are not actual DPF failures, these sensors play a critical role in diesel soot control, so when one or both pressure sensors fail, the engine control module cannot assess the efficiency of the DPF, which from an emissions control perspective, is the same thing as a DPF failure.

The use of substandard aftermarket, or “rebuilt” DPFs

Since OEM or OEM-equivalent DFPs are very expensive, unscrupulous supplies of aftermarket parts have taken to importing and distributing cheap, but substandard DPFs that do not conform to any acceptable standards in terms of performance and durability. So, while such products might work satisfactorily, they will do so only for a strictly limited time, which can sometimes be measured in days, or even hours of operation before failing.

“Rebuilt” DPFs, on the other hand, sometimes do not work at all because in many cases, a “rebuilt” DPF is just an old, failed unit that had been cleaned off and repainted to look new.

Note though that none of the above is the same as saying that all aftermarket DPFs are bad or substandard- far from it. In fact, several aftermarket DPF manufacturers fabricate and supply high-quality products for use in competition and/or severe service applications, and many of these units typically meet and often exceed OEM specifications in terms of fit, form, and function.

However, in practice, it is often very difficult to distinguish between a high-quality special application aftermarket DPF and cheap, illegal knock-off DPFs. Therefore,  be sure to consult with recognized experts in diesel exhaust components before purchasing or installing any aftermarket DPF to avoid ending up with a DPF that may not only not work, but may also be illegal to fit to any standard road-going vehicle.

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