As experienced technicians, we have all seen how defective turbochargers can damage catalytic converters and diesel particulate filters (DPF), but have you ever wondered if the opposite is true; can a defective or clogged DPF damage a turbocharger? You may be surprised to learn that clogged or defective DPF’s do in fact damage more turbochargers than you might have thought, and in this article we will take a closer look at how this process works, starting with this question-
Diesel particulate filters were first introduced in 2005 as part of the Euro 4 emissions regulations, and as their name suggests, their purpose is to filter out or entrap solid particulate matter (aka soot) in diesel exhaust.
However, being filters, diesel particulate filters have a limited capacity to hold on to trapped soot particles before they reach the point where they become so clogged up that the free flow of exhaust gas through the device is impeded or restricted. To prevent this from happening, car makers have adopted various strategies that are designed to clean out the accumulated soot in a process called “regeneration”, which although important, does not overly concern us in this article.
As a practical matter, the emission control systems on diesel vehicles use input data from sensors such as exhaust gas backpressure and exhaust gas temperature sensors to determine the efficiency of diesel particulate filters in terms of the amount of accumulated soot the device holds, which is expressed as a percentage of the devices’ theoretical efficiency. Thus, when the efficiency of a diesel particulate filter is deemed less than about 80% - 85%, the ECU will initiate a process of regeneration to burn off the accumulated soot, which then restores the diesel particulate filters’ efficiency, as well as its ability to allow the exhaust stream to pass through it freely.
Nonetheless, all diesel powered vehicles also use turbochargers to boost power, and it is the relationship between the turbocharger, diesel particulate filter, and the overall ability of the exhaust system to extract exhaust gas from the engine that determines whether or not a clogged or defective diesel particulate filter can, or will damage the turbocharger, which brings us to-
Since a clogged diesel particulate filter prevents the free flow of exhaust gas through the exhaust system, the restriction can have several effects on a turbocharger, although the severity of one or more effects may vary according to the application, and the degree to which the diesel particulate filter is restricting the flow of exhaust gas. Nonetheless, as a rule of thumb, any one, or more of the following things can happen-
Reduced turbocharger efficiency
Exhaust gas needs the pass through the exhaust system freely and at a certain rate in order for an engine to breathe freely. However, since a clogged diesel particulate filter restricts the exhaust flow, the resulting backpressure acts as a brake on the turbocharger that both reduces the turbocharger’s efficiency, and increases the temperature of the exhaust gas in and immediately after the turbocharger, which has several negative consequences, including-
Carbonisation of oil in the turbocharger
During normal operation the oil that seeps past turbocharger shaft seals remains liquid, but at elevated temperatures caused by excessive exhaust backpressure, oil within the CHRA (Centre Housing Rotating Assembly) can be carbonised (which is often misdiagnosed as insufficient lubrication of the rotating assembly), with severe mechanical wear of the turbo shaft, thrust plate, seals, and bearings being the result.
Oil leaks
If exhaust backpressures become excessive, it can happen that exhaust gas is forced past the shaft seals on the turbine side into the rotating assembly, which has the effect of displacing lubricating oil into the compressor side through the shaft seal on the compressor side. This is often misdiagnosed as worn shaft seals
Wastegate failure
One of the characteristics of exhaust gas is that can seep into, and past the tiniest of gaps. Thus, when excessive backpressure is present, exhaust gas can be forced past the wastegate, which not only reduces the overall efficiency of the turbocharger, but can also cause carbon to build up around the wastegate and its control mechanism(s). In severe cases, the wastegate door can become stuck as the result of excessive carbon build-up, which can make it impossible for the ECU to control the boost pressure.
Turbine wheel failure
Over prolonged periods, excessive exhaust gas temperatures caused by excessive exhaust backpressures can raise the temperature inside the turbine housing to the point where the turbine wheel can fail as the result of high cycle fatigue. This is similar to failures caused by metal fatigue, but in this case, the failure is the result of excessive temperatures, as opposed to repeated mechanical loads.
Sticking oil rings
Although this does not affect the turbocharger directly, excessive exhaust gas pressures and temperatures can cause carbon to build up in the turbine side of the oil rings, making the rings stick in the ring grooves which obviously prevents effective oil control on the cylinder walls, which brings us to-
Experienced technicians will know that several of the issues outlined above can have causes that do not involve diesel particulate filters, but if you have not had much exposure to diesel vehicles, it can be tricky to determine which is which. Did the diesel particulate filter cause the turbocharger failure, or did the defective turbocharger cause the diesel particulate filter to fail?
Fortunately, there are several ways to tell which is which, with the most unambiguous clues being-
Discolouration of rotating turbocharger parts
Although discolouration of rotating parts as the result of excessive heat is an important clue, the pattern of discoloration is more important. For instance, discolouration that starts at the turbine wheel and extends though the shaft towards the compressor side is clear evidence of excessive temperatures caused by excessive exhaust backpressure and hence, a clogged or defective diesel particulate filter, which should be confirmed by scanning the vehicle for at least one or more of the following generic-
Fault codes
If you are new to diesel emission control systems, you should take note that although the list of fault codes above offers the best indications that a diesel particulate filter may be clogged or defective, the list is not complete, nor exhaustive. Be aware also that while all the codes on this list will illuminate the DPF warning light, many other codes, including generic and/or manufacturer-specific codes that relate to the EGR and/or DPF regeneration systems/processes may be present as well, any of which could turn out to be the root cause of the diesel particulate filter issue. Additionally, one or more turbocharger boost control issues may also be present, some of which may be manufacturer-specific, which begs this question-
As a rule, diesel particulate filters seldom, if ever, fail outright or without warning, which means that most, if not all diesel particulate filter issues are caused by problems in other systems and/or emission control processes*. In fact, if everything in a diesel emission control system works as designed and expected, an OEM diesel particulate filter should provide reliable and dependable service for at least 160 000 to 200 000 km before showing signs of failing.
* It should be noted that on applications that use a liquid reductant to initiate DPF regeneration, failures and defects in this system often produce a limp mode and/or a no-start condition that will persist until the fault in the reductant injection system is corrected. Note also that some types of failures in the reductant injection system can cause an excess of ammonia to be deposited onto the DPF substrate, which in some cases, can have the same effect on DPF operation as an excessive soot load.
However, several factors can dramatically reduce the useful service life of a diesel particulate filter and by extension, the useful life of a turbocharger. The most common such factors generally include the following-
Multiple short trips
One particular regeneration process, known as “passive regeneration” depends on the temperature of the exhaust gas and adaptations to the injection timing to burn off accumulated soot in the diesel particulate filter, which usually requires sustained high-speed driving to be initiated. Therefore, since the exhaust gas almost never reaches the required temperature in urban driving conditions, passive regeneration can never take place, or if it does, it may not complete successfully. In both scenarios, a diesel particulate filter can become clogged in a relatively short time, with possible damage to the turbocharger as a result.
Failed or failing sensors
ECU’s cannot monitor the efficiency level of a diesel particulate filter directly, but can only infer an efficiency value based on input data from pressure and/or temperature sensors that are located upstream and downstream from the diesel particulate filter. Thus, if one or more of these sensors produce inaccurate, invalid, implausible, or no input data at all, the ECU cannot initiate, control, or regulate regeneration processes, which in turn, causes the diesel particulate filter to clog up, with possible turbocharger damage again being the result.
EGR issues
EGR systems are inherently unreliable, given the conditions these systems are expected to cope with. Nonetheless, since correct EGR operation is a critical component of effective emission control on diesel vehicles, almost any malfunction or failure in an EGR system is bound to affect the operation of a diesel particulate filter directly. In fact, the most common effect of most EGR failures is incomplete combustion that produces excessive levels of soot in the exhaust gas, thus clogging up the diesel particulate filter.
It should be noted that correct EGR operation is just one of the many preconditions that must be met before DPF regeneration can take place. Therefore, if regeneration cannot take place over extended periods, the soot load in the diesel particulate filter could increase to the point where not even a forced regeneration process can clear out the soot, assuming of course, that a forced regeneration is still possible, which it may not be, depending on the soot load.
Note that incomplete combustion caused by EGR failures can produce turbocharger failures quicker than almost any other cause that affects the operation of diesel particulate filters, including defective intake manifold runner flaps (aka swirl flaps).
Leaking injectors
Diesel engines run with excess air, and can therefore operate on a wide range of air/fuel mixtures without performance, fuel economy, or emission levels being affected in meaningful ways. Therefore, leaking injectors could go unnoticed for a period, especially if the injectors are only marginally defective, and therefore do not leak much. Nonetheless, even marginally leaking injectors on common rail diesel injection systems affect the combustion process directly, with increased soot production, clogged diesel particulate filters, and damaged turbochargers being the most common results.
One final thing to keep in mind when dealing with defective or damaged turbochargers is that all reasonable efforts must be made to either confirm, or eliminate a defective or clogged diesel particulate filter as the cause of the turbocharger failure. In some cases though, the distinction between cause and effect is not at all clear, so when in doubt, err on the side of caution and replace both the turbocharger and the diesel particulate filter.
