There are many articles and technical documents relating to how a faulty turbo can lead to DPF damage. However, the DPF is actually responsible for more turbo-related failures than you might think. Here we explore what effect a blocked DPF can have on a turbocharger.

DPFs (Diesel Particulate Filters) were first introduced in January 2005 with the Euro 4 emission standard, where diesel particulate levels were reduced to extremely low levels to reduce the allowable amount of particulate matter (PM) released into the atmosphere. Reducing the size of PM from the combustion process to this level was not technically possible, so this meant all diesel vehicles after September 2009 were fitted with a filter to capture soot and other harmful particles, preventing them entering the atmosphere. A DPF can remove around 85% of the particulates from the exhaust gas.

A blocked DPF will not work correctly. To clear this blockage there are two types of regeneration which are commonly used to remove the build-up of soot. Newer vehicles engage active regeneration, which is the process of removing the accumulated soot from the filter by adding fuel post combustion to increase exhaust gas temperatures and burn off the soot, providing a temporary solution. Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Many manufacturers have moved to using active regeneration as many motorists do not often drive prolonged distances at motorway speeds to clear the DPF. Constant short distances are not good for the turbo or exhaust system.

So, what happens to the turbo when a DPF is blocked?

A blocked DPF prevents exhaust gas passing through the exhaust system at the required rate. As a result, back pressure and exhaust gas temperatures increase within the turbine housing. Increased exhaust gas temperature and back pressure can affect the turbocharger in a number of ways, including:

• Problems with efficiencies
• Oil leaks
• Carbonisation of oil within the turbo
• Exhaust gas leaks from the turbo

How to spot a turbocharger that has suffered from DPF problems:

Discolouration of parts within the core assembly (CHRA)

Usually with evidence that the heat is transferring through the CHRA from the turbine side. This excessive temperature within the CHRA is caused by back pressure forcing the exhaust gas through the piston ring seals and into the CHRA. The high temperature exhaust gas can prevent efficient oil cooling within the CHRA and even carbonise the oil, restricting oil feeds and causing wear to the bearing systems. This type of failure can often be mistaken as a lack of lubrication or contaminated oil.

Carbon build-up in the turbine side piston ring groove

Caused by the increased exhaust gas temperatures.

Oil leaks into the compressor housing

Can be seen as a consequence of exhaust gas forcing its way into the CHRA from the turbine side and forcing oil through the oil seal on the compressor side.

Exhaust gas forced through smallest gaps

A blocked DPF can force exhaust gas through the smallest of gaps, including the clearances in the bearing housing VNT lever arm and turbine housing waste gate mechanisms. If this occurs, carbon build-up in these mechanisms can restrict movement of the levers, affecting performance of the turbo. In some cases soot build-up can be seen on the back face of the seal plate where the exhaust gas has been forced through.

Turbine wheel failure

Caused by high cycle fatigue (HCF) due to temperature increase.

How can you prevent these failures from occurring?

As a starting point, it is essential to identify the failure mode and determine whether a DPF-related issue is the root cause. If the entire rotor assembly is okay, and there are some signs of overheating towards the turbine side of the core assembly, then the failure is likely to be caused by excessive exhaust gas temperatures. High amounts of carbon build-up within the VNT mechanism and lever arms indicate a blocked DPF, and the driver may experience turbo lag or over boost of the turbo.

To help prevent turbo failure caused by DPFs:

• Determine whether the DPF is blocked.
• Contact a DPF specialist for advice.
• Replace the DPF with a high quality replacement — lower cost DPFs will often not operate as efficiently as the original. This can replicate the environment of a blocked DPF.
• If the DPF is blocked, always replace the turbocharger core assembly to prevent possible oil leaks.
• Check the actuator achieves its full range of movement, particularly if electronic, as internal components could be worn.

Important: It takes time for a DPF to block, sometimes years. Once blocked though, turbo failure can occur very quickly. If you don't check for a DPF issue when installing a replacement turbo, there is a very high chance the replacement turbo will suffer the same failure, as it will be subject to the same operating environment as the previous unit.

Turbochargers have come a long way in recent years. With modern engines demanding higher performance, turbochargers are spinning faster than ever and operating under extreme temperatures. This evolution has driven significant advancements in compressor wheel design to handle these more demanding conditions.

Flatback: The Original Design

The Flatback compressor wheel is one of the earliest designs in turbocharging. While simple, it laid the foundation for modern wheel development and is still used by some manufacturers today.

Stepped Back: Strengthened for Durability

The Stepped Back design was a small but important improvement over the original flatback. Its primary purpose was to reinforce the compressor wheel, adding strength to withstand higher loads. However, this design is now rarely used in modern turbocharger applications.

Superback: Reinforcing for Speed

As turbochargers began spinning at higher speeds, the Superback design emerged. The increased rotational speed puts tremendous force on the compressor wheel, particularly at the exducer—the outer edge of the wheel that rotates fastest. The Superback design reinforces the back face of the wheel, preventing it from tearing under stress and improving reliability under extreme conditions.

Deep Superback: Meeting Modern Demands

The Deep Superback is an exaggerated version of the Superback, used in many recent turbo applications. With increasing rotational speeds in modern engines, this design provides even greater durability and performance.

Deep Superback – Extended Tip: Boosting Efficiency

The Deep Superback with Extended Tip goes a step further by optimizing airflow. This design allows for faster boost response at lower engine speeds while maintaining efficiency at higher boost pressures. By effectively amplifying the performance of a smaller wheel, extended tip technology enables smaller turbochargers to perform like larger ones, delivering better airflow and boost capabilities.

Other Compressor Wheel Designs

Threaded Wheels

• Features threads that extend all the way up the shaft
• Ideal for applications with increased load
• Eliminates the need to source a separate shaft nut

Boreless Wheels

• Threads extend only halfway up the shaft
• Provides added strength for high-load applications
• A stronger, more durable design for modern turbochargers

Conclusion

From flatback to deep superback with extended tips, compressor wheel designs have evolved to meet the extreme demands of today's turbocharged engines. Understanding these designs helps automotive enthusiasts and engineers appreciate how modern turbos achieve higher speeds, improved efficiency, and greater durability.

REA (Rotary Electronic Actuator) and SREA (Simple Rotary Electronic Actuator) electronic actuators are fitted to a variety of different variable geometry turbos and control the variable vane movement. The following questions provide answers to common REA/SREA issues.

What are the symptoms of possible electronic actuator failure?

There are a few factors which determine an actuator failure:

• Flashing engine management light
• A complete loss of power, causing the vehicle to go into limp home mode
• Low boost
• Over boost
• Noise from the turbocharger
• Fault codes

How to identify a REA/SREA connector?

There are two types of Electronic Actuators. The two types (SREA/REA) can be identified by the different orientation of their connectors.

Please note: Avoid touching the connectors to reduce the risk of damage.

My diagnostic equipment has identified a fault with the turbocharger — is an electrical fault responsible?

Checking ECU error codes and researching these codes is critical and can help you identify the root cause very quickly. Turbos are on the same circuit as other sensors, and it may well be those at fault, not necessarily the turbo. We have seen examples of turbo faults being registered on the diagnostic tool where, upon investigation, the injectors were actually at fault.

Check the vehicle history for past issues, such as previously recorded ECU error codes and replaced engine components — especially those linked to the turbo.

With the engine turned off and cold, check if the linkage can be seen or felt between the SREA/REA (a small black box attached to the intake side of the turbo) and the bearing housing lever arm. If accessible, check for free movement at each end of the linkage — there should be a small amount of play. Also check for corrosion which may be restricting movement, and confirm the linkage is not detached at either end.

Please note: Use the locking tabs to release the electrical connector to avoid damage.

Further investigation:

• Is the REA/SREA electrical connector firmly located? Avoid pulling the wiring directly as this could cause damage.
• Are the locking tabs on the connector damaged?
• Are there signs of water ingress? Check for water or staining below the connector seal and in the REA/SREA connector (removal of the connecting wires is required to do this).
• Are the connector walls or seal damaged? Check the REA/SREA connector walls for damage or cracks.
• Are the wires within the harness connected properly and unused positions sealed?

Check the REA/SREA connector wall for damage or cracks.

Finally…

• Switch on the ignition without cranking the engine — does the electronic actuator move freely? Note: some actuators will not move until the engine is running.
• Do any warning lights appear on the dashboard?
• Start the engine and again listen for actuator noise; if possible, visually check for movement of the actuator.
• Check again for any warning lights on the dashboard. If lights appear and movement of the actuator can be seen, there may be an electronic fault elsewhere on the vehicle.
• Switch the ignition off — the actuator arm should move rapidly to the "safe" position. On some occasions the actuator arm may continue its sweep to clear the nozzle vane path, depending on the application.

If the steps above have been followed and all connectors are in good condition, movement of the actuator is free, and there are no signs of water ingress, then it is highly likely the fault lies somewhere other than the turbo.

If the electronic actuator has failed, should I check the turbocharger?

It is vital to check the turbocharger even if the problem appears to be an electronic actuator-only issue.

SREA and REA actuators can fail because the variable geometry mechanism sticks due to sludging or carbon build-up. When this happens, it pulls a higher current through the motor than it is designed for, causing the motor to burn out or the plastic worm gears to fail. This restriction can reduce boost pressure and may result in the vehicle being put into limp home mode.

When repairing the electronic actuator, the worm gear and motor must be the correct ratio to avoid immediate failure. In most failures, the black cap and electronics within are unaffected by gearbox failure and can be reused.

If the actuator has failed to open the nozzle ring assembly vanes under acceleration, the turbo will also fail to operate efficiently. Vanes set to a closed position can cause choking of the engine or overspeeding of the turbine. Conversely, vanes that are open more than required will cause excessive lag and slow turbo response.

Are electronic actuators interchangeable?

SREA/REA electronic actuators are highly complex and intricate. They are not interchangeable with different gearboxes or black caps. The calibration settings are programmed in the software within the black cap and are unique to each individual turbocharger. Even loosening the retaining bolts on the actuator will take it outside of specification.

The black cap is damaged — can I repair it?

Each turbo is calibrated differently and the electronics within each cap are programmed specifically for that turbo, making it a non-serviceable part. In this instance you would need to replace the complete SREA or REA unit with the correct calibration settings. Incorrect calibration of the electronic actuator when assembled on the turbocharger can result in poor performance or complete failure.

Can I use solder to fix broken connections in the black cap?

Soldering to repair broken connections in the gearbox and black cap is not advised. Solder is susceptible to cracking in environments with temperature variance and vibration. For this reason, the motor, motor choke assembly, and connectors must not be spot welded.

What other factors can cause my actuator to fail?

Mishandling of the turbo

If the turbo connector is knocked or banged it will break, and the whole unit will need replacing.

Water ingress

The location of a turbo in the engine compartment can make the electronic actuator susceptible to water ingress. The actuator can become rusty and contaminated, giving incorrect signals and ultimately failing.

Board connectors

The wire connectors can expand and contract over time, eventually breaking and causing actuator failure. This fault can often go undetected during repair and workshop testing, as it may only become apparent once a temperature change occurs.

Engine vibration

Constant vibration from the vehicle can gradually wear the electronic actuator out, causing it to fail over time.

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Signs You May Need a New Turbocharger

If you are experiencing:

• Loss of power under acceleration
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