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Turbocharger failure and root causes

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Blowing Bad…

With turbochargers delivering the emissions demanded by legislation and performance desired by owners, Rob Marshall finds out how failure is usually the symptom of a problem, not the root cause.

Interestingly, failure of the main turbocharger core tends to be the consequence of something else. Replacing the turbocharger alone, without investigating any underlying causes, risks the return of a disgruntled customer, a host of unnecessary remedial work and a destroyed profit margin.

Understandably, many suppliers of turbocharger cores, i.e. the main mechanical chamber that houses the shaft, compressor and turbine wheels, have strict warranty stipulations. This is not because they are supplying low-quality parts; the real reason is to cover themselves against unreasonable claims.

Considering that typical turbine shaft speeds can exceed 300,000 times per minute, repairs of the associated components should be conducted by specialists. Melett highlights that a turbocharger is a precision-engineered product, repair of which requires specialist parts, training and equipment. Therefore, it is essential to source components via vendors that do not cut corners. This also justifies Borg Warner’s stance that non-specialised garages and car owners should not attempt to dismantle a turbocharger core. Should any relevant nuts and screws be loosened, for example, its warranty would be voided.

The sacrificial lamb…

A weakness in the lubrication system tends to be highlighted by turbocharger failure. Carwood reports that, because turbochargers place the engine oil under enormous strain, should the supply be restricted, expect catastrophic damage to the bearings, which destroys a turbo within seconds. In severe cases, it can even ruin the engine. The remanufacturing specialist also highlights that running a turbocharger without oil for only five seconds is said to be as harmful, if not more, as running an engine without lubrication for five minutes.

Technicians should not forget this, especially when installing a replacement unit. Dependent on application, many turbochargers require the technician to pre-lubricate the core’s bearings, so they are protected while the engine’s oil pressure builds. Cold-start oil starvation is a further reason why OE quality oil filters are mandatory.

This sectioned turbocharger core highlights the moving parts. Either side of the shaft is a rotating wheel – one, the turbine, is spun by exhaust gas. The other, the compressor, develops the intake boost pressure. The oil seals are situated just behind each wheel. Positioned above the shaft centre is the oil feed – note the tiny galleries that supply the bearings, before lubricant exits the unit and returns
to the sump.

Nissens reminds us, however, that temperature is a big enemy. While hot exhaust gases are responsible for much of the heat within the turbocharger core, the compressor must also pressurise the air intake and intercooler, which creates another heat source. The oil system is mainly liable for not only lubricating between the shaft and bearings but also transferring heat from the turbocharger core. Admittedly, some vehicles possess additional water cooling (sometimes with an auxiliary electric water pump) but this cannot compensate fully for deficiencies in the lubrication circuit. Nissens adds that an inadequate oil supply causes the lubricant within the turbocharger core to overheat and carbonise, which damages the shaft and its bearings. Aside from this happening during premature engine shutdown, a restricted oil pipe can be to blame. Some oil feeds contain a filter that can also block, starving the turbocharger of its lifeblood. Consequentially, many turbocharger suppliers insist such pipes are renewed, at the very least. Nissens explains that, even if the oil pressure is satisfactory, and both the oil feed and return lines are in good order, degraded oil that has passed its drain interval cannot fulfil its turbocharger protecting function. Aside from the natural denigration of the oil’s structure and its additive packs, dirt, water and fuel contamination can cause rapid wear of the highly-stressed parts of the turbocharger. Running a low oil level degrades the remaining lubricant even faster. Melett finds that oil leaks are one of the most common reported issues and, while the turbocharger might not be at fault, it is the first item to suffer the consequences.

Coming to the crunch…

Understandably, the result of introducing foreign particles into an intake containing a compressor wheel that is spinning at hundreds of thousands of times per minute, operating at temperatures approaching close to 1,000 oC, is not going to produce an outcome that is anything less than catastrophic. Yet, such damage remains a common problem.

Melett reports that these particles can enter the inlet via a damaged air filter, or intake hoses, so technicians must ensure that these parts are serviceable. Debris that was not cleared from a previous turbocharger can also damage the replacement unit. While poor workshop practice can also cause bolts, nuts and even rags to enter the intake, ice can also be a problem, although many cars possess a means of heating the inlet. Yet, you can still check that a fitted electrical heating element is working with your multimeter.

Melett also underscores shaft overspeeding. The situation is a consequence of air leaks between the compressor and the engine, due to cracks, failing seals, or even wastegate failure. The turbocharger then must work much harder to deliver the required pressure levels. The resulting overspeeding causes the compressor wheel to break apart, fragments of which can then be ingested by the engine.

Nissens adds that over-pressure brings its own problems. Caused typically by a malfunctioning blow-off valve, or MAP sensor, the situation creases a pressure imbalance between the turbine and compressor wheel housings, which are designed to work at similar pressures. This causes the compressor wheel to strike its housing, causing rapid turbocharger failure. Borg Warner adds that overspeeding increases the turbocharger’s operating temperatures, too.

Turning rogue…

When the turbocharger breaks, the consequences are also far-reaching. Carwood finds that, should shaft failure occur, metallic debris is forced under pressure into the combustion chamber, damaging the piston, valves, liners and cylinder-head. Borg Warner adds that, further down the line, catalysts can also be damaged.

Aside from physical abrasion, turbocharger failure can also cause the oil to bypass the internal oil seals and will contaminate other components. On the intake side, the pipework and intercooler will be affected. This is one reason why many companies recommend that intercoolers and their pipes are replaced following turbocharger failure. Should oil bypass the turbine seals, lubricant can enter the exhaust system, damaging either the catalytic converter, the DPF and any associated sensors. Therefore, it is worth reporting to the owner that these extra parts may also need attention, which will increase the repair cost.

Preventative measures…

The importance of regular oil changes, with a lubricant that meets the required specifications, along with OE quality filtration, is well-understood. Checking the engine oil pressure is also prudent, especially on a car that has endured neglected servicing. Should the turbocharger feature water cooling, ensure that the system is in good order and that any auxiliary electric water pumps, if fitted, are operational.

Driving technique also plays a significant role in turbocharger life. While Melett’s recommendation, that drivers allow the engine to warm up on idle for two minutes before settling off, may be impractical in the real world, it urges technicians to remind drivers about the importance of driving with a degree of mechanical sympathy. This includes driving steadily before accelerating too hard, especially on colder days, or if the engine has been idling for a while. Melett reasons that many drivers are unaware that it can take up to 20 minutes of running for the engine oil to reach its optimum working temperature from cold, unlike the coolant.

Carwood adds that, short trips aside, idling for long periods means that the turbocharger reaches neither its optimum speed, nor temperature. Consequently, carbon deposits can accumulate, allowing oil to pass through the compressor/ turbine wheels and into the housings. The company also advises selecting a lower gear, instead of making the engine labour at low RPMs and high boost pressures.

While turbochargers have improved immensely throughout the decades, ‘hot shut down’ remains a problem. Allowing the engine to idle for a time allows the turbocharger shaft to slow and shed some heat, while oil pressure is still present. Should the engine be switched off, the oil pressure is cut and the residual oil can overheat and carbonise, creating abrasive particles that can damage the shaft and its soft bearings. Borg Warner advises that an extended idle time before shut down is only really necessary after driving with high loads for over five minutes, such as driving at high motorway speeds, before entering a service station. Driving for several minutes under part-load conditions are usually sufficient to reduce turbo temperatures to a moderate level.

Nissens recommends that intercoolers and oil feed pipes (pictured) are replaced at the same time as the turbo.

Fitting consequences…

To bestow a replacement turbocharger with a long service life, and minimise you having to repeat the replacement task under warranty, pay extra attention to installation literature. The part’s guarantee may be invalidated if you choose not to follow the instructions. Replacing the engine oil and filter is a common requirement.

Prior to fitting the replacement unit, Carwood states that you inject 10mm of engine oil into the return line on the turbo’s underside. While still holding the unit upside-down, turn the wheel and shaft, by gripping the centre nut between the thumb and forefinger and rotating it gently for 30 seconds, while moving the turboshaft from side to side. Repeat the bearing pre-lubrication exercise on the oil inlet. As you rotate the shaft manually, should you notice any rubbing or sticking, reject the turbocharger.

Even if you have pre-lubricated the turbocharger and ensured that the whole lubrication circuit is unblocked, turbocharger dry starting remains a danger. Carwood notifies technicians to allow oil pressure to build before the engine fires, by cranking the engine ten times in single second bursts. Should the engine fire, switch it off immediately – it may be worth disabling the fuel pump to assist with this priming exercise. Once completed, start the engine and run it at idle for three minutes before accelerating, to purge any airlocks in the lubrication system.

While you should investigate any issues that cause low oil pressure, do not forget to check if micro filters are positioned within the turbocharger oil feed pipe.

Tuning consequences…

Borg Warner comments that tuning shortens turbocharger life, because it needs to operate at higher speeds and temperatures than those intended by its design and testing parameters. Melett agrees, adding that the normal working parameters of the engine should be modified only by professionals, because it can cause several turbocharger complications, such as temperature and backpressure issues. Carwood adds that, while most turbos have some tuning potential, they are designed to work within optimum airflow and boost pressure ranges. Yet, performance tuning alone is not the only reason why turbochargers can become overstressed. While blanking the EGR valve should be picked up by MOT Testers, in practice, this modification is almost impossible to spot visually. Even so, EGR bypassing increases not only NOx emissions (which are not assessed by the annual inspection) but also turbocharger temperatures.

Carwood advises budding tuners that turbocharger internals rotate at designated speeds that are intended for the vehicle to which they are fitted. While they possess a design tolerance, the shaft, turbine and compressor can only spin so fast until overspeeding causes mechanical failure, or the air exiting the compressor becomes excessively hot. At this point, installing a larger turbocharger is the only option. However, the rest of the intake system (and, possibly, the engine) might need upgrading to prevent other consequences that result from excessive boost pressures.

TURBO TRENDS…

Now that Peugeot/Citroen’s 1.4/1.6 HDI engine has been out of production for several years, it is unsurprising that replacing this unit’s turbocharger is not as popular as it once was. Carwood tells ATthat the DV6’s Garret turbocharger is still in its top ten of most popular units but the Mitsubishi unit that was fitted to earlier engines languishes in 33rd place. Currently, Carwood’s top five remanufactured turbochargers are:

  • 1. TUR786880-0006/ TUR854800-0001 – Ford Transit/Tourneo Custom 2.2 2013-
  • 2. TUR5438-970-0005 – Various applications – Mercedes, Nissan, Vauxhall and Renault 1.6 2014-
  • 3. TUR795367-0001 – Renault Master/Traffic 2.3 dCi, Vauxhall Vivaro 2.0 CDTi
  • 4. TUR5435-970-0027 – Alfa Romeo Mito, Chevrolet Aveo, Fiat Doblo, Idea, Punto, Qubo, Vauxhall Astra, Combo, Corsa and Meriva 1.3 MultiJet 2013-
  • 5. TUR724930-0012 – Audi A3, Seat Altea, Leon and Toledo, Skoda Octavia, Superb, VW Golf, Passat, Touran and Jetta 2.0TDi 16V 2003-

TURBO TRAINING…

Nissens offers a pair of turbo e-learning courses; the first focuses on theory, the second on best practice diagnostics and installation. It is offered through the Nissens Training Concept (NTC) training platform, which is being developed continuously and is designed to help technicians overcome the current lack of live training meetings. It can be accessed via this online activation weblink: https://ntc.nissens.com/muc/turbo21uk

TURBOCHARGER HOSES…

A consequence of being exposed constantly to temperature and pressure changes mean that turbocharger hoses will need replacing. Even a small split leads to a loss of boost, which reduces performance, increases exhaust emissions, harms fuel economy, causes rapid DPF filling and can result in limp home mode being instigated. The turbocharger internals can also overspeed, introducing the possibility of permanent damage or failure of the turbo. Should this happen, it is recommended to renew all the hoses (and the intercooler) to ensure no debris remains that could damage the replacement turbocharger assembly.

While garages tend to approach main dealers for replacements, consider an alternative option. First Line has introduced a range of hoses that are made from materials and utilise production processes that meet, or exceed, OE standards. They are backed-up by a two year, 24,000-miles warranty.

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