Why can a new turbocharger fail? 7 causes outside the turbocharger itself

A new turbocharger can be damaged if the actual cause of the previous unit’s failure is left in the vehicle. The new component will use the same lubrication, intake and exhaust systems, and will operate under the same combustion conditions. This is why, before fitting another turbocharger, you need to establish not only what was damaged, but also the mechanism that led to it. The condition of the removed turbocharger can point the diagnosis in a particular direction, but on its own it does not yet confirm a specific cause.

The conclusions below are based on recurring cases of turbocharger damage and on information provided by workshops specialising in forced induction systems. Wiatreo’s more than 20 years of experience in the industry make it possible to view a damaged turbocharger not merely as a part to be replaced, but also as a source of information about problems in the systems that work together with the engine. This practice shows why, before a new component is fitted, you must check the systems that could have created the conditions leading to the failure of the previous one.

7 causes of turbocharger failure outside the turbocharger itself

Worn bearings, damaged compressor wheel blades and an overheated hot side lead the diagnosis in different directions. Before fitting another turbocharger, you therefore need to check the systems that could have altered its lubrication, air flow, exhaust gas pressure or combustion conditions. The same symptom can have several sources, so finding the damage does not yet end the search for the cause.

1. Lubrication problems: oil quantity, pressure and properties

The shaft and the plain bearings of the turbocharger are separated by an oil film. Insufficient oil, pressure that is too low, incorrect viscosity or insufficient resistance of the oil to high temperature all degrade lubrication. In an extreme case the shaft begins to run without an adequate protective layer, which leads to wear of the bearing surfaces, increased clearance and seizure, and in severe cases also to damage to the shaft.

The problem also works the other way round. Excessive pressure, for example associated with incorrect operation of the oil pump, can encourage oil to pass into the intake or hot side and form deposits. Oil used for too long loses its protective properties, oxidises and cokes, and the deposit impairs flow and the operation of moving parts. Coking is also encouraged by switching off a thoroughly heated engine immediately after heavy load, when oil circulation stops while the turbocharger is still at high temperature.

Before fitting another unit you must therefore check the oil level and specification, the pressure in the system, the operation of the pump and the patency of the oil feed line. Simply replacing the turbocharger will not improve any of these parameters.

2. Blocked oil return and disturbed pressure difference

A properly functioning oil supply is not enough if the lubricant cannot leave the turbocharger freely. A return line that is restricted, coked, kinked or incorrectly routed creates back pressure inside the housing. Oil can then pass into the intake or exhaust, where the high temperature encourages it to burn and form hard deposits.

A similar picture can occur where there is an incorrect pressure difference between the intake and exhaust sides. Resistance on one side can encourage oil to be pushed out on the other, which is why oil in the pipework, smoking and a drop in boost pressure do not yet prove that the internal components of the turbocharger are damaged. In addition, oil drainage can be impaired by elevated crankcase pressure, so with such symptoms the inspection should not stop at the return line alone.

The coke that forms under such conditions can, over time, restrict the movement of the mechanisms controlling exhaust gas flow. Before fitting a new turbo you must therefore establish why the oil was not draining correctly, instead of treating the leak itself as a complete diagnosis.

3. Leaking intake, air filter and contamination

The intake system can place a load on the turbocharger in several ways. A dirty filter restricts air flow, while a damaged or poorly seated element, a leaking housing or a loss of sealing upstream of the compressor can allow unfiltered air, dust and small foreign bodies to reach the impeller. Leaks in the pipework also change the air flow and make it harder to achieve the expected boost pressure.

Hard particles striking the rapidly rotating compressor wheel act as an abrasive. They gradually destroy the edges of the compressor wheel blades or the shaft, create micro-damage and change the mass distribution of the rotating assembly, which can worsen its balance and increase the load on the shaft and bearings. Another problem is created by oil and carbon deposits building up in the intake, because a large amount of deposit is a signal that its source must be identified before it starts to restrict flow or travel further into the system.

Traces of mechanical impact on the cold side therefore direct the diagnosis towards the filter, its housing and the section of the intake upstream of the compressor. A drop in boost pressure without such traces, on the other hand, calls for a check of the tightness of the pipework and connections.

4. DPF, catalytic converter and GPF: when the problem starts in the exhaust

A clogged DPF or excessively restricted flow through the catalytic converter increase exhaust back pressure. As the resistance rises, so does the thermal load on the hot side, and the exhaust gases do not leave the system under the conditions foreseen by the engine’s design. The result can be overheating, oil leaks, fluctuations in boost pressure, accelerated wear of the bearing system and a loss of power.

High temperature and soot can also lead to coking of the oil and restrict the movement of the VNT mechanism or wastegate. The original exhaust flow problem then begins to affect boost control as well. In petrol engines the GPF calls for similar attention, because a disturbance of flow under high thermal load can worsen the operating conditions of the turbine housing, the hot side and the wastegate.

A separate indication is a cracked or leaking exhaust manifold. Exhaust gas escaping upstream of the turbine reduces the energy reaching the rotating assembly and can manifest itself as underboost, a loss of power or a P0299 fault code. This is why, with an overheated hot side or unstable boost, the inspection should cover the patency of the DPF and catalytic converter, the condition of the GPF in the relevant engines, and the tightness of the manifold and the rest of the exhaust.

5. EGR and disturbed exhaust gas flow

The EGR system changes the quantity of exhaust gas directed back into the engine, which is why its correct operation also matters for the operating conditions of the turbocharging system. A contaminated, blocked or erratically operating valve can disturb flow and pressure on the exhaust side. The result can be boost deviations, drops in power and smoking.

In many modern diesels, low-pressure and high-pressure EGR systems work together with the DPF and the remaining emission control components. This does not mean that every fault in one of these systems can automatically be regarded as the cause of the turbo failure. You need to establish whether exhaust gas flow, back pressure or boost parameters have actually changed.

Similar symptoms can be produced by a faulty VNT/VGT mechanism or electronic valve. These components should not, however, be classed as external sources of damage, because they belong to the turbocharger system itself and require separate assessment.

6. Injectors, fuel metering and fuel parameters

A modern injection system controls the timing, quantity and manner of fuel delivery with great precision. In Bosch Common Rail solutions the injection pressure reaches 2,500 bar. Wear of an injector or a deviation in its corrections can noticeably change the course of combustion. A worn nozzle tip may deliver an incorrect quantity or atomise the fuel poorly, and the temperature and composition of the exhaust gas driving the turbine then change.

Too small a quantity can cause a loss of power and disturb the interaction between the engine and the turbocharging system. An excessive quantity or incorrect atomisation encourage incomplete combustion of the fuel, the formation of soot and an increase in the thermal load on the exhaust. This is why a case in which the DPF or turbocharger has been replaced but the injectors have not been checked can end with the problem returning.

Fuel that complies with the requirements of the specific engine also matters. Incorrect parameters, including an unsuitable cetane number in a diesel, an elevated sulphur content or contaminants that encourage deposit formation, can worsen the course of combustion and the cleanliness of the system. With overheating or heavy carbon build-up you should therefore check the injector corrections and combustion parameters, instead of starting with another turbocharger.

7. Engine damage and foreign bodies on the hot side

Not every external cause develops gradually. A fragment of a damaged component of the piston and cylinder assembly can pass with the exhaust gas directly into the hot part of the turbocharger. One example is damage to an oil scraper ring, a fragment of which can strike the turbine wheel, damage the blades and lead to further damage to the rotating assembly or the shaft.

Traces of such an impact are different in character from gradual wear caused by oil or temperature. Fitting another turbocharger without finding the source of the fragments means leaving a more serious failure inside the engine itself. Mechanical damage from the hot side therefore requires a check of what was happening in the cylinders and along the exhaust gas path upstream of the turbine.

The seven areas described here do not exhaust the faults that can arise inside the turbocharger itself. A manufacturing defect, material fatigue, the bearings’ own wear, overheating, poor balancing of the rotating assembly, blocked variable geometry or a faulty actuator form a separate group and can produce similar symptoms. Where an external cause is not confirmed, the condition of the old component becomes the starting point for distinguishing between these two groups of failure.

What the marks on the old turbocharger say about the direction of further diagnosis

A mark on a damaged turbocharger indicates the direction of the diagnosis, but on its own it does not confirm the source of the failure. The same type of damage can arise under several different conditions, which is why the condition of the component must first be documented in detail.

During the inspection it is worth recording:

•         the condition of the compressor wheel and the blade edges

•         the condition of the rotating assembly, the bearings and the CHRA

•         axial and radial clearance

•         the presence of oil, carbon and deposits

•         traces of overheating on the hot side

•         the condition and range of movement of the VNT/VGT and the result of the actuator check.

A single mark is usually not enough to identify the source of the failure, so it must be set alongside an inspection of the relevant vehicle systems.

The further procedure can be summed up in five steps:

1.       Document the damage to the old turbocharger.

2.       Identify the systems that could have created such conditions.

3.       Confirm or rule out the hypotheses through workshop diagnostics.

4.       Eliminate the confirmed cause and check the system again.

5.       Only then fit another turbocharger and verify the boost parameters.

Another turbocharger should be fitted only after the source of the previous failure has been confirmed and eliminated, or after an external cause has been reliably ruled out. Without this, the next component is exposed to an unexplained damage mechanism.

Frequently asked questions before and after fitting a new turbocharger

What should be done with the oil system before the new turbocharger is started for the first time?

You must use fresh oil of the required specification and a new filter, and confirm that the feed and return lines are clear. Before the first start the turbocharger should be filled with fresh oil, and oil should then be supplied to its bearing system in accordance with the manufacturer’s procedure. The first few seconds of operation without correct lubrication can be enough to damage the bearing system.

What about the intercooler after a turbocharger failure? Clean it or replace it?

That depends on the type of damage. Oil and deposits must be removed from the system in accordance with the manufacturer’s procedure. If, however, mechanical damage to the compressor wheel has left metal fragments behind, flushing the intercooler alone may not remove all of them. In such a case the intercooler may need to be replaced so that the contaminants do not later reach the engine.

Can you drive straight away after replacing the turbocharger?

No. Once the oil system has been correctly prepared, the engine must first be run at idle and the oil, intake and exhaust connections checked for leaks. After this inspection the oil level should be checked again. Only when there are no leaks and the system is operating correctly can you move on to a test drive and verification of the boost parameters in accordance with the manufacturer’s procedure.

Does smoking after a turbocharger replacement mean the new component is faulty?

Not always. Oil left in the intercooler or intake after the previous failure can cause smoking even after a new turbocharger has been fitted. If the symptom persists, you need to check the cleanliness of the system, the patency of the oil drain, and the crankcase pressure and the operation of its breather, before the new component is regarded as the source of the problem.

Does a whistling noise after a turbocharger replacement mean the unit is damaged?

Not always. A loud or new whistle can be caused by a leak in the pipework, the intercooler, the connections of the charge air system or the exhaust system. You should therefore first check the tightness of the air and exhaust gas path. Only once these causes have been ruled out should the turbocharger itself be diagnosed.