Turbochargers
Many drivers are unaware that their car may have a turbocharger, until it goes wrong. Rob Marshall looks at common reasons for failure and investigates the replacement procedure on the most popular engine that is prone to turbo issues.
We look at the common reasons for turbo failure and investigate the replacement procedure on the most popular engine with turbo problems – the DV6.
Once the virtual sole preserve of the high-performance scene, turbochargers have become adopted by virtually every mainstream car manufacturer.
Not only are they ubiquitous in modern diesels but many low-capacity petrol engines would also be unable to develop their impressive high power and low emissions figures without them.
Consequently, the turbocharger has become a victim of its own success. Once a sporting enthusiast’s status symbol, most drivers today are unaware about how unintentional neglect can shorten the turbo’s life. Sensitivity to oil quality/ drain intervals and neither running the turbocharger at high boost pressures immediately following a cold start, nor allowing its core shaft to slow and cool before cutting the ignition, remain relevant today for optimised turbo life.
How the turbocharger works
In basic terms, engine power is related to how much oxygen is available. Theoretically, if the engine cylinders are bigger (ie, have an increased cubic capacity - cc) more oxygen can be drawn in, more fuel burnt and greater power figures result. However, should air be forced into the engine under pressure, more oxygen is ‘squeezed’ into the cylinders, more fuel burnt within them and more torque results.
This means that engines with reduced cylinder sizes but featuring forced induction can produce the same (or greater) output than larger-capacity units that run at atmospheric pressure. Yet, compressing incoming air reduces its density and increases its temperature; the result lowers the amount of oxygen available. On most cars, the pressurised air is cooled by being directed through a series of small tubes within an air-toair intercooler. The heat transfers from the compressed air into the cooling fins, which are cooled subsequently by airflow that is a byproduct of the car moving forward. This explains why intercoolers tend to be mounted at the front of the vehicle and, like engine cooling radiators, why their efficiency becomes compromised, should their fins become blocked by leaves, for example.
The turbocharger forces air into the cylinders, under certain conditions. Under idle and light throttle loads, the inlet manifold may experience negative pressure (ie, lower than atmospheric), because the pistons are still ‘sucking’ air into the cylinders. However, under positive ‘boost’ circumstances, the
turbocharger pressurises the air after it has passed through the air filter, therefore raising the inlet manifold pressure above that of atmospheric.
A significant advantage is that flowing exhaust gases power the turbocharger, the energy from which would have been wasted otherwise. A supercharger, incidentally, is driven directly by the engine, which makes it less efficient. The exhaust gases enter the turbocharger’s turbine housing and cause a specially shaped turbine wheel to rotate, which is fixed to the end of a shaft, often by a retaining nut. The shaft is supported at its centre by a bearing assembly, while a compressor wheel is fitted to the opposite end.
The compressor, incidentally, not only draws air into the turbocharger but also compresses it into the engine’s inlet tract, increasing the pressure, or 'boost'. However, as the engine speed increases, excessive pressures might result, which are controlled and reduced by a wastegate. Traditionally, the turbine speeds are related directly to engine RPM and throttle position but modern engines can manipulate this through variable vane and variable geometry technologies. How these systems operate and why they fail is not relevant to this article but we shall detail them thoroughly in a future issue.
Despite being a straightforward device in theory, the turbocharger is a precision-made assembly. As the turbine/compressor wheels can experience speeds above 150,000rpm, the unit must be well made and balanced accurately. Consider this when buying replacement turbochargers and be wary of the many cheap copies that are on the market. Choosing a genuine remanufactured turbocharger from a reputable supplier that offers a decent warranty is the best way forward.
Colin Cottrell, Marketing & Central Operations Director at Euro Car Parts told Car Mechanics:
“The use of remanufactured turbos is increasingly common, mainly because they are priced more competitively, without compromising on performance, or reliability. Our TURBOJETZT range of 900-plus remanufactured turbos is tested to OE standards, and all units come with a standard two-year and optional lifetime warranty.”
The ancillary components
More recent technologies have created additional challenges. Diesel particulate filters (DPFS) can be mounted very close to the turbochargers, to get as hot as possible to ‘regenerate’ and vapourise the trapped soot content, as per their design intentions. When the filter becomes overloaded, it creates a partial blockage in the exhaust system. The resultant back-pressure increase within the turbo’s housing can not only
Many current turbocharged cars aren't fitted with boost pressure instruments, despite the engine management system featuring pressure sensors in the inlet manifold. On this manometer instrument, atmospheric pressure is achieved with the engine off, so the needle rests between the white and yellow segments. With the engine running with little/no load, the needle deflects into the white segment, because the pistons are sucking in air and the manifold pressure is below that of the outside. Under acceleration, the turbocharger increases intake manifold pressure above atmospheric, so the needle enters the yellow zone. Should it enter the red area, the boost pressure is too high – this tends to be caused by a fault, such as a seized wastegate.
cause the turbine blades to stall but the pressures can also cause the securing nut to unscrew, which can cause the turbine wheel to detach from its shaft. The high pressure can also force excessive quantities of lubricating oil into the air intake side of the system.
These are reasons why the commonlyheld view of taking a car with a blocked DPF for a high-rpm, high-speed ‘thrash’, to try and clear the blockage, is not an especially wise idea. Additionally, blocked DPFS promote higher exhaust gas temperatures within the turbo’s turbine housing, increasing further bearing failure risks. Should you notice carbon deposits around the wastegate levers on the outer casing, excessive exhaust gas pressures may be to blame.
While neglected oil changes are known as turbocharger ‘killers’, so too is premature oil contamination. Any component, therefore, that increases deposit build within the engine oil must be addressed. Stuck-open exhaust gas recirculation valves, faulty/leaking fuel injectors, contaminated inlet manifolds, damaged air pipes, clogged air filters and faulty glow plugs all contribute to oil ageing and turbocharger failure.
Consequences of turbocharger failures
If your turbocharger has failed, you should establish not only the underlying reason but also the impact on the other components as part of your assessment before starting work.
After you've done your calculations, you may find the cost of parts alone to be unviable. If turbocharger failure has caused oil to enter the exhaust system, you may also have to replace the DPF and catalytic converter.
Should the intake be filled with oil (although light oil deposits are normal), the turbocharger pipes will need either cleaning, or replacement, as will the intercooler. If its bearings have failed, the resultant lateral movement on the turbo shaft may have caused the spinning turbine/compressor wheel blades to contact their respective housings, introducing fine metal filings into the intercooler and pipes. Should you not check the intercooler, all of those abrasive particles risk being forced into the cylinders, increasing the likelihood of causing further mechanical damage. Nissens, for example, is one respected parts manufacturer and supplier that recommends that an intercooler be renewed, so you can guarantee that no debris lingers within the intake system.