What does a turbocharger do, and what makes one fail?
What forced induction actually does, why oil supply is the thing that kills turbochargers, and what the symptoms of a failing one feel like.
- Difficulty
- intermediate
- Time
- 20 min
- Read
- 6 min
- Safety
- caution
Short answer
A turbocharger uses the energy in the exhaust gas to spin a turbine, which drives a compressor that pushes more air into the engine than it could draw in by itself. More air means more fuel can be burnt, so a small engine makes the power of a larger one. Almost every turbo failure traces back to oil — the wrong grade, an overdue change, a blocked feed pipe, or being switched off hard after a long run.
Turbocharging is why a modern 1.0-litre engine can pull a family car and why manufacturers were able to shrink engines without shrinking performance. The principle is elegant: the exhaust gas leaving the engine still contains useful energy, so a turbine in the exhaust stream harvests it and a compressor on the same shaft pushes more air in. What makes turbochargers demanding is not the concept but the conditions — a shaft spinning at enormous speed, sitting in the exhaust stream, floating on a film of engine oil that is doing both the lubricating and much of the cooling.
Safety
Step by step
- Understand the two halves.A turbocharger is two wheels on a common shaft in separate housings. Exhaust gas spins the turbine on the hot side. The compressor on the cold side draws in fresh air and pushes it towards the engine. Nothing drives it from the engine mechanically, which is what distinguishes it from a supercharger — a supercharger is belt or gear driven off the engine, so it responds instantly but costs engine power to run.
- Understand why the intercooler is there.Compressing air heats it, and hot air is less dense, which undoes part of the benefit. An intercooler is a radiator in the intake path that cools the compressed air before it reaches the engine. A split intercooler hose is a common and relatively cheap fault that produces a dramatic loss of power and often a warning light.
- Understand what controls the boost.Left alone, a turbo would produce more pressure than the engine can safely take. A wastegate or variable-geometry mechanism regulates it, and on modern engines that mechanism is controlled electronically. Sticking or seized boost control, common on diesels that do a lot of short low-load journeys, is a frequent cause of limp mode.
- Know that lag is a property, not a fault.The turbo needs exhaust flow before it can spin up, so there is a delay between opening the throttle and the boost arriving. Small modern turbos have far less of it than older large ones. A car that has always had a slight pause low down is behaving normally; one that has recently developed a hesitation has a problem.
- Treat oil as the maintenance item that matters.The bearings are lubricated and largely cooled by engine oil under pressure. Old, contaminated or wrong-specification oil coming through a small feed pipe is the leading cause of turbocharger failure. Use the specification the manufacturer names, not just the viscosity, and do not stretch the interval on a turbocharged engine.
- Let it idle briefly after a hard run.Switching off immediately after sustained high load stops the oil flow while the turbo is still extremely hot and still spinning. That bakes oil in the bearing housing over time. Thirty seconds to a minute of idling after a motorway run, or simply driving the last mile gently, avoids it. Many modern cars run an electric pump afterwards to do this for you.
- Recognise the symptoms of a failing turbo.A noticeable loss of power, often with a warning light or limp mode. A whining or siren-like noise that rises with engine speed, distinct from normal induction noise. Blue or grey smoke from the exhaust under load. Rising oil consumption without an external leak. Any of these together with a boost fault code is a strong signal.
- Do not confuse it with the cheaper faults that mimic it.A split intercooler or intake hose, a stuck boost control actuator, a blocked particulate filter and a faulty boost pressure sensor all produce loss of power on a turbocharged engine and all cost a fraction of a turbo. Get the fault codes read and the intake system pressure tested before anyone quotes for a turbocharger.
- Understand what a replacement involves.It is a significant job on most cars, and a good garage will also change the oil and filter, clean or replace the oil feed and return pipes, and check why the original failed. Fitting a turbo without addressing the cause is how people buy two.
- Know the diesel runaway risk.If a turbocharger's seals fail badly on a diesel, engine oil can enter the intake and the engine can run on it, revving uncontrollably and ignoring the ignition key. It is rare and it is serious. If it happens, stop the car, get everyone out and away, and call the emergency services rather than attempting anything.
Tips
- On a turbocharged car, check the oil level regularly rather than only at services. Many modern turbo engines legitimately use some oil, and running low is precisely what damages the turbo.
- A short-journey life is hard on a turbocharged diesel. If most of your driving is a few miles at a time, that is worth knowing when choosing a car, not after buying one.
- Whining that rises smoothly with engine speed is worth recording on a phone. It is one of the few noises a garage can identify from a clip.
Common mistakes
- Using the right viscosity oil but the wrong specification — Modern engines name an approval code as well as a grade. Turbocharger bearings and particulate filters both depend on the specification, not just on the thickness.
- Replacing a turbo without finding out why it failed — A blocked oil feed, a worn engine or contaminated oil will destroy the new one just as fast. The diagnosis matters more than the part.
- Switching off immediately after sustained hard driving — The turbo is still hot and still spinning when the oil supply stops, which cokes oil in the bearing housing over time.
If it doesn't work
Sudden loss of power with a warning light, clears on restart
Cause: Boost control fault, a split intake hose, or a sensor out of range — Fix: Get the codes read and have the intake system pressure tested. This pattern is far more often a hose or an actuator than a turbocharger.
Whining or siren noise rising with revs
Cause: Worn turbocharger bearings — Fix: Have it inspected properly. Record the noise first — it is distinctive and it helps a garage separate it from ordinary induction noise.
Blue smoke under acceleration and oil use with no leak
Cause: Turbo seals passing oil, or engine wear — Fix: Needs diagnosis rather than assumption, because both produce the same smoke. Keep a record of how much oil you are adding and how often — that is genuinely useful evidence.
Power loss only on a diesel, with black smoke
Cause: Blocked particulate filter, sticking boost actuator or EGR fault — Fix: Three distinct faults with three distinct fixes. Codes first, parts second.
Questions people ask
What is the difference between a turbocharger and a supercharger?
A turbocharger is driven by exhaust gas, so it costs no direct engine power but takes a moment to spin up. A supercharger is driven mechanically from the engine, so it responds instantly but consumes some of the power it produces. A few engines use both.
What is the most common cause of turbo failure?
Oil. Old or wrong-specification oil, an overdue service, a blocked or coked oil feed pipe, or running the level low. The bearings depend entirely on clean oil at pressure, and there is very little margin.
Do I need to let a turbo car idle before switching off?
After sustained hard driving it genuinely helps, because it lets the oil carry heat away from a turbo that is still very hot. After ordinary town driving it makes little difference. Many modern cars run a pump after shutdown to do it automatically.
Can I fix a turbo myself?
No. It is a precision assembly running at extreme speed and temperature in the exhaust stream, and it needs the cause of failure diagnosed as well as the part changed. What you can usefully do yourself is keep the oil right and the level correct.