Turbo losing power: what to check before the turbo

What it means

A boost leak, meaning air escaping between the turbo and the engine, is one of the most common reasons a turbo engine feels flat. If pressurized air escapes on its way in, or the boost control sticks, the car feels down on power even though the turbocharger itself may be fine.

How the turbo makes power

Exhaust gas leaving the engine spins a turbine, and a shaft carries that motion to a compressor that pushes more air into the engine. More air lets the engine burn more fuel and make more power. Every factory system limits that boost, with a wastegate on most petrol turbos or movable vanes on many diesels.

Common causes

• A boost leak: a split hose, a loose clamp or a failed gasket
• A wastegate stuck open, giving underboost and sometimes a fault code
• A wastegate stuck closed or a control fault, often followed by the engine cutting power to protect itself
• On a diesel, variable-geometry vanes sticking from soot and carbon
• An intercooler fault, with reduced power especially after sustained load
• Oil starvation in the turbo, from low or degraded oil or a restricted feed

What to do now

1. Note whether the power loss came with a new noise, and whether the noise has changed.
2. Listen for a hiss, which sometimes comes with a boost leak.
3. Check the oil level, and use the exact oil specification in your owner's manual.
4. Note the color of any new smoke and when it appears.
5. Ask for boost leaks to be checked first, then the control system, and the turbo last.

Turbo, or only looks like it?

• Flat, down on power, sometimes a hiss: a boost leak, so check this first
• Flat performance with a fault code: possibly a wastegate stuck open
• Engine cuts power to protect itself: possibly overboost from a wastegate stuck closed or a control fault
• Diesel with erratic boost, poor low-speed response and limp mode: sticking variable-geometry vanes
• A new or rising-pitch whine with reduced power: a change in the rotating assembly, or a leak
• Blue-grey exhaust smoke: oil burning, and the turbo is only one candidate

What people often get wrong

That every flat-feeling turbo engine has a failing turbo. Boost leaks and boost-control faults are more common, and a turbocharger replaced without fixing what killed the first one, such as a restricted oil drain or dirty oil, will fail the same way.

When the car must go to a workshop

When power loss or a new whine persists, have it diagnosed, and ask one specific question: has the cause been identified, or only the symptom?

Read next

• Exhaust smoke color: white, blue or black explained
• Car burning oil: how to tell and what it means
• DPF light on: drive, do not park

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.