Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician is diagnosing an engine that experiences a significant loss of power under heavy load, accompanied by a 'whooshing' sound from the engine bay. There are no DTCs related to engine misfires or fuel system issues. A boost pressure test reveals that the turbocharger's maximum boost pressure is significantly below specification. What is the MOST likely cause?
- AA restricted air filter.
- BA faulty wastegate actuator.
- CA leaking intake manifold gasket.
- DA clogged catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty wastegate actuator.
If the wastegate actuator is faulty and stuck open (or partially open), it will allow exhaust gases to bypass the turbocharger turbine, preventing the turbo from spinning up to full speed and thus failing to build specified boost pressure. The 'whooshing' sound could be exhaust gases escaping through the open wastegate or air escaping due to low pressure not being properly sealed, compounded by the loss of power under load.
Why the other options are wrong
- A. A restricted air filter would cause a vacuum at the turbo inlet and reduced power, but not typically a 'whooshing' sound or a direct failure to build boost pressure in this manner.
- C. A leaking intake manifold gasket would cause a vacuum leak, leading to a lean condition and rough idle, but not primarily a 'whooshing' sound or low turbo boost.
- D. A clogged catalytic converter would cause back pressure, leading to power loss, but typically without a 'whooshing' sound and it wouldn't directly explain low *boost* pressure.
Wastegate Function
The wastegate controls turbocharger boost pressure by diverting exhaust gases away from the turbine. If stuck open, it prevents the turbo from building full boost.
- Regulates turbo speed and boost pressure.
- Prevents over-boosting.
- Stuck open: low boost; stuck closed: over-boost.
Memory trick: Low boost means the turbo's 'air-pump' is either broken or bypassed.