Wastegate

Without a wastegate, boost would keep rising with engine speed. Opening the wastegate sends part of the exhaust around the turbine, so the turbo spins slower. Most modern petrol turbos use an electrically or vacuum-operated wastegate controlled by the engine control unit.

Internal wastegates are built into the turbo housing; external wastegates are separate valves often found on high-performance engines. Many diesel engines use a variable geometry turbocharger (VGT) instead of a wastegate. Movable vanes around the turbine change how the exhaust gas hits it, giving quick response at low speed and limiting boost at high speed. Both systems solve the same problem: keeping boost pressure within safe limits.

The control unit compares actual boost with the target and moves the wastegate actuator to correct it. A vacuum or pressure actuator, or an electric motor, opens the wastegate flap. The control unit uses a solenoid or motor to set the target boost.

A sticking wastegate or rattling linkage causes over- or underboost codes such as P0234 or P0299, loss of power and sometimes a rattle at idle. A stuck wastegate can cause overboost or underboost codes, limp mode and low power. A rattling noise from the turbo area may be a worn wastegate linkage.