Turbochargers have a wastegate to control the boost pressure. When opened, the wastegate allows exhaust gases to flow past the turbine wheel, reducing boost. Slowing the turbine wheel slows the compressor wheel and reduces the boost in the intake manifold.
Without the wastegate, the turbocharger would produce maximum pressure at all times. This excessive pressure results in damaging engine detonation.
The wastegate is controlled by a diaphragm that moves an adjustable rod. In most of today’s vehicles, the wastegate is computer controlled. The computer uses signals from different sensors like the knock and oxygen sensor to make adjustments.
A sticking wastegate or faulty diaphragm can result in performance issues. A stuck open wastegate results in poor high-end performance, allowing too much exhaust pressure to bypass the turbine wheel. A stuck closed wastegate results in damaging overboost by providing maximum pressure at all times.