A wastegate is a device mounted internally or external to a vehicle's turbocharger that ensures the turbocharger does not produce more boost than desired. When boost exceeds the preset target, the wastegate will bleed off any additional boost. A malfunctioning or improperly assembled wastegate can cause a variety of boost issues.
Two types of wastegates exist as of 2011: internal and external. Built onto the turbocharger housing and typically found in smaller turbochargers, internal wastegates use a diaphragm that responds to boost pressure provided by a boost line. External wastegates are much larger units designed to withstand high boost levels by using a valve instead of a diaphragm design.
While the specific design varies, the purpose of either an external or internal wastegate is twofold: protecting the engine from overboost, which can cause damage, and staying fully closed until the engine reaches the target boost.
Symptoms of a Failing Wastegate
Stress caused by an ageing turbocharger or by a motorist increasing boost pressure from the turbocharger can overwhelm the spring that opens the wastegate at a specified pressure. In these case, the wastegate will flutter because the spring cannot keep the gate fully shut. The boost will oscillate and the engine will stumble and surge as the boost varies rapidly (any given throttle input should create steady boost levels). The engine may also respond sluggishly to throttle input since the wastegate is likely bleeding air.
Protecting the Engine
If you believe your wastegate is failing, take care not to generate high levels of boost until you can replace the wastegate or have the turbocharger examined by a professional. Since some of the turbocharger's output is bleeding out through the fluttering wastegate, the turbocharger will have to work harder to maintain target boost, which will generate heat, increase air intake temperatures and, as a result, harm the engine.