The pressure relief valve is a backup in case the wastegate in the turbocharger can no longer do its job. A wastegate is a valve in the exhaust of a turbocharged engine, this valve bypasses the turbocharger and prevents the turbo from building up too much pressure when the engine revs higher or is under heavier load. When the relief valve is not working, the turbo overpressure can no longer escape, and this overpressure control valve takes over.
This valve is often removed to increase turbo pressure, which is not a good idea, as you run the risk that, should the relief valve fail, there will be no back-up and the damage will be greater than a simple broken relief valve.
If the pressure relief valve is no longer present, or due to continuous overheating caused by temperature differences, the plastic becomes brittle, you will need to install or replace a new one.
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