A turbo GTI that's down on power or logging an under/over-boost code often just has a boost leak — a split pipe or loose clamp in the charge system.
Where they leak:
- Intercooler hard-pipe couplers and clamps (the plastic pipes crack at the necks).
- The diverter valve (Diverter Valve) and its seals.
- The PCV (EA888 PCV Breather) and breather hoses.
- Split intercooler end-tanks on hard-driven or tuned cars.
How to find it:
- A boost-leak test — cap the intake, pressurise with shop air (~15–20 psi) and listen or spray soapy water for the hiss.
- Read fuel trims and requested-vs-actual boost on VCDS.
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