Over pressure down the hose to wastegate actuator would only reach the actuator and the TVSV valve - assuming boost is still controlled by the ECU. It's not uncommon to have an aftermarket boost controller fitted which will have it's own valve normally between turbo and actuator, then the second pipe from actuator to the standard VSV is normally blocked off.
If the wastegate jammed open, it's quite likely you have an aftermarket downpipe and the wastegate is jamming on it - not uncommon with many aftermarket pipes.
Is the boost gauge 'staying at the bottom' meaning right at the bottom (negative pressure) or at zero (where the gauge sits with ignition on and engine stopped) - this is important as staying negative would indicate a sensing problem rather than an actual loss of boost.
On a 205, the fuelling is set from manifold pressure so the sensing is critical - and the boost gauge is fed from the same sensor as the ECU uses. The pipe to the sensor (purple coloured, mounted on bulkhead) should come from the 'orrifice' fitting in the middle of the inlet manifold by the throttle body, and should not have any other pipes attached on this path. Any leaks / other sensors attached on this line can affect fuelling.
Another line to check is the small vac. line from the top of the fuel pressure regulator (on end of fuel rail) to the inlet manifold. This sets fuel pressure relative to manifold pressure to keep a constant pressure across the injectors. Without this, it will go rich on idle and weak on boost.
The final check is the small vac. line from inlet manifold to the dump valve. This holds the valve shut on boost as well as pulling it open when the throttle is released.
To describe symptoms of wastegate stuck open - you still get boost, in fact near full boost at highest RPM, but boost is much lower than it should be at lower RPM and especially at the 3000RPM mark where it's only just making good boost with the wastegate shut.
The other thing to be aware of, the ECU deliberately inhibts boost above basic actuator boost when cold. This is to protect the engine from owners with no mechanical sympathy.
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