Wastegate here isn't leaking, it's being opened too soon by a weak spring. Most 617 engines were set fairly low at the factory (10-11 psi boost) and the spring is going to allow lower boost as it ages.

When the boost line is clamped shut, the wastegate won't be opened, and since the turbo produces more and "faster" boost under that condition, the wastegate is fine. The wastegate actuator is opening the gate too soon.

Note that a wastegate is NOT a pressure relief valve -- it opens to "waste" the exhaust around the turbine. If it won't seal, the boost will be low from excessive flow around the turbine no matter what the actuator is doing. Usually a poppet type valve, so flow is proportional to opening amount -- this allows a constant pressure in the intake with variable flow through the turbine.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!