CORRECTION: These instructions are for a 123. I do not know if an 87 300TD has the mentioned port.
You could hook up a pressure gauge to your intake Manifold to see if your Turbo is boosting. Forced Induction on another Forum said words to this effect: Port for installing a Boost Gauge at rear of Intake manifold on the Air Filter Housing Side.
M10x1.0 port behind the air filter housing.

I believe max normal boost is 10-12 psi.

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84 300D, 82 Volvo 244Gl Diesel