Excessive black smoke won't be produced by a stuck wastegate -- the would give you more power, but fuel delivery will still be properly controlled.

Dirty fuel filters won't give black smoke, either -- no fuel, no smoke.

The only other thing that could have this effect is a stuck injection timer, giving over-advanced timing at low speeds. Not an issue on the Volvo, as the timer is driven by the intergral lift pump on the VE pumps, not a cetrifugal device. Will also make it sound like a Mack truck, too -- lots of combustion noise at idle.

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!