Diesel knock is the sound of the fuel igniting when sprayed into the hot air in the cylinder. The later the ignition, the more fuel there is to "knock" and the louder the sound.

Three things have a large influence on diesel knock at idle: condition of injectors, compression, and injection timing.

An injector that sprays a solid stream of fuel instead of the desired cone of atomized fuel will cause a hard knock, as will an injector with the nozzle stuck open. Usually also causes the engine to vibrate excessively. Sounds a great deal like someone is driving a large nail into a slab of oak on top of the head. Cured by replacing nozzle ($50).

Low compression will also cause injector knock, and replacing the nozzle won't fix it. Low compression temp results in slow ignition. Usually causes lumpy idle as the cylinder in question will be low on output too. You will see heavy black smoke at speed, worse under full load. If the valves are out of adjustment, you will have low compression, and setting them properly will fix the problem. If the rings or cylinder walls are shot, you will need to rebuild the engine. A compression check and leakdown test will tell you which is the problem, although I'd bet valves unless you also have excessive blowby and oil consumption.

Fast injection timing will cause excessive noise and poor performance at low speeds, including excessive smoke that goes away at high speeds.

Hope this gives you a place to start! Don't fiddle the ALDA until you get the engine working!

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!