Whayne:

"Nailing" is actually usually called injector knock. It is in fact LATE fuel ignition, not "pre-ignition" -- you can't get pre-ignition in a diesel unless you do something silly like use ether to start it, or run propane or acetylene into the intake.

There are several causes of injector knock:

Bad injector (the usual cause) -- pintle sticking open or closed, worn nozzle not giving a good spray, etc. Cured by replacing nozzle.

Low compression, causing low initial heat and poor ignition -- large cloud of fuel mist ignites all at the same time instead of buring as injected, goes "bang". Cured by fixing the cause of low compression.

Missing or broken ball pin in prechamber, or badly burned prechamber. In really serious cases, chunks can fall into the combustion chamber and ruin the piston, rod, and sleeve. Cured by replacing pre-chamber, best done with head off.

Bad seal on pressure valve holder on IP. This causes serious vibration/shaking problems at idle, too as very little fuel gets injected at low speeds. Cured by replacing the seal in the IP.

Defective IP, cured by replacing. Very uncommon.

To determine which is which:

You've already isolated one cylinder (#3), so here goes:

Low compression or a bad injector will usually also result in excess black smoke, and large amounts of white smoke on startup. If you pump the accelerator with the car out of gear, intermittant black smoke usually means a bad injector.

Bad prechamber is similar, but you can pull the offending injector and look in so see if the ball pin is in place or if the pre-chamber has eroded. Check for burned out glow plug parts in the bottom of the prechamber, too, as that can cause a similar problem.

If you have IP problems, usually only a bad pressure valve holder seal, you will have low power, vibration, injector knock, but no excess smoke. When you crack the injector line, you will have normal fuel delivery around the nut. Engine runs normally above 1500 rpm or so, has good power once wound up, and doesn't smoke excessively at speed. The quick and dirty check is to loosen the injector line at the pump then tighten the lower nut -- should be 20 ft/lbs or so. The seal is a copper washer, and if re-used can fail to seal properly.

On my 300D the pressure valve holders were so loose I could unscrew two of them by hand, no wrench! Ran badly, shook like crazy, knocked something awful on #4, with some knock on #2 and #3. I replaced the pressure valve holder seals and it doesn't knock at all at idle warm!

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!