Has anyone ever had a situation where the delivery valves were NOT leaking externally, but replacing the crush washer & o-rings resolved a nailing condition at idle? (Or has anyone ever encountered a broken delivery valve spring? (Assuming correctly or not that a broken or out-of-spec DV spring could possibly be causing nailing at idle?)

Reason for asking: My '98 E300 (OM606.92) has been nailing at idle on one cylinder. It's especially bad when cold, and it actually misfires under cold-start conditions. (The glow plugs all ohm out OK - but even if a glow plug was faulty, that wouldn't cause it to nail at idle while the engine is hot.)

I've pulled the factory original injectors out (to have Greazzer go through them) and I temporarily replaced them with a spare (used) set for the time being, but the nailing persists. Granted, I don't know the condition of the spare injectors, but the nailing is consistent – therefore my suspicion about the delivery valve.

It is definitely not a tank of bad fuel, as it has persisted through several tanks. (I also run Power Service at relatively high doses with every fill up.)

Any thoughts, opinions, conspiracy theories, etc. are much appreciated. Oh, and Merry Christmas too!!!

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.