intermittent misfire '82 300td

[Admiral Ahani]

I've had my 300td for 12 years, and it's been running well (I won't say smoothly... it's a OM617 LOL) for all that time. Recently, it will start missing at random times for random durations. It can happen at idle, at 25mph, when cold or warm. When it misses at idle, it shakes pretty harshly as you can imagine.

Engine has 345k miles on it, original timing chain. IP has never been touched except for a shimmed ALDA. I haven't adjusted the valves for 2 years, the fuel filters are about 3 years old, and the injectors have never been cleaned. The vacuum locks were replaced with electric, and the rest of the vacuum system is leak-free. Shutoff via the key works instantly.

I don't think it's the IP rack damper - it's not a rough idle, it's a misfire and is rpm independent.

If this were a gasser, I would pull a plug lead at a time to see if it's cylinder specific, and if so, which one. But on this diesel I don't know how to diagnose. I don't like throwing parts at problems, I want to at least narrow it down first. TIA!