300D Rough Idle when HOT "Diesel Purge" did nothing

[skibum13]

I have an '84 300D with over 220k miles on her. the girl is having problems idling. she starts right up in the morning (glow plugs are good) and idles at about 800 rpm without too much roughness (normal for a diesel when cold). but once she gets up to operating temperature, the idle speed increases to about 900 rpm (not sure if that's normal or how to adjust that) and she idles really rough. i have to give her enough gas to keep the RPMs above 1000 to make it bearable.

here's what I've tried: i cracked open each of the fuel injector lines. three of the cylinders weren't providing power. bought one new injector and installed it; no effect. I swapped a "bad cylinder" injector with one that wasn't bad; the "bad cylinder" remained where it was (which I'm pretty sure means it's not the injector). Bought two more injectors just in case and replace two of the "bad cylinder" injectors; it ran WORSE, so I put the old injectors back in.

Here's the interesting part: I ran an entire bottle of Lucas Fuel Treatment undiluted 100% through the engine and the girl ran as smooth as a baby's butt! But as soon as I switched it back to diesel she ran just as rough as before. I got my hands on some Lubro Moly Diesel Purge hoping it would do the trick. While running one bottle undiluted 100% through the engine, the girl ran WORSE! And it ran the same as before afterwards.

I also tried adjusting the rack dampener screw, but it didn't make any noticable difference adjusting it all the way down to about 1/4" out. I'm at a loss for what to try next. My guess is that the Injection Pump is bad and needs to be calibrated/replaced. One mechanic I talked to thought it was just the low cetane rating of truck stop gas that I use that is causing the rough idle. I doubt it, but I got some cetane boost to add to the diesel to see if that makes a difference.

Can anybody think of anything else to try before I start trying to get the injection pump off? Thanks for your help!