I've been working on my basket-case of an '86 300SDL and trying to track down and solve the deeply disappointing running quality at idle and low RPM's.

As a back story (cut very short, I'll detail in the project car forum later on), the car was traded into a used car lot in '07, car lot went bankrupt and was sold to another owner, new owner was going to refurb the car for his own use, fell ill, and the car sat in a back field until January of this year (9 years sitting for those keeping track). Since this is Texas, it survived largely intact and I basically stole it for a song with the intention of using it as a training bed to learn to work on a diesel. (I specifically wanted a fixer-upper).

Cut to the chase, everything in the car works, and it seems to be a restorable car with a minimum of money outlaid, but lots of elbow grease. I've been working on the engine on the weekends to get it going again, it ran on maybe 3.5 cylinders when I got it and is now running somewhat on all 6, although far from acceptable (or driveable) condition.

Important things first:

I've done a Diesel purge. I've replaced the copper washers in the delivery valves and the related O-rings. I rebuilt the original injectors, but they were too far gone to keep the halves from leaking. Installed 6 new Bosch injectors. Head is original #14 head, however it is crack-free, has new valve guides, new valve guide seals, and valve seats ground/machined.

Prior to removing the head I did a compression test (stone cold, not warm/hot) and got:
Cyl 1: 385 PSI
Cyl 2: 380 PSI
Cyl 3: 385 PSI
Cyl 4: 370 PSI
Cyl 5: 380 PSI
Cyl 6: 365 PSI

Cylinder 4 and 6 both had poor seal of the exhaust valve due to worn valve guides, explaining their lower pressures.

The engine is burning some oil (I assume stuck rings from sitting nearly 10 years with old/crappy/sludgy oil in it), but has virtually no blowby coming out of the breather, what little blowby it does have is not even enough to pressurize the valve cover if the breather hole is blocked. While the head was off the cylinders were checked for wear/scratches/pitting and found to be in good shape. This largely tells me the bottom end is pretty solid.

I just can't seem to get the thing to idle or run at low RPM's worth a crap! It feels like it's missing out randomly on 1 or more cylinders. It'll run smooth, then suddenly miss out and surge a bit (almost like a governor hunt), romp around, engine rocking around, smooth out, repeat, etc. If you give it a bootful, it is very hesitant to rev, almost stalling, until the RPM's get above 2000, then it begins to smooth out, then becomes perfectly smooth around 3000 RPM or higher, if the RPM's are backed off and held around 2000 RPM, it'll be perfectly smooth for a few seconds, then that random missing starts showing back up. As the RPM drops back down below 1800 RPM, it becomes more noticeable again.

I've tried cracking injector lines, and all 6 make the same difference. It becomes VERY obvious when the line is cracked and you have the dead miss.

It all screams to me fuel-delivery issues. The other big red flag (to me at least) is that if the car sits overnight, it'll fire right up the next morning after maybe 2-3 compression hits. However if it sits for a few days, it'll fire, die, then act as if it has to build prime again (15-20 secs of crank). I have it running from a temporary bottle right now as I need to clean the fuel tank, and I've confirmed that I have no injector lines leaking (nuts dry, injectors dry, delivery valves dry). All of the rubber fuel hoses are new as well, and the fuel heater is bypassed.

Could this be a lift-pump issue? It certainly seems to be pumping fuel through the clear filter, but the leaking down and random soft missing just screams air in the lines, leaking injection valves, mucked up injection pump, something! I would expect a mechanical fault with the engine to produce something obvious, excessive blowby, low compression, a consistent soft miss or dead skip, etc, but this seems to be completely random, I can't nail it down to a single cylinder.

Trials? Suggestions? Tips? I'm fresh out of ideas!

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]