Hi everyone! Great to be part of this forum.


A couple of weeks ago, I got an 85 300SD W126, Automatic, the California turbo model. It's showing 384K miles, but the engine is obviously fresher than that. No paperwork, no history. The car is very straight and rust free, and a lot of stuff works. The dash needs some help- ignition switch area is broken and a temporary switch has been cobbed in. It may well have overheated at some point in it's history- the coolant pressure cap was down to bare brass- all the rubber gone. Drove it 6 miles and shut it down before the temps went into the red. Got a pressure cap, tightened the fan belt. It also had a sticking brake, and a couple of other little issues- but I drove it 350 miles home.



Got it to the shop and pressure washed it, changed oil, adjusted valves, did diesel purge, fuel filter replacement, replaced return lines, got fan clutch working (so greasy it wasn't sensing temps), new valve cover gasket, 1 motor mount, lots of little things. What I've done has made big improvements- the car now kicks quite well, sprays gravel, chirps tires, driving quite well.


Here comes the 'But...' There's a noise, and a miss. You cannot hear anything except the usual OM617A rattle out under the hood. Back at the tailpipe, you hear a miss, probably an injector not firing very well. But in the drivers seat, in time with the miss, you hear a quiet dull thud. Cracking injector lines has not yet revealed which cylinder it is- you can't hear it under the hood. Each injector line affects the idle, as if they were all firing. An infrared camera shows each cylinder top at 216 - 220 degrees F. Oil pressure is excellent. Timing chain has virtually no slack, and blow-by is minimal. Sometimes I catch a little blue smoke from the tailpipe, sometimes it's very clean.


There's another occasional oddity: operate the pump linkage, and it runs one way up to about 1200 RPM: perhaps a little rough and erratic. Then continue to increase throttle very gently, and between 1200 and 1500 there is no control. Something suddenly changes and it's at 1500 RPM. I cannot get it to stay steady at say 1300 or 1400 RPM. From 1500 up, it runs very smoothly, no missing.



I'm hoping it's an injector, but it's kind of a bigish thud. Will get a stethoscope on the bottom end looking for rod knock. Will pull oil sump, check for metal (none in the old filter).


Any suggestions are much appreciated.