Hello -- I'm new to forum and looking for advice on my 1992 300GD SWB W463.327 OM603.931 engine. ~140k km.

Some months back, I started seeing/smelling faint smoke out the exhaust and an oily, dark residue being left on my rear driver's wheel near exhaust outlet. Seemed to happen at both idle and freeway speeds, cold and warm weather and engine temps. My fuel economy had been notably less than expected as well.

Mistaking for white smoke and concerned about a leaking gasket or head issue, I first used one of those chemical combustion leak tester kits at coolant reservoir. No chemical change in the combustion gas leak tester and no bubbles.

I did a diesel purge and no change in faint exhaust smoke. As the second owner of this G without a detailed service history, I did a few things that I thought might be related:

-- replaced diesel injectors with new Bosio nozzles, carefully lapping surfaces, checking spray pattern, pop pressure (proper shims), etc.
-- replaced glow plugs, reaming out carbon from glow plug holes
-- replaced fuel delivery valve o-rings (mostly preventative since I wasn't seeing fuel leaks on top of governor)
-- replaced coolant pump with recommended compatible, larger-flow pump and new thermostat at same time
-- did a combustion test and good, normal compression on all cylinders, values close to each other. I'm a bit slow at all reapirs and so by the time I removed intake manifold and got access to glow plug holes, engine wasn't technically warm, but pressures were still very good.
-- another diesel purge

All OEM parts and being careful to protect all the various chambers and openings while exposed (no specks or debris falling into fuel delivery valve, prechambers, etc).

After getting air out of lines, the rig certainly started up much perkier and idles smoother, but still the faint smoke and residue on the wheel. And no improvement in fuel economy.

Even though there aren't a lot of hoses or components in the vacuum system and no EGR or turbo or anything like that on this engine, I then thought I might have a vacuum leak somewhere and paid for an hour of shop time for a nearby mechanic to hook up to a smoke machine to look for leaks. But vacuum is good, and no smoke.

So, I'm wondering if the fuel governor is giving too much diesel, making the fuel/air mixture rich, yielding un-combusted diesel coming out exhaust and leaving black/soot/residue on wheel (?)

Any advice or thoughts on where to look next or if there is a way to turn down the fuel mixture at the governor? Or a way to set correct air/fuel mix? I have access to repair guide/manual, so chapter & verse or what to search for related to governor servicing or tests would be greatly appreciated. Or if I'm way out in left field based on others similar experiences, I'd welcome any advice or thoughts.

Thank you!

Ben