S320 CDi Black Smoke - Cured + Some Hints


Jun 19, 2013
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Leicestershire, UK
Your Mercedes
2002 s320 CDi
My recently acquired 2002 W220 was running in LIMP mode when I got it - very stately progress, but not much fun going up the hills around here - until after 50 miles when it cleared and the turbo kicked-in big time; but that wasn't much fun either though due to the prodigious cloud of black smoke it left behind on kick-down - a real 007 'trick' not very popular in town... So, having little experience of Mercs (my speciality is classic British stuff - and yes, I'm an accomplished welder(!)) I looked over the MBO forums and saw all this stuff about EGR's, MAF's, Cats etc and hoped there was a simpler solution before going for any of that. And there was. The fault was a loose hose clip on the air duct that goes from the aircleaner to the turbo inlet, at the turbo end, probably has been like it since the last £90-phr-'technician' to work on it forgot to tighten it up.
What makes this clip so important is that air being drawn into the turbo this way, ie around the loose duct rather than the air filter housing, prevents a sufficient level of vacuum to exist in the whole vacuum system and so prevents operation of some pretty important components - the turbo actuator/regulator, the EGR valve actuator, the crankcase breather/oil separator and perhaps most significantly, the MAF sensor simply won't be giving an accurate reading to the ECU and I think this induces the ECU to dump a load of fuel through the injectors, as it 'thinks' the air flow is low.
Talking of the crankcase breather/oil separator - this is the black plastic thing that sits at the front of the r/h cam cover and has a hose outlet that goes to the aforementioned duct just before the turbo inlet; it contains a diaphragm and spring (yes, the top can be carefully prised off to check the diaphragm for holes/splits/perishing) and under light 'throttle' openings allows fumes form the crankcase to be sucked through; however, at at high levels of vacuum (i.e. when the turbo is spinning at speed/under load) the diaphragm is drawn down and shuts off the breather/oil separator. This is important, as the only other way air can enter the crankcase is via the dipstick tube and therefore will thoroughly aerate the oil in the sump, which can give a "Reduce Oil Level" error message,. Also, oil mist in crankcase air is a very bad thing if it gets drawn in to the combustion chambers - Youtube has a few examples of 'diesel-run-on-self-destruct' cases where the engine runs on it's own engine oil and - er- self destructs (!)
So, when replacing/refitting these parts, make sure the hoses and ducts are in good condition, all clips are tight, and the O-rings associated with the breather/separator are properly fitted, and finally, check the O-ring on the dipstick - it's there for a very good reason :)
Anyway, the car now runs as it should and doesn't smoke, though I do miss the wide-eyed, push-back-in -the-seat grunt of that un-fettered, un-controlled turbo effect that really has no place on a diesel car!


Mar 4, 2010
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Your Mercedes
2005 CL500.
Nice report...thanks.

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