MajorCommonEngine

Mercedes-Benz S-Class: OM642 3.0 V6 CDI Oil Separator Failure and DPF Blockage

OM642 3.0 V6 CDI Oil Separator Failure and DPF Blockage is a common engine fault on the Mercedes-Benz S-Class on the 2005–2020 cars, and we rate it major. Putting it right costs $700 to $3,200 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 20 documented faults on the Mercedes-Benz S-Class.

Vehicle: Mercedes-Benz S-Class
Engine: 3.0L
Years: 2005–2020
Fuel: diesel
Gearbox: auto
System: Engine

This page covers the Mercedes-Benz S-Class as a model. Some faults only affect particular engines or build years — enter a plate below to narrow the list to one car.

All Mercedes-Benz S-Class faults20 documented faults on this model

What you are looking at

No photograph available

We do not hold a verified photograph of the PCV / oil separator or Diesel particulate filter yet. We would rather show none than show a picture of a different part.

What goes wrong

The OM642 3.0-litre V6 diesel engine used in both the W221 (S350 CDI) and W222 (S350d) suffers from two well-documented failure modes. First, the oil separator (crankcase ventilation valve) fails and causes excessive oil consumption, oil ingestion into the inlet manifold, and eventual inlet valve carbon fouling with oil sludge requiring manifold removal. Second, the DPF is susceptible to blockage in low-duty-cycle city use. Both faults are confirmed by AU and NZ independent workshop reports. The oil separator failure is the more serious issue as it can disguise itself as general oil consumption until inlet contamination causes power loss.

What you would notice

Blue or white smoke from exhaust under load indicating oil burning
Elevated oil consumption between services
DPF warning light on dashboard
Oily residue inside the air intake pipe or intercooler hoses
Power reduction or limp mode from DPF blockage

How to check it

Where to look. Oil separator on top rear of engine block; DPF under vehicle in exhaust midpipe section

Check the oil level after a warm test drive.
Look for blue smoke on a cold restart.
Check inlet manifold at service history for any decoking work.
If a workshop inspection is possible: Check the DPF soot load via a diagnostic scanner.
Remove the air intake pipe from the turbo inlet and inspect inside for oil sludge coating — a clear sign of oil separator failure.

Parts involved: PCV / oil separator, Diesel particulate filter.

What it costs to put right

$700 to $3,200 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

Ask the seller

“Has the oil separator been inspected or replaced?”
“Has oil consumption between services been noted?”
“Has the DPF ever been cleaned, regenerated, or replaced?”

Now check the Mercedes-Benz S-Class you are actually looking at

Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.

Not sure what you would be paying for? See what a full KickTyres check includes.

Where this comes from

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.

Mercedes-Benz S-Class buyer questions

Is om642 3.0 v6 cdi oil separator failure and dpf blockage serious on a Mercedes-Benz S-Class?

We rate it major and common on the Mercedes-Benz S-Class. The OM642 3.0-litre V6 diesel engine used in both the W221 (S350 CDI) and W222 (S350d) suffers from two well-documented failure modes. First, the oil separator (crankcase ventilation valve) fails and causes excessive oil consumption, oil ingestion into the inlet manifold, and eventual inlet valve carbon fouling with oil sludge requiring manifold removal. Second, the DPF is susceptible to blockage in low-duty-cycle city use. Both faults are confirmed by AU and NZ independent workshop reports. The oil separator failure is the more serious issue as it can disguise itself as general oil consumption until inlet contamination causes power loss.

How much does it cost to fix on a Mercedes-Benz S-Class?

$700 to $3,200 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.

Which Mercedes-Benz S-Class years are affected?

Our research records this fault on the 2005–2020 Mercedes-Benz S-Class. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.

How do I check for it before I buy?

Check the oil level after a warm test drive. Look for blue smoke on a cold restart. Check inlet manifold at service history for any decoking work. If a workshop inspection is possible: Check the DPF soot load via a diagnostic scanner. Remove the air intake pipe from the turbo inlet and inspect inside for oil sludge coating — a clear sign of oil separator failure. Look here: Oil separator on top rear of engine block; DPF under vehicle in exhaust midpipe section