OM642 engine: faults and reliability

• OM642 Euro 4, first generation (280/300/320 CDI, 2005-2009)

  2005–2009 · 190–224 hp

  With caution

  It shares all the typical OM642 weak points (oil cooler, intake flaps, injectors) and, according to one specialist source, it is the generation with the most documented cases of the less durable oil cooler seal and of chain tensioner failure. It is worth checking the service history and the condition of these parts before buying.^[1][2]

• OM642 Euro 5, second generation (300/350 CDI and BlueTEC, 2009-2014)

  2009–2014 · 211–258 hp

  With caution

  The oil cooler seals improved over the first generation, but the engine's other weak points (intake flaps, injectors at higher mileages) are still present. There is no documented design change that eliminates them in this generation.^[2]

• OM642 Euro 6 BlueTEC (350 BlueTEC/d, 2014-2021)

  2014–2021 · 258–265 hp

  With caution

  It is the generation with the best oil cooler behaviour, but it adds the risk of NOx sensor failure and is the one at the centre of the investigation and the software update campaign over emissions defeat devices. It is worth checking whether the vehicle has already received the software update.^[2][6][7]

Buy: No known serious faults, or they have already been corrected, in this version. With caution: Worth buying if something specific is verified: history, a replaced part or a completed campaign. Avoid: High risk of a serious fault that is hard to check for or prevent.

1. 1Critical severityFrequency: CommonIntakeBroken swirl flaps in the intake manifoldYes: the plastic rods of the intake manifold's swirl flaps become brittle and can break at around 100,000-200,000 km. If a loose flap is sucked into a cylinder, it can hit the piston and destroy the engine; this is why removing them preventively is recommended.Repair (garage): €600–€1,200Appears at around 100,000 km
2. 2High severityFrequency: Very frequentLubricationOil cooler seal leak (between the cylinder heads)Yes: the OM642's oil cooler, between the two cylinder heads, has seals that harden with heat and eventually leak oil. It is the engine's most frequently cited weak point, especially before 2009-2010, and the repair is expensive because the intake manifold, turbo and several injectors have to be removed.Repair (garage): €1,500–€2,200Appears at around 120,000 km
3. 3High severityFrequency: CommonInjectionWear and failure of the Bosch piezo injectorsThe OM642's Bosch piezo injectors can fail from around 150,000-200,000 km, especially with poor-quality diesel; they cannot be repaired, only replaced. Because it is a V engine, those on the rear bank are harder to reach, which pushes up labour costs.Repair: €4,500–€6,500
4. 4Critical severityFrequency: OccasionalTimingRear timing chain wear and tensioner failureThe OM642's timing chain, at the rear next to the gearbox, usually lasts a long time: normal stretch begins at around 300,000-350,000 km. On older units (up to 2009) cases of premature tensioner failure have been documented, uncommon but serious if the chain skips.Repair (garage): €3,500–€5,000Appears at around 300,000 km
5. 5Medium severityFrequency: Very frequentEmissions controlEmissions defeat device (AdBlue/EGR) and software update campaignGermany's KBA found indications of emissions manipulation in cars with the OM642 engine (alongside the OM651): AdBlue dosing and the EGR changed between the test bench and the road. Mercedes-Benz denied it, but in 2018 it recalled hundreds of thousands of diesels in Europe.

Severity colours: low, medium, high, critical. How we rate them

Manufacturer

Mercedes-Benz (Daimler)

Family

OM642

Fuel

Diesel

Displacement

2,987 cm³

Cylinders

6

Fuel system

Turbo

Injection

Common rail

Timing

Chain

Power

184–265 hp

Peak torque

620 Nm

Production

2005–2021

Makes

Mercedes-Benz, Jeep, Chrysler, Dodge and Freightliner

Technical data^[3][4][5]

1. 2005–2009

   OM642 Euro 4, first generation (280/300/320 CDI, 2005-2009)

   First generation of the engine, approved to Euro 4 and informally known in the trade as "642.85x". According to several specialist sources, these units were fitted with silicone (orange) oil cooler seals that are less durable than those of later generations, and they account for the documented cases of premature timing chain tensioner failure.^[1][2][3]

2. 2009–2014

   OM642 Euro 5, second generation (300/350 CDI and BlueTEC, 2009-2014)

   Generation approved to Euro 5, informally known as "642.92x"/"642.94x". According to one specialist source, from around 2010 Mercedes-Benz improved the material of the oil cooler seals (Viton seals, purple in colour), which reduced the leak problem compared with the first generation, although it did not eliminate it entirely.^[2]

3. 2014–2021

   OM642 Euro 6 BlueTEC (350 BlueTEC/d, 2014-2021)

   Final generation, approved to Euro 6 with the BlueTEC system (AdBlue injection) and informally known as "642.96x". One specialist source points to it as having the best oil cooler seals, but it introduces a weak point of its own: the durability of the NOx sensors, with documented failures before 150,000 km. This generation is also the one involved in the German KBA investigation into possible emissions defeat devices.^[2][6]

• Timing chain kit

  • Buy part at Autodoc (affiliate link, opens in a new tab)

• Oil filter

  • Buy part at Autodoc (affiliate link, opens in a new tab)

• Air filter

  • Buy part at Autodoc (affiliate link, opens in a new tab)

• Diesel fuel filter

  • Buy part at Autodoc (affiliate link, opens in a new tab)

• Glow plugs

  • Buy part at Autodoc (affiliate link, opens in a new tab)