OM642 vs OM651: Which V6/I4 Mercedes Diesel Is in Your Car?

When a customer phones our Centurion yard asking for "a Merc diesel engine", the first question we ask is always the same: OM642 or OM651? These are the two diesels under the bonnet of almost every modern passenger Mercedes on South African roads - but they share almost no parts. Get the wrong one and you've bought a paperweight.

Short version: the OM642 is a 3.0-litre V6 fitted to bigger cars (E-Class, ML, GL, S-Class, Sprinter, G-Wagen). The OM651 is a 2.1-litre four-cylinder fitted to the C-Class, smaller E-Class variants, GLK, GLC and Vito vans. Different blocks, gearboxes and mounts. Below is what we tell customers before we quote.

How to tell them apart at a glance

Open the bonnet. The OM642 is a 72-degree V6 - two cylinder banks angled outwards in a V, with the turbocharger in the valley between them. A wide, square-looking lump filling the bay corner-to-corner. The OM651 is an inline-four - a single row of four mounted longitudinally in rear-drive cars, transversely in front-drive variants like the CLA 220 CDI.

The OM642's plastic cover usually carries the V6 CDI or BlueTEC badge. Count four injector lines on top: it's the OM651. Two banks of three: OM642.

Where each engine is fitted

The OM642 (production roughly 2005 through 2018+, Sprinter use into the early 2020s) was Mercedes' V6 diesel for anything that needed real torque:

• E-Class - W211 E320 CDI, W212 E350 CDI / BlueTEC
• S-Class - W221 S320 / S350 CDI
• ML / GLE - W164 and W166 ML280, ML300, ML320, ML350
• GL / GLS - X164 and X166 GL320, GL350
• R-Class - W251 R320 CDI
• G-Wagen - W463 G350 BlueTEC
• Sprinter - W906 and W907 318 / 319 CDI panel vans

The OM651 launched in 2008 in the W204 C220 CDI and rolled out across the smaller range and the vans:

• C-Class - W204 and W205 (C200, C220, C250 CDI / BlueTEC)
• E-Class - W212 and W213 E200, E220, E250 CDI (the four-pot variants)
• GLK / GLC - X204, X253 (200/220/250 CDI)
• Vito and V-Class - W639 (later) and W447
• Sprinter - W906 / W907 211, 213, 313, 316 CDI
• CLA / GLA / B-Class - transverse fitment in the 220 CDI variants

Need a used OM642 or OM651? We stock both with delivered prices typically 30-50% under main agent. Request a quote or WhatsApp us with your VIN.

Common faults - OM642

The OM642 is strong when maintained, but it has three well-documented weaknesses. Buyers should budget for them.

• Oil cooler seals. The headline fault. The oil cooler sits in the V between the banks and its rubber seals harden and weep oil - usually onto the starter motor or down the bell housing. Mercedes revised the compound (later "Viton" purple seals replaced the original orange ones around 2010) but it's still common. Symptoms: dropping oil level, staining at the back of the engine, sometimes a Check Oil Level message. Parts are cheap; labour is heavy because the turbo and intake manifolds all have to come off to reach it.
• Swirl flap failure. The intake manifold has motorised swirl flaps for low-RPM combustion. The actuator motor wears, the flap linkages snap, and in worst cases a flap breaks off and gets ingested. Many workshops do a "swirl flap delete" preventatively when the inlet is off for the oil cooler job.
• EGR cooler cracks. The EGR cooler can crack internally and leak coolant into the intake - symptoms are coolant loss with no external leak, white smoke and rough idle.

Common faults - OM651

The OM651 is more reliable on paper but has its own issues, mostly clustered in the early production years (roughly 2008-2011).

• Timing chain stretch. Early OM651s had under-spec chain tensioners and plastic guides that crumbled with heat and missed oil services. The classic symptom is a rattling or chattering noise from the front of the engine on cold start. Left long enough, the chain jumps a tooth and you're in for a head rebuild. Strict 10,000-15,000 km oil intervals on MB 229.51 spec oil are non-negotiable.
• Piezo injectors (early units). 220 CDI and 250 CDI variants up to mid-2010 used Delphi piezo injectors that develop a knock or strong vibration on acceleration. Mercedes switched to magnetic injectors progressively from 2010-2011 - far more durable. If you're buying a pre-2011 C220 CDI, factor injector replacement as a possibility.
• Elevated oil consumption. Some early 2.1L units developed unusual oil consumption. We don't see this as often as the timing chain complaint, but check the dipstick on a test drive.

SA parts pricing - what to expect

Realistic Centurion-yard ranges as of 2026. Quotes vary with mileage, variant and whether the engine comes complete with injectors, turbo and ancillaries.

• Used OM642 (complete unit): roughly R28,000 - R55,000. ML/GL units sit at the higher end because demand is steady.
• Reconditioned OM642 long block: typically R65,000 - R95,000, depending on whether injectors and turbo are included.
• Used OM651 (complete unit): roughly R18,000 - R38,000. Vito and Sprinter units cluster in the middle; W205 C-Class units at the top.
• Reconditioned OM651 long block: typically R45,000 - R75,000.
• Timing chain kit (OM651): kit plus labour at an independent diesel specialist typically lands between R12,000 and R22,000 fitted.

Always ask whether a price is engine-only or includes turbo, injectors, flywheel, harness and ECU - bare vs complete drop-in can be R10,000+ apart on the same chassis. While you source the engine, budget for fresh oils and fluids at fitment, and a timing chain kit on a high-mileage OM651.

Which one is in YOUR car?

Three quick checks, in order of reliability:

• Read the chassis code. Every Mercedes carries a 3- or 4-character chassis code (W212, W204, W463 etc.) on the door jamb sticker and the engine-bay plate. We've written a full guide on decoding your chassis code from the VIN. Once you have the chassis code, the engine almost always falls into place - a W204 C220 is OM651; a W164 ML350 is OM642.
• Check the boot or fuel-flap label. Many Mercedes diesels have an emissions sticker inside the fuel-filler flap or under the boot lid that lists the engine code outright (e.g. OM 642.882 or OM 651.924).
• Count the cylinders. Two banks of three in a V = OM642. One straight row of four = OM651. Takes ten seconds.

Still not sure? Send us your VIN - fastest way for our parts team to confirm the engine code and quote the right unit. Request a quote here or WhatsApp us your VIN and we'll come back inside our usual two-hour window with a delivered price.