Engines · Mercedes-Benz
Mercedes OM642 3.0 V6 diesel common problems
The OM642 is a 3.0 litre (2,987cc) 72-degree V6 turbodiesel with common-rail piezo injection, a variable geometry turbo, chain-driven camshafts and a balance shaft. It arrived in 2005, replacing the five- and six-cylinder OM647 and OM648, and was used across the Mercedes range for well over a decade, later in BlueTEC form with AdBlue, as well as in the Jeep Grand Cherokee and Chrysler 300C. It is a strong, smooth engine that covers big mileages when looked after.
Common problems and what to look for on the OM642
The known issues most worth checking when you view a car with this engine. None of them means a car is a bad buy, but they're worth knowing about before you buy, and a good history often shows they've been dealt with.
Oil leaking from the oil cooler seals
What to look for: With the engine warm, open the bonnet and sniff for a burning oil smell, and notice it through the vents on the drive too. With a torch, look down the back of the engine and at the top of the gearbox for oil running down. If the engine cover lifts off, look for oil pooled in the V between the cylinder banks. Ask whether the oil cooler seals have been replaced and for the invoice.
Why it matters: The oil cooler sits in the V of the engine under the intake manifolds and turbo, and its seals harden with heat and leak oil into the valley, from where it runs down towards the gearbox. Engines before 2010 are the most prone; from 2010 Mercedes fitted purple Viton seals in place of the earlier orange ones. The seals themselves are small parts but reaching them is a long job, so a car with the work on invoice, ideally with the later seals, is in a good position.
Swirl flap motor faults and limp mode at 3,000rpm
What to look for: On the drive, accelerate firmly through the gears. If the engine refuses to rev beyond about 3,000rpm, or feels flat and hesitant, and the engine warning light is on, that is the classic sign. Ask whether the swirl flap motor or intake manifolds have been replaced, and whether the intake was cleaned at the same time.
Why it matters: Swirl flaps in the intake help low-speed running. Oily vapour from the crankcase breather and soot build up as a tar-like coating that jams the flaps and their linkages, and the motor that works them can fail, which can blow a fuse and put the car in limp mode at about 3,000rpm. Typical codes are P2004 to P2007. The fix is a new motor and, often, cleaned or new manifolds; replacing the motor alone tends to fail again if the carbon is left.
Ticking from the front of the engine at idle
What to look for: Listen at the front of the engine with the bonnet open, warm and idling. A rhythmic ticking or knocking at idle and up to about 1,500rpm is what to note. Ask whether a front main bearing has been replaced under Mercedes' bulletin.
Why it matters: Mercedes issued a service bulletin in February 2013 for an intermittent ticking at idle from the first crankshaft main bearing, fixed by replacing that bearing's shells, which can be done with the engine in the car. Earlier engines are more prone. Caught early it is a contained repair; left alone, bearing wear can lead to engine failure, so get any ticking diagnosed before buying.
More known issues, covered in the full guide for each car
- Leaking injector seals
- AdBlue and NOx sensor warnings on BlueTEC models
Cars with the OM642
- Mercedes-Benz C-Class W204: C320 CDI, replaced by the C350 CDI from 2009
- Mercedes-Benz CLK C209: CLK320 CDI coupé
- Mercedes-Benz CLS C219: CLS320 CDI, renamed CLS350 CDI in 2009
- Mercedes-Benz CLS C218: CLS350 CDI, later CLS350 BlueTEC and 350d
- Mercedes-Benz E-Class W212: E350 CDI, and E350 BlueTEC after the facelift
- Mercedes-Benz M-Class W164: ML280 CDI and ML320 CDI
- Mercedes-Benz M-Class / GLE W166: ML350 BlueTEC and GLE 350d
- Mercedes-Benz GL-Class X164: GL320 CDI, later GL350 BlueTEC
- Mercedes-Benz GL / GLS X166: GL350 BlueTEC and GLS 350d
- Mercedes-Benz S-Class W221: S320 CDI, and S350 CDI after the facelift
- Mercedes-Benz S-Class W222: S350 BlueTEC, 2013 to the 2017 facelift
- Mercedes-Benz G-Class W463: G320 CDI and G350 CDI/G350d V6 diesels
- Mercedes-Benz Viano W639: 3.0 V6 CDI
A pre-purchase inspection guide for your car
The CrankSight guide is a pre-purchase inspection (PPI) you do yourself, written for the exact model and generation you're viewing. It walks you round the car on your phone, with every check for that version, engine included: what to look for, where to find it, and what anything you find is likely to cost. As you go it builds an assessment of that car, and if you view more than one, it lines them up side by side.
£29 for the model, used on as many cars as you like, delivered within 72 hours or your money back.
General points for the engine, last checked October 2026. Every car is different, so treat this as a guide to what to look at, not a verdict on any car. CrankSight is independent and has no connection with any manufacturer.