Mercedes CLS OM642/M276/M278 Engine SolutionsUK Specialists

A rebuilt Mercedes Benz CLS engine typically costs £2,847 to £9,500 supply-only, depending on engine code and variant. The OM642 diesel (CLS 350 CDI) averages £3,200-£5,200, while the M156 AMG 6.2L ranges from £6,500 to £9,500.

• CLS 250 CDI (OM651) rebuilt: £2,800-£4,500
• CLS 350 petrol (M272) rebuilt: £2,847-£5,200
• CLS 63 AMG (M157) rebuilt: £7,500-£12,000
• Additional £450-£900 for supply and fit by AMG-trained technicians

The OM642 3.0 V6 diesel is fundamentally robust and capable of 200,000+ miles, but it has a well-documented Achilles heel: oil cooler seal failure that allows coolant to mix with oil. Catch it early and it's a great engine.

• Oil cooler seals harden and fail at 70,000-120,000 miles
• Turbocharger can be damaged if oil contamination is ignored
• CLS 350 CDI is considered the best diesel of C219/C218 generations
• Preventative seal replacement costs £650-£950 at a specialist

The M156 6.2L V8 is a masterpiece but has a critical flaw: head bolts on pre-2011 engines can snap, allowing coolant into cylinders. Camshaft adjuster wear and hydraulic tappet noise are also common on higher-mileage cars.

• Head bolt failure typically occurs between 40,000-60,000 miles
• Symptoms include coolant loss, white smoke, and misfire on cold start
• Preventative head bolt upgrade costs £2,200-£3,200 at an AMG specialist
• Post-2011 cars have the updated bolt design

A well-maintained CLS diesel (OM642 or OM651) typically reaches 200,000-300,000 miles before major work is needed. Petrol V6 engines (M272/M276) often last 150,000-200,000 miles. The M156 AMG can reach similar figures if head bolts are addressed.

• OM642 diesel regularly exceeds 250,000 miles with proper maintenance
• OM651 four-cylinder diesel is known for 300,000+ miles on some examples
• Most CLS cars average 8,600 miles per year
• Only 0.8% of CLS vehicles reach 200,000 miles based on market data

For OM642 oil seal failure caught early, repair is excellent value (£650-£950). For M272 timing chain tensioner failure, replacement timing components cost £900-£1,400. For M156 head bolt failure with coolant contamination, full rebuilt engine is usually the smarter option.

• Preventative repairs are always better value than reactive ones
• If the bottom end is damaged, replacement is the only reliable fix
• Specialist rebuilt engines include upgraded components addressing original failure points
• On a CLS worth £4,000-£8,000, weigh repair cost against car's overall condition

The CLS 350 CDI with the OM642 V6 diesel (2010-2014) is widely considered the most reliable CLS engine, combining robust engineering with strong fuel economy. The later OM654 in C257 models also shows excellent reliability but is newer with less long-term data.

• CLS 350 CDI (OM642) is the "best diesel of all CLS generations"
• CLS 350 (M276 petrol, post-2014) is very reliable with few major issues
• CLS 350d (OM654, C257) praised as "one of the most modern diesels ever"
• Avoid early M272 petrol engines (pre-2008) with balance shaft issues

The OM651 2.1L diesel does not have the catastrophic timing chain failures of some competitors, but it does suffer from EGR valve failure, swirl flap mechanism breakdown, and injector seal leaks. Timing chain issues are rare when oil changes are done on schedule.

• EGR valve failure causes rough idle and check engine light
• Swirl flap failure in intake manifold reduces power significantly
• Injector seal leaks create ticking noise and diesel smell
• With proper maintenance, OM651 can exceed 300,000 miles

If your CLS is otherwise in excellent condition - no rust, good gearbox, well-maintained interior - a rebuilt engine from a specialist is often worthwhile. On a car worth £4,000-£12,000, a £3,000-£5,000 rebuilt engine with 12-month warranty can give you another 100,000+ miles.

• Check for rust on subframes and rear arches before committing
• Gearbox condition matters - 7G-Tronic and 9G-Tronic are generally robust
• A specialist rebuilt engine includes upgraded components to prevent original failures
• Compare the cost of replacement against buying another used CLS

Diesels (OM642, OM651) dominate UK CLS sales and offer 45-55 mpg real-world economy, making them cheaper to run for high-mileage drivers. Petrols (M272, M276, M278) are rarer, smoother, and more enjoyable to drive, but return 25-35 mpg and have different failure patterns.

• Diesel: higher torque for towing, better motorway cruiser, AdBlue system maintenance required on BlueTEC models
• Petrol: smoother power delivery, no DPF or AdBlue issues, lower mileage examples available
• OM642 diesel oil seal failures are predictable and preventable
• M276 petrol direct injection requires carbon cleaning periodically

The M256 3.0L inline-six mild hybrid is generally reliable, but early C257 models (2018-2020) have reported 48V electrical system faults and water pump failures. The combustion engine itself is robust, with most issues being electrical rather than mechanical.

• 48V battery module failure affects EQ Boost function
• Water pump leaks reported on some early M256 engines
• Software updates resolve many electrical gremlins
• Full engine replacement is rare - most problems stop at the 48V system