Mercedes Benz CLS 450/500 Modern Engine ReplacementCompare Prices & Get Quotes

A Mercedes CLS 450 or CLS 500 replacement engine typically costs between £3,200 for a used M256 unit and £7,500 for a fully rebuilt M278 V8 engine. Specialist fitting adds £800-£1,500.

• Used M256/M278 engines: £3,200-£5,500 depending on mileage and condition
• Reconditioned units: £4,500-£6,500 supply only
• Rebuilt units: £5,500-£7,500 with 12-month warranty
• Specialist fitting adds £800-£1,500; dealer labour is significantly higher
• 12-month warranty included on all rebuilt units through EnginesMarket

The M256 inline-six is generally robust and reliable, but it has known weak points. 48V electrical system faults, coolant pump failures, timing chain noise, and oil consumption are the main concerns.

• 48V battery module and electric coolant pump failures have been reported
• A recall was issued for coolant pump wiring harness chafing
• Timing chain rattle on cold start can indicate tensioner wear
• Oil consumption can increase past 60,000 miles
• Carbon cleaning (walnut blasting) is recommended every ~60,000 miles

The M278 twin-turbo V8 is less reliable than the M256. Timing chain tensioner failure and cylinder wall scoring are the main concerns.

• Timing chain rattle on cold start is common - don't ignore it
• Cylinder wall scoring can cause high oil consumption and eventual engine failure
• Have cylinders bore scoped if oil consumption is excessive
• Oil leaks from cam sensor covers and cooler seals are common
• Regular oil changes every 10,000 miles are critical for longevity

Cylinder wall scoring (bore scoring) is a serious issue affecting the M278 V8 engine. The Alusil cylinder coating can wear, causing deep scores in the cylinder walls.

• Symptoms: high oil consumption, piston slap on startup, loss of compression
• If suspected, have the cylinders bore scoped
• Repairs can cost £4,500-£6,500+, often requiring engine-out work
• A rebuilt engine with upgraded cylinder work is the only reliable fix if scoring is severe

The 48V electrical system that powers the EQ Boost integrated starter-alternator can suffer from battery module degradation, software corruption, or wiring issues.

• Symptoms: erratic cooling, warning lights, loss of EQ Boost functionality
• A recall was issued for electric coolant pump wiring harness chafing
• Specialist 48V battery module replacement costs £1,000-£1,600
• Most issues stop at the 48V system - full engine replacement is rare

Well-maintained M256 engines can exceed 150,000-200,000 miles. M278 V8s can also reach high mileage, but timing chain and cylinder scoring issues can occur earlier.

• M256: 150k-200k+ miles with regular servicing
• M256 timing chain inspection recommended from ~100,000 miles
• M278: 100k-150k+ miles if timing chain and cylinder scoring are addressed
• Regular oil changes every 10,000 miles are critical for longevity

The CLS 450 uses the M256 3.0-litre inline-six with 48V EQ Boost (362-367 hp + 21 hp electric), while the CLS 500 uses the M278 4.7-litre twin-turbo V8 (408-449 hp). The CLS 500 offers more power but is less reliable and has higher running costs.

• CLS 450 (M256): 362-367 hp, inline-6, 48V mild hybrid, more economical
• CLS 500 (M278): 408-449 hp, V8, twin-turbo, more power but higher running costs
• The CLS 450 is the modern replacement for the CLS 500 in the C257 generation
• The CLS 500 was produced in the C218 generation (2011-2018)