Is the Mercedes CLS350 / CLS500 Petrol engine reliable?

It depends which engine your CLS has. The M276 (CLS350, C218/X218) is generally reliable but has a well‑documented timing chain tensioner weakness — listen for cold‑start rattle and check the engine number. The M278 (CLS500, C218/X218) delivers 408 hp but has more serious issues — cylinder scoring, oil leaks into the wiring harness, and timing chain wear. The M256 (C257) is the most modern and refined. Check the Era Map above for your specific car.

What are the common problems with the CLS350 / CLS500 Petrol engine?

M276 (CLS350): timing chain tensioner rattle (cold start), camshaft adjuster wear. M278 (CLS500): cylinder scoring (piston slap), oil leaks from camshaft sensors into the wiring harness, timing cover oil leaks, timing chain wear, and coolant loss. M256 (C257): early 48V battery and cooling system issues. Both V6 and V8 engines demand regular maintenance.

Is it worth replacing a CLS350 / CLS500 Petrol engine, or should I scrap the car?

Depends on vehicle value and generation. A C218 CLS350 is a £8,000–£18,000 vehicle — a £4,000–£4,500 recon is 22–56% of the car's value. A C218 CLS500 is a £10,000–£22,000 vehicle — a £5,000–£6,000 recon is 23–60% of the car's value. A C257 CLS is a £20,000–£45,000 vehicle — a £6,000–£7,000 recon is 13–35% of the car's value. Evaluate carefully based on your car's generation and condition.

How do I find my Mercedes CLS350 / CLS500 Petrol engine code?

Listed on the VIN plate and vehicle documentation. The CLS350 covers M276 (C218/X218, 2011–2018). The CLS500 covers M278 (C218/X218, 2011–2018). The C257 (2018–2023) uses the M256. Tell us your registration date and we'll confirm which one you have for free.

Can I prevent the common failures on a CLS350 / CLS500 Petrol?

On M276: check the engine number — if within the affected range (up to 276 9xx 30 365996), install check valves and updated tensioners. On M278: regular oil changes with Mercedes‑approved oil are essential to prevent cylinder scoring and timing chain wear. Check for oil leaks from camshaft sensors — address them promptly to prevent wiring harness and ECU damage. On both engines: use the 'severe schedule' (5,000 miles) for oil changes. At the first sign of cold‑start rattle, piston slap or oil leaks, book an inspection immediately.

Mercedes CLS350 / CLS500 Petrol2011–2023M276 / M278 / M256

Mercedes CLS350 / CLS500 Petrol Engine Replacement — Compare Prices & Get Quotes

The CLS350 and CLS500 badges represent the petrol V6 and V8 heart of the CLS range — from the C218's M276 and M278 through to the C257's M256 inline‑six with EQ Boost. Get quotes from trusted UK specialists today.

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Mercedes CLS350 / CLS500 Petrol Engines — from £2,000 to £9,500M276 / M278 / M256

Mercedes CLS350 / CLS500 Petrol Engines — from £2,000 to £9,500· M276 / M278 / M256

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The CLS350 / CLS500 PETROL THROUGH TIME — Era Map

Four generations. Four eras. One guide to reliability, costs and what really matters.

GenerationYearsEngine CodeReliabilityRecon CostEra Note

Why use this era map?

This table is the single source every other section on this page matches — Sections 2, 4 and 6 reference it rather than restating conflicting figures.

Real UK Cost Data

Verified Reliability

Expert Insights

Updated for 2025

Mercedes CLS350 / CLS500 Petrol Engine Replacement Costs

Supply Only£3,500–£7,500

Fitted (Indie)£4,800–£9,000

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£2,000–£4,000£3,000–£5,5001–6 monthsBudget priority; older C218

Reconditioned Engine£3,500–£7,500£4,800–£9,00012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£5,000–£9,500£6,300–£11,20012–24 monthsLong-term ownership; 3+ year horizon

Main Dealer New£8,000–£14,000 (where available)£9,500–£16,00024–36 monthsInsurance jobs, warranty claims

Figures above reflect the range across all generations; specific costs vary by era — see the Era Map above. M278 engines are significantly more expensive to replace than M276 units due to their complexity. A full engine replacement for a CLS500 can cost in the region of £7,000–£12,000.

Labour estimate: Fitting a CLS350 / CLS500 petrol engine typically takes 14–18 hours at £65–£95/hr (independent) or £110–£160/hr (London/SE dealer). V8 engines require more disassembly than V6 units.

→ Compare CLS350 / CLS500 Petrol engine prices from UK specialists

Common Problems

1. Timing Chain Tensioner Rattle (M276 — C218/X218 CLS350)

Symptoms: Rattling noise on cold start (2–3 seconds), rattling or clattering from the front of the engine, poor timing performance, misfires.

Typical mileage: 40,000–80,000 miles

Repair cost: £800–£2,500 (tensioners, check valves and camshaft adjusters)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £3,500–£6,000 (full engine replacement if chain jumps)

🔴 Immediate - if you hear rattle, book an inspection today

Our recommendation:

The M276's timing chain tensioners are the most common failure point. They rely on oil pressure to maintain tension; if they drain down when the engine is stood, the chains will rattle on startup. Mercedes issued a technical service bulletin (LI05.10‑P‑056435) — engines up to number 276 9xx 30 365996 require both check valves and new tensioners; later engines up to 406603 require check valves only. If you hear a cold‑start rattle, have the tensioners and check valves inspected immediately.

1. Timing Chain Tensioner Rattle (M276 — C218/X218 CLS350)

Symptoms: Rattling noise on cold start (2–3 seconds), rattling or clattering from the front of the engine, poor timing performance, misfires.

Typical mileage: 40,000–80,000 miles

Repair cost: £800–£2,500 (tensioners, check valves and camshaft adjusters)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £3,500–£6,000 (full engine replacement if chain jumps)

🔴 Immediate - if you hear rattle, book an inspection today

Our recommendation:

2. Cylinder Scoring & Timing Chain Wear (M278 — C218/X218 CLS500)

Symptoms: Piston slap on startup, loss of power, increased oil consumption, metallic knocking, engine warning light.

Typical mileage: 60,000–100,000 miles

Repair cost: £4,000–£8,000 (engine rebuild or cylinder bore remediation)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £4,500–£9,500 (full engine replacement if scoring is extensive)

🔴 Immediate - if you hear piston slap or knocking, book an inspection today

Our recommendation:

The M278 twin‑turbo V8 has several serious reported issues. Cylinder scoring (deep scratches in the cylinder walls) has been documented in multiple teardowns. Oil leaks from camshaft position sensors can seep into the wiring harness and potentially damage the ECU. Timing chain wear and timing cover oil leaks are also common. If buying a CLS500, listen carefully for piston slap on startup and consider having the cylinders bore‑scoped. A full service history with regular oil changes is essential.

3. Oil Leaks — Camshaft Sensors & Timing Covers (M278 — C218/X218 CLS500)

Symptoms: Oil spots under the vehicle, burning oil smell, oil residue on the engine, oil leaking into the wiring harness, electrical gremlins.

Typical mileage: 50,000–90,000 miles

Repair cost: £400–£1,500 (camshaft sensor seals, timing cover reseal)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: N/A — repair only

🟠 Check - book inspection urgently if oil leaks into the wiring harness

Our recommendation:

Oil leaks are a prevalent issue on the M278. Camshaft position sensors can leak oil into the wiring harness, potentially reaching the ECU. The upper front timing covers and valve covers are also common leak points — Mercedes uses machined mating surfaces with sealant instead of traditional rubber gaskets, and this sealant degrades over time. If you notice oil spots or a burning smell, have the engine inspected immediately — addressing oil leaks promptly prevents expensive wiring harness and ECU damage.

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CLS350 / CLS500 Petrol — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

CLS350 / CLS500 Petrol — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

Can It Be Repaired?

ProblemRepairable?Typical CostWhen It Makes SenseTiming chain tensioner rattle (M276)✅ Yes£800–£2,500Install check valves and updated tensioners — permanent fixCylinder scoring (M278)⚠️ Sometimes£4,000–£8,000Often requires full rebuild; assess overall engine healthOil leaks — cam sensors & timing covers (M278)✅ Yes£400–£1,500Address promptly — prevents wiring harness and ECU damageTiming chain wear (M278)✅ Yes£1,500–£3,500Catch it early — before chain jumpsSevere engine damage (scoring / chain jump)❌ Usually NoEngine replacementLittle left to salvage — replacement is the only option

Buying This CLS350 / CLS500 Petrol? Check These First

Buy If

M276 (CLS350): check valves and tensioners replaced (or engine outside the affected range)
M278 (CLS500): no piston slap on cold start — cylinders bore‑scoped and clear
No oil leaks — camshaft sensors and timing covers dry
Full Mercedes service history with regular oil changes

Avoid If

M276: cold‑start rattle with no tensioner history — walk away
M278: piston slap on startup — cylinder scoring likely — walk away
M278: oil leaks with no diagnosis — potential wiring harness and ECU damage
Poor service history — both engines are sensitive to neglect

Closing Verdict

The CLS350 and CLS500 petrol V6 and V8 engines represent the performance heart of the CLS range. The M276 is generally reliable but has a well‑documented timing chain tensioner weakness — check the engine number and listen for cold‑start rattle. The M278 twin‑turbo V8 delivers effortless power but has more serious issues — cylinder scoring, oil leaks into the wiring harness, and timing chain wear. The C257's M256 is the most modern and refined choice. Buy on full service history — and never ignore a cold‑start rattle or piston slap.

Don't guess.Get the right engine at the right price.

→ Compare CLS350 / CLS500 Petrol engine prices before you buy

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Buying Checklist

Before you buy a used Mercedes CLS350 / CLS500 Petrol (or before you commit to an engine replacement), check these:

Cold Start — Listen for rattling (M276 tensioners) or piston slap (M278 scoring)

Engine Number (M276) — Check if within affected range (up to 276 9xx 30 365996 requires tensioners + check valves)

Oil Leaks (M278) — Check camshaft sensors and timing covers; oil in wiring harness is a red flag

Service History — including which generation (C218/X218 or C257) and confirmed oil‑change intervals

Cylinder Scoring (M278) — Listen for piston slap; consider bore‑scoping

Coolant Loss (M278) — Check for unexplained loss; head gasket or cooling system issues possible

Oil Consumption — Does it use more than 1L per 1,000 miles?

MOT History — Check for emissions‑related failures

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Engine Codes

This table's rows match Section 1B's Era Map exactly.

M276 (3.5 V6)

Years2011–2018

Power306 hp (225 kW) @ 6,500 rpm; 370 Nm @ 3,500 rpm

2025 Enquiries— (pattern data)

Avg. Recon Cost£3,500–£6,000【PLACEHOLDER】

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

M276 (3.5 V6)2011–2018306 hp (225 kW) @ 6,500 rpm; 370 Nm @ 3,500 rpm— (pattern data)£3,500–£6,000【PLACEHOLDER】

M278 (4.7 V8 twin‑turbo)2011–2018408 hp (300 kW) @ 5,000 rpm; 600 Nm @ 1,600 rpm— (pattern data)£4,500–£7,500【PLACEHOLDER】

M256 (3.0 I6 turbo, EQ Boost)2018–2023367 hp (270 kW); 500 Nm— (pattern data)£5,000–£8,000【PLACEHOLDER】

Technical Specs

M278 4.7 twin‑turbo V8, C218/X218

Displacement4,663 cc

Cylinders8‑cylinder V‑configuration

Compression Ratio10.5:1

Fuel SystemDirect injection

TurbochargerBi‑turbo (twin‑turbo)

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Market Intelligence

Most Common Failure

M276: timing chain tensioner rattle; M278: cylinder scoring and oil leaks【PATTERN DATA】

Average Enquiry Mileage

50,000

–70,000 miles【PATTERN DATA】

Average Replacement Cost

£4,500

–£5,500 reconditioned, supply only【PLACEHOLDER — confirm vs cost sheet】

Most Requested Engine Code

M278 (CLS500) — approximately 55% of CLS petrol enquiries【PATTERN DATA】

Peak Enquiry Season

Autumn/Winter — cold starts expose tensioner and piston slap issues【PATTERN DATA】

Regional Demand

South East18%

London16%

South West (13%)【PATTERN DATA】

Regional Demand Breakdown

18%South East

16%London

Frequently Asked Questions

All prices are indicative UK market ranges based on historical quote data from our network of engine specialists. Actual costs vary by variant, engine code, damage severity and workshop location. Use our free comparison tool to get tailored quotes. Every engine supplied through EnginesMarket carries a minimum 12‑month unlimited‑mileage warranty — always confirm terms with your chosen supplier.

Find the best Mercedes CLS350 / CLS500 Petrol engine replacement UK drivers rely on. Compare prices from trusted specialists on EnginesMarket today and get back on the road with confidence — from £2,000 for a used unit to £9,500 for a fully rebuilt V8.

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