Is the Mercedes E350d / E400d (OM656) engine reliable?

The OM656 is a sophisticated and powerful straight‑six diesel, but it has several known issues. The tandem oil/vacuum pump failure is the most critical — it can cause loss of braking and requires an engine‑out repair. Cam rocker wear causes ticking/popping noises. AdBlue system failures are also common. With regular maintenance and early intervention, it can be a reliable engine.

What are the common problems with the E350d / E400d engine?

Tandem oil/vacuum pump failure (limp mode, loss of braking — safety critical), cam rocker wear (ticking/popping from intake), AdBlue system failures (sensors, heaters, SCR), turbo actuator vacuum hose collapse, increased oil consumption, and EGR valve clogging.

Is it worth replacing an E350d / E400d engine, or should I scrap the car?

Depends on vehicle value and the nature of the failure. A W213 E350d / E400d is a £10,000–£25,000 vehicle — a £6,000–£7,000 recon is 24–70% of the car's value. For a tandem pump failure (up to £12,000 at a dealer), replacement may approach or exceed our 70% Mercedes scrap threshold. Evaluate carefully based on your car's condition and the specific repair needed.

How do I find my Mercedes E350d / E400d engine code?

Listed on the VIN plate and vehicle documentation. The E350d and E400d engine code is OM656 — a 2.9‑litre straight‑six turbo‑diesel. Tell us your registration date and we'll confirm it for free.

Can I prevent the common failures on an E350d / E400d?

On the vacuum/oil pump: check for TSB completion — if the pump fails, it is an engine‑out job. On cam rocker wear: regular oil changes with Mercedes‑approved MB 229.51/229.52 oil and shortened intervals (5,000–8,000 miles) are essential. On the AdBlue system: use high‑quality AdBlue from reputable sources and have the system serviced regularly. At the first sign of limp mode, ticking from the intake or AdBlue warnings, book an inspection immediately.

Mercedes E‑Class E350d / E400d2018–2023OM656

Mercedes E‑Class E350d / E400d Engine Replacement — Compare Prices & Get Quotes

The E350d and E400d badges represent Mercedes’ most advanced straight‑six diesel — the OM656 2.9‑litre turbocharged engine, with 286–340 PS and up to 700 Nm of torque. Get quotes from trusted UK specialists today.

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Mercedes E350d / E400d Engines — from £2,500 to £9,500OM656

Mercedes E350d / E400d Engines — from £2,500 to £9,500· OM656

Instant QuotesNo Obligation100% FreeReconditionedRebuiltUsedSupply & Fit AvailableUK-Wide Network

The E350d / E400d 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 E‑Class E350d / E400d Engine Replacement Costs

Supply Only£5,500–£8,500

Fitted (Indie)£6,800–£10,000

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£2,500–£4,500£3,700–£6,0001–6 monthsBudget priority; lower‑mileage donor

Reconditioned Engine£5,500–£8,500£6,800–£10,00012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£6,500–£9,500£7,800–£11,20012–24 monthsLong‑term ownership; 3+ year horizon

Main Dealer New£9,000–£13,000£10,500–£15,00024–36 monthsInsurance jobs, warranty claims

Figures above reflect the current OM656 generation. A tandem oil/vacuum pump failure repair alone can cost upwards of £12,000 at a main dealer. A reconditioned engine is a more viable alternative in many cases.

Labour estimate: Fitting an E350d / E400d engine typically takes 14–18 hours at £65–£95/hr (independent) or £110–£160/hr (London/SE dealer). The OM656’s timing chain is at the back of the engine, making many repairs engine‑out jobs.

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Common Problems

1. Tandem Oil/Vacuum Pump Failure (<a href='https://mercedesengines.uk/engine/om654-om656'>OM656</a> — All Years)

Symptoms: Limp mode, loss of braking assistance (servo failure), engine stalling, rough running, P0299E6 error code (boost pressure too low).

Typical mileage: 25,000–60,000 miles

Repair cost: £1,500–£5,000+ (pump replacement, vacuum system clean, engine out)

Replacement cost: £5,500–£9,500 (full engine replacement if damage is extensive)

🔴 Immediate - if you experience loss of braking or limp mode, stop driving and book an inspection today

Our recommendation:

The OM656’s tandem oil and vacuum pump is the single most critical issue. The seal between the oil and vacuum sides degrades over time, allowing oil to enter the vacuum system. This leads to limp mode and, in severe cases, complete loss of servo‑assisted braking. The repair requires the engine to be removed and can cost over £12,000 at a main dealer. If you experience any of these symptoms, do not drive the car — have it recovered to a specialist immediately. There is a TSB (Technical Service Bulletin) for this issue.

1. Tandem Oil/Vacuum Pump Failure (<a href='https://mercedesengines.uk/engine/om654-om656'>OM656</a> — All Years)

Symptoms: Limp mode, loss of braking assistance (servo failure), engine stalling, rough running, P0299E6 error code (boost pressure too low).

Typical mileage: 25,000–60,000 miles

Repair cost: £1,500–£5,000+ (pump replacement, vacuum system clean, engine out)

Replacement cost: £5,500–£9,500 (full engine replacement if damage is extensive)

🔴 Immediate - if you experience loss of braking or limp mode, stop driving and book an inspection today

Our recommendation:

2. Cam Rocker Wear / Camshaft Scoring (OM656 — All Years)

Symptoms: Ticking or popping noise from the intake, rough idle, loss of power, metallic ticking.

Typical mileage: 60,000–120,000 miles

Repair cost: £2,000–£5,000 (cam rockers, camshaft replacement)

Replacement cost: £5,500–£9,500 (full engine replacement if damage is extensive)

🟠 Check - book inspection urgently if you hear ticking or popping from the intake

Our recommendation:

Cam rocker wear is a known issue on the OM656. If left unaddressed, it can lead to scoring of the cam lobes, requiring expensive camshaft replacement. The popping noise from the intake is the early warning sign. Regular oil changes with Mercedes‑approved oil (MB 229.51 or 229.52) are essential. If you hear a ticking or popping noise, have the valvetrain inspected immediately.

3. AdBlue System Failures (All OM656)

Symptoms: Check engine light, AdBlue warning, reduced power, countdown to start restriction (usually 500 miles), engine warning light.

Typical mileage: 30,000–80,000 miles

Repair cost: £500–£2,500 (sensor replacement, AdBlue heater, SCR system repair)

Replacement cost: N/A — repair only

🟠 Check - book inspection urgently if AdBlue countdown appears

Our recommendation:

The AdBlue (SCR) system on the OM656 is a common failure point. Faults include AdBlue level sensors (P203B31), concentration sensors (P206B31), and heaters. If the AdBlue system fails and the countdown reaches zero, the car will not start. Mercedes has issued technical bulletins for these issues. Use high‑quality AdBlue from reputable sources and have the system serviced regularly. At the first sign of an AdBlue warning, book an inspection immediately.

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E350d / E400d — 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.

E350d / E400d — 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 SenseTandem oil/vacuum pump failure✅ Yes£1,500–£5,000+Engine‑out job — catch it early before complete failureCam rocker wear / camshaft scoring✅ Yes£2,000–£5,000Catch it early — before cam lobes are scoredAdBlue system failure✅ Yes£500–£2,500Sensor or heater replacement — use quality AdBlueEGR valve clogging✅ Yes£400–£1,200Routine maintenance — clean or replaceSevere engine damage (pump failure / chain jump)❌ Usually NoEngine replacementLittle left to salvage — replacement is the only option

Buying This E350d / E400d? Check These First

Buy If

No history of vacuum/oil pump failure — or the pump has already been replaced under TSB
No ticking/popping from the intake — cam rockers in good condition
Full Mercedes service history with correct MB‑spec oil
AdBlue system serviced — quality AdBlue used

Avoid If

Vacuum/oil pump failure history with no documented repair — walk away
Ticking or popping from the intake — cam rocker wear may be present
AdBlue warning light with no diagnosis — potential SCR system failure
Poor service history — OM656 engines are sensitive to neglect

Closing Verdict

The OM656 is one of Mercedes’ most advanced straight‑six diesels — smooth, powerful and efficient. But it has several well‑documented weaknesses: the tandem oil/vacuum pump (which can cause loss of braking), cam rocker wear and AdBlue system failures. The pump failure is safety‑critical and can occur at relatively low mileage. Buy on full service history, check for TSB completion and never ignore a ticking noise or AdBlue warning. A well‑maintained E350d / E400d is a rewarding, long‑lived executive express; a neglected one is a £10,000+ bill waiting to happen.

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

→ Compare E350d / E400d engine prices before you buy

Don't Guess. Compare.

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→ Compare E350d / E400d engine prices before you buy

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Buying Checklist

Before you buy a used Mercedes E‑Class E350d / E400d (or before you commit to an engine replacement), check these:

Vacuum Pump — Check for any history of vacuum/oil pump failure — safety critical

Cold Start — Listen for ticking or popping from the intake — cam rocker wear indicator

AdBlue System — Check for warning lights — AdBlue system failure indicator

Service History — including engine era (OM656) and confirmed oil‑change intervals

Turbo Health — Listen for whistling or whining; turbo actuator hose issues possible

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

Coolant Level — Check for unexplained loss; EGR cooler issues possible

MOT History — Check for emissions‑related failures (AdBlue/SCR)

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.

OM656 (2.9 I6 turbo diesel)

Years2018–2023

Power286–340 PS (210–250 kW); 600–700 Nm

2025 Enquiries— (pattern data)

Avg. Recon Cost£5,500–£8,500【PLACEHOLDER】

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

OM656 (2.9 I6 turbo diesel)2018–2023286–340 PS (210–250 kW); 600–700 Nm— (pattern data)£5,500–£8,500【PLACEHOLDER】

Technical Specs

OM656 2.9 I6 diesel

Displacement2,925 cc (or 2,989 cc in some variants)

Cylinders6‑cylinder inline

Compression Ratio15.5:1

Fuel SystemCommon‑rail direct injection

TurbochargerBorgWarner R2S® twin‑scroll turbo with eBooster

ValvetrainDOHC with CAMTRONIC variable valve lift

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Market Intelligence

Most Common Failure

Tandem oil/vacuum pump failure — the most critical OM656 issue【PATTERN DATA】

Average Enquiry Mileage

40,000

–60,000 miles【PATTERN DATA】

Average Replacement Cost

£6,500

reconditioned, supply only【PLACEHOLDER — confirm vs cost sheet】

Most Requested Engine Code

OM656【PATTERN DATA】

Peak Enquiry Season

Autumn/Winter — cold starts expose vacuum pump and cam rocker issues【PATTERN DATA】

Regional Demand

South East19%

London17%

South West (14%)【PATTERN DATA】

Regional Demand Breakdown

19%South East

17%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 E‑Class E350d / E400d engine replacement UK drivers rely on. Compare prices from trusted specialists on EnginesMarket today and get back on the road with confidence — from £2,500 for a used unit to £9,500 for a fully rebuilt straight‑six diesel.

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12-Month Warranty

As standard

Nationwide Delivery

Supply & fit available