OM654 vs OM651 — Which Should You Buy?

The OM654 wins for most buyers — it's more fuel‑efficient, cleaner, and more refined than the older OM651. The OM654 is the more modern, more advanced engine and is used in newer Mercedes models. The OM651 is a proven workhorse, but its age shows in higher emissions, lower efficiency, and the risk of balance shaft wear.

OM651 (2.1L 4‑cyl diesel): 1,250 enquiries in 2025 [EM-VERIFIED], typical reconditioned engine cost £2,800–£4,800 [EM-QUOTE], 136–204 bhp, known for balance shaft wear and timing chain tensioner issues
OM654 (2.0L 4‑cyl diesel): 520 enquiries in 2025 [EM-VERIFIED], typical reconditioned engine cost £3,000–£5,000 [EM-QUOTE], 150–265 bhp, known for AdBlue system issues, EGR cooler leaks, and injector seal problems
OM654 delivers better fuel economy (50–55 mpg vs 48–52 mpg) and significantly lower CO₂ emissions
OM651's balance shaft wear can be a costly repair; OM654's AdBlue/EGR issues are generally less severe but require specialist diagnostic equipment
OM654 is used in newer C‑Class, E‑Class, GLC, GLE and CLS models, with better parts support and a more modern driving experience

Full spec comparison, five‑year ownership cost analysis, and a clear verdict for your budget and driving needs below.

At a Glance

	OM651 (2.1L Diesel)	OM654 (2.0L Diesel)
2025 UK Enquiries	1,250 [EM-VERIFIED]	520 [EM-VERIFIED]
Displacement	2.1L 4‑cyl diesel	2.0L 4‑cyl diesel
Power Output	136–204 bhp	150–265 bhp
Recon Engine Supply & Fit	£2,800–£4,800 [EM-QUOTE]	£3,000–£5,000 [EM-QUOTE]
Typical Real‑World MPG	48–52	50–55
Defining Failure Points	Balance shaft wear, timing chain tensioner, injector seals	AdBlue/SCR system, EGR cooler leaks, injector seal issues
Best For	Proven reliability, lower entry price, older C/E‑Class models	Modern efficiency, lower emissions, newer C/E/GLC/GLE models

The Verdict

The OM654 is the better engine for the vast majority of buyers. It's more efficient, cleaner, more refined and used in newer, more desirable Mercedes models. The OM651 is a proven engine that still has a loyal following, but its age shows in higher emissions and the risk of balance shaft wear. Unless you're on a very tight budget or specifically want a simpler, older diesel, the OM654 is the clear winner.

OM651 — The Proven 2.1 Diesel

The OM651 is Mercedes' 2.1‑litre four‑cylinder diesel, introduced in 2008 and used until 2021. It's one of the most widely used diesel engines in the Mercedes range, fitted to C‑Class (W204/W205), E‑Class (W212/W213), GLC (X253), GLE (W166), CLS (C218), S‑Class (W222), Sprinter and Vito. Power outputs range from 136 to 204 bhp, with peak torque of 350–500 Nm. The engine is known for its excellent fuel economy (48–52 mpg in real‑world driving) and strong performance. Key issues include balance shaft wear (especially on earlier models), timing chain tensioner failure, and injector seal leaks. These are generally well‑understood by independent specialists. Replacement cost is £2,800–£4,800 [EM‑QUOTE], making it a relatively affordable engine to maintain. The OM651 is the engine to choose if you want proven diesel performance at a lower entry price. See our OM651 Knowledge Centre and OM651 balance shaft failure page for more detail.

OM654 — The Modern 2.0 Diesel

The OM654 is Mercedes' 2.0‑litre four‑cylinder diesel, introduced in 2017 and still in production today. It's a significant evolution of the diesel concept, with better fuel economy, lower emissions and improved refinement. The OM654 is used in the C‑Class (W205/W206), E‑Class (W213/W214), GLC (X253/X254), GLE (V167), CLS (C257) and S‑Class (W223). Power outputs range from 150 to 265 bhp depending on the variant — the 265 bhp version in the E300d and GLC300d is particularly impressive. The OM654 is more fuel‑efficient (50–55 mpg vs 48–52 mpg) and has significantly lower CO₂ emissions than the OM651. Known issues include AdBlue/SCR system faults, EGR cooler leaks, and injector seal problems. These can be more expensive to diagnose and fix than the OM651's issues, requiring specialist diagnostic equipment. Replacement cost is slightly higher at £3,000–£5,000 [EM‑QUOTE]. The OM654 is the engine to choose if you want modern diesel performance, refinement and lower running costs. Our OM654 Knowledge Centre and OM654 injector seal failure page have full details.

Cost & Ownership Comparison

The OM654 is cheaper to run than the OM651 over a typical five‑year period, despite slightly higher replacement costs. The OM654's better fuel economy saves around £200–£400 per year in fuel costs, and its lower CO₂ emissions mean lower VED. The OM651's lower entry price is tempting, but the risk of balance shaft wear (costing £1,000–£2,000 to fix) can quickly erode any upfront saving. The OM654's AdBlue/EGR issues are generally less severe and more predictable. Over 50,000 miles, the OM654 typically costs its owner £500–£1,000 less to run than the OM651, making it the more financially sensible choice for buyers looking at newer models. For those on a tight budget, a well‑maintained OM651 with documented balance shaft work can still be a good option.

Metric (5 yrs, 50k miles)	OM651 (2.1L)	OM654 (2.0L)
Fuel cost	£5,600–£6,200	£5,200–£5,800
Servicing (scheduled)	£900–£1,300	£900–£1,300
Tax (VED)	£700–£900	£600–£800
Insurance (estimated)	£2,200–£3,000	£2,300–£3,100
Total estimated	£9,400–£11,400	£9,000–£11,000

These figures are [ESTIMATED] based on UK market data and our proprietary enquiry patterns. Actual costs vary by driving style, location and individual vehicle condition.

Which Should You Buy?

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Best Value & Lower Entry Price

OM651 — older C‑Class and E‑Class models are significantly cheaper to buy. Just budget for balance shaft work (if not already done) and factor in slightly higher running costs.

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Modern Performance & Efficiency

OM654 — more powerful (in most variants), more efficient and more refined. Used in newer, more desirable models like the W206 C‑Class, W214 E‑Class and X254 GLC.

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Lowest Emissions & Cleanest Running

OM654 — significantly lower CO₂ emissions and better fuel economy. The cleaner, greener choice for the environmentally conscious buyer.

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Simpler Diagnostics & Parts Availability

OM651 — well‑understood by every Mercedes specialist, with abundant parts and cheaper diagnostic equipment. The OM654 requires more specialist knowledge and equipment.

Whichever you choose, our repair vs replace calculator can help you decide what to do if you face a major engine failure.

Other Diesel Comparisons

If you're still weighing up your options, these comparisons cover related Mercedes diesel engines.

OM654 vs OM651 →

You're here — new vs old 4‑cyl diesel.

OM651 vs OM642 →

4‑cyl vs V6 diesel comparison.

OM646 vs OM651 →

Older vs newer 4‑cyl diesel.

Diesel vs Petrol Running Costs →

Which fuel type suits your mileage?

For more detailed engine‑specific information, visit our OM651 Knowledge Centre or OM654 Knowledge Centre.

Frequently Asked Questions

Which diesel is more reliable, OM654 or OM651?

The OM651 is arguably more robust in some ways — its main issues (balance shaft, chain tensioner) are well‑understood and fixable. The OM654's AdBlue/EGR issues can be frustrating and expensive to diagnose. However, the OM654 is a more modern engine with better overall engineering and fewer catastrophic mechanical failure modes.

Which is cheaper to run, OM654 or OM651?

The OM654 is cheaper to run — it uses less fuel (50–55 mpg vs 48–52 mpg) and has lower emissions, resulting in lower VED. The OM651's lower purchase price may be offset by slightly higher running costs.

Which has more tuning potential, OM654 or OM651?

The OM654 has more tuning potential due to its more advanced engine management and common‑rail system. Up to 300 bhp is achievable with a remap and hardware upgrades. The OM651 is less tunable.

Can I fit an OM654 into an older Mercedes?

No — the engine mounts, ECU, gearbox and cooling systems are completely different. An engine swap between generations is not practical or cost‑effective.

Is the OM654 AdBlue issue as bad as the OM651 balance shaft?

No — the OM654 AdBlue issues are generally less catastrophic and more predictable. They can be frustrating and costly to diagnose, but they rarely destroy the engine. The OM651 balance shaft failure can be catastrophic if left unchecked.

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1,770 Combined EnquiriesOM654 and OM651 analysed from real 2025 UK data.

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A Real VerdictWe name a winner for most buyers, and explain when the OM651 still makes sense.

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15+ Vetted Mercedes SpecialistsExperienced with both diesel engines.

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100% IndependentNo sponsors, no manufacturer influence.