There are two ways to read the story of Mercedes' four-cylinder diesels. One is a cautionary tale. The other is a comeback. The OM651 is both — depending entirely on which year you buy — and the OM654 is its better, but not trouble-free, successor.
If you're shopping a used C-Class, E-Class or GLC diesel, the most important facts on the windscreen are the year and the variant. Get them wrong and you inherit a four-figure injector or timing-chain bill. Get it right and you avoid the worst of it.
Quick answer: The OM654 (2016→) is the better buy at 76/100 (ACCEPTABLE), but it has weak points of its own: rocker-arm wear on engines built before mid-2018 and a timing chain at the gearbox end. The early OM651 (2008–2014) scores 66/100 (CAUTION) on its timing chain and, on some variants, its piezo injectors. Later OM651s score 72/100. Never buy an OM651 without knowing its build year, its variant and its injector history.
Act one: how the OM651 went wrong
The OM651 arrived in 2008 as Mercedes' new mainstream 2.1-litre diesel, and it brought two expensive habits with it.
OM651 (early) — 66/100 (CAUTION)
The pre-2014 cars are the problem children. Two failures define them, and both are wallet-emptying:
| Problem | Severity | Repair cost |
|---|---|---|
| Timing chain stretch and tensioner wear (rear chain, from ~150,000 km) | Critical | €1,200–1,800 |
| Piezo injector failure — 220 CDI to 2010/11 and every 250 CDI | High | €1,400–2,000 |
| DPF + EGR fouling (city use) | High | €500–1,200 |
| Water pump leak (2009 builds) and coolant stub | Moderate | — |
The injectors were the headline act, but only on some cars: the 200 CDI always had solenoid injectors, the 220 CDI switched to them in 2010–11, and Mercedes replaced many piezo sets free. Failures start from about 50,000 km, and you're usually replacing all four. The chain is the bigger long-term risk.
Found in: Mercedes C-Class (W204), E-Class (W212), GLK (2008–2014).
OM651 (late) — 72/100 (ACCEPTABLE)
Here's the nuance most buyers miss. The later W205/W176-era cars are a better proposition, but not because of one clean 2014 fix. Most 136 and 170 PS versions run solenoid injectors instead of the failure-prone piezo units — the 204 PS 250 d kept piezo. The rear-mounted chain still wears from around 150,000 km, and cars built February–November 2014 were recalled for a leaking chain-tensioner seal:
| Problem | Severity | Repair cost |
|---|---|---|
| Piezo injector failure — 250 d (204 PS) only | High | — |
| Timing chain and tensioner wear (rear chain) | High | — |
| Chain-tensioner seal leak — recall, Feb–Nov 2014 builds | High | free under recall |
| DPF blockage (city driving) | High | €500–1,500 |
| AdBlue / SCR faults | Moderate | €290–590 |
| EGR valve fouling | Moderate | €300–700 |
Only six points separate the two OM651 halves — the late engine kept the rear chain, and the 250 d kept its piezo injectors. This is why "is the OM651 reliable?" has no single answer.
Found in: Mercedes A 180d, B 180d, C 200d, C 220d, E 200d, E 220d, GLA 200d, GLC 220d (2014–2020).
Act two: the OM654
In 2016 Mercedes threw out the architecture and started again — an all-aluminium block, stepped-bowl combustion, and a clean-sheet design. The result is the quietest, cleanest diesel the company has ever sold to the public.
OM654 — 76/100 (ACCEPTABLE)
No critical faults, and none of the early OM651's piezo-injector history — but not trouble-free either. Two high-severity faults and two recalls bring it down to ACCEPTABLE:
| Problem | Severity | Repair cost |
|---|---|---|
| Roller rocker arm wear (engines built before mid-2018, ~95,000–120,000 km) | High | — |
| Timing chain stretch (chain at the gearbox end, ~120,000–180,000 km) | High | — |
| Coolant pump can leak into the vacuum system (recall, fire risk) | Moderate | free under recall |
| DPF and EGR clogging, oil diluted by diesel (short trips) | Moderate | — |
| Crankshaft out of specification (recall, engines built July–December 2020) | Minor | free under recall |
| Oil leak at the front crankshaft seal | Minor | — |
Found in: Mercedes A 180d, C 220d, C 300d, E 220d, GLC 220d, GLE, S-Class (2016–present).
The verdict
| OM651 early | OM651 late | OM654 | |
|---|---|---|---|
| Score | 66/100 | 72/100 | 76/100 |
| Verdict | CAUTION | ACCEPTABLE | ACCEPTABLE |
| Years | 2008–2014 | 2014–2020 | 2016→ |
| Critical faults | 1 | 0 | 0 |
The OM654 comes out ahead, but only four points clear of the late OM651 — its rocker arms and gearbox-end chain are real costs to plan for. The more useful takeaway is about the OM651: the badge tells you little — the build date and the variant tell you more. A 2009 C 250 CDI with piezo injectors and a 2012 C 200 CDI wear the same engine name and carry very different repair risks.
What to do at the dealer
- Buying an OM654? On an engine built before mid-2018, ask for proof the rocker arms were checked or replaced; past ~120,000 km, listen for a chain rattle for a few seconds after a cold start. Confirm the coolant-pump recall was done (cars built 2017–2021), that servicing used Mercedes-approved oil on time, and that the DPF regenerates (motorway miles, not pure city use).
- Tempted by a cheap OM651? Check the year and variant first. Pre-2014: a 200 CDI, or a 220 CDI from 2011, avoids the piezo injectors; on a piezo car, ask for injector invoices, and on any car past 150,000 km ask for proof of timing-chain work. Post-2014: acceptable — confirm the tensioner recall was done on 2014 builds, avoid the piezo 250 d unless the injectors have a history, and budget for DPF/EGR.
- Cross-shop the OM654 against BMW's B47 in our OM654 vs B47 comparison.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →