Quick Summary<\/strong><\/p>\n\n\n\n The OM651 powered C 220d and C 250d models from 2014 to 2018. It’s a 2.1-litre four-cylinder with a single roller chain driving the cams and common-rail injection. On paper, it’s torque-rich and efficient. In practice, early examples earned a reputation for expensive failures.<\/p>\n\n\n\n The first generation of OM651 (roughly 2008-2012) has two well-documented weaknesses. The camshaft sprocket on the chain side wears prematurely, and the Delphi injectors become problematic with age. A full injector set replacement runs \u00a3800-\u00a32,000 according to UK supplier data, and timing chain work isn’t cheap either.<\/p>\n\n\n\n Later OM651s (2014 onward in the W205) received reinforced chain components and Bosch injectors on some versions, making them noticeably better. But you need to verify which version you’re looking at. For a deeper technical breakdown of this engine family, see our OM651 diesel engine guide<\/a>.<\/p>\n\n\n\n Cold start is your best diagnostic tool.<\/strong> Listen for a metallic rattle lasting 2-4 seconds. Any chain noise means walk away or negotiate heavily. Check the service history for timing chain replacement invoices \u2014 if there’s no documentation, assume it hasn’t been done.<\/p>\n\n\n\n Also inspect the intake manifold swirl flaps and EGR cooler. These clog with soot on town-driven cars and can crack, leaking coolant into the intake. A DPF that never completes regeneration is another red flag on low-mileage city cars. According to GOV.UK MOT emissions standards<\/a>, a failed DPF regeneration cycle can also trigger an MOT failure if visible smoke is present.<\/p>\n\n\n\n The M274 is a 1.6 or 2.0-litre turbocharged petrol four-cylinder used in C 180, C 200, and C 250 models from 2014 to roughly 2019. It’s smoother and quieter than the diesel, with decent performance and reasonable fuel economy for a petrol. You can browse used Mercedes C-Class engines<\/a> to compare current UK stock and pricing.<\/p>\n\n\n\n The M274 uses a single-row timing chain that stretches prematurely, particularly on engines with delayed oil changes or higher loads. Independent Mercedes workshops report chain elongation measurable between 90,000 and 140,000 km. A full repair \u2014 chain, tensioners, guides, and both camshaft adjusters \u2014 is the only sensible fix. Replacing just the tensioner rarely lasts beyond 12 months.<\/p>\n\n\n\n Repair costs<\/strong> for timing chain and balance shaft issues can reach \u00a33,500-\u00a36,500 at UK indies. That’s a significant chunk of a used C-Class’s value. For a detailed breakdown of M274 reliability and known faults, see our M274 petrol engine guide<\/a>.<\/p>\n\n\n\n Some M274 engines consume oil at 1 quart per 1,000-2,000 miles, linked to piston ring design and cylinder bore glazing. Direct injection also causes carbon buildup on intake valves over time.<\/p>\n\n\n\n The BorgWarner turbo wastegate actuator can develop play, causing a rattle on acceleration that eventually affects boost control. Turbo replacement runs \u00a31,200-\u00a32,500. If you’re weighing repair against replacement, our guide on engine replacement vs rebuild<\/a> breaks down the cost-benefit for UK drivers.<\/p>\n\n\n\n Best years for the M274:<\/strong> Post-2017 models have revised parts and software that reduce \u2014 but don’t eliminate \u2014 chain risk. If buying earlier, insist on documented chain work.<\/p>\n\n\n\n The OM654 replaced the OM651 from 2016 onward (C 220d and C 300d). It’s a clean-sheet design with an aluminium block, 1,950cc displacement, and a timing chain relocated to the front of the engine. For full technical specifications, see our OM654 diesel engine page<\/a>.<\/p>\n\n\n\n The OM654 scores 85\/100 on independent reliability databases, beating BMW’s equivalent B47 diesel. Known issues are minor: DPF clogging around 80,000 km and mild EGR fouling around 100,000 km, both manageable with correct driving habits and maintenance.<\/p>\n\n\n\n There are no widespread timing chain failures, no injector dramas, and no balance shaft catastrophes like the M274. For a used diesel C-Class, this is the engine to target.<\/p>\n\n\n\n Early OM654s (mid-2018 to early 2019) had some teething issues \u2014 check for completed software updates and recalls before buying. The “Bauart 2” revisions from late 2019 onward are the most trouble-free. The DVLA vehicle recall checker<\/a> lets you verify outstanding recalls by registration.<\/p>\n\n\n\n AdBlue system faults<\/strong> (NOx sensors, dosing injectors) are the most common OM654 workshop visit. A pre-purchase XENTRY scan will reveal any stored faults. If you need a replacement, our reconditioned Mercedes engines<\/a> come with warranty and fitment verification.<\/p>\n\n\n\n The answer depends on your priorities and budget.<\/p>\n\n\n\n For diesel buyers:<\/strong> The OM654 is the clear winner. A 2019 or later C 220d with full service history gives you modern diesel efficiency without the OM651’s expensive failure modes. You’ll pay more upfront but save on repairs.<\/p>\n\n\n\n For petrol buyers:<\/strong> Look for a 2017 or later M274 with documented timing chain inspection or replacement. The C 200 and C 250 offer a good balance of performance and refinement, but budget for oil consumption and potential chain work if the history is thin.<\/p>\n\n\n\n Budget option with caution:<\/strong> A late OM651 (2015-2016) with Bosch injectors and documented chain replacement can be a sensible buy if the price reflects the risk. Just don’t buy blind. Our guide on the most reliable Mercedes engines<\/a> ranks these units against the rest of the range.<\/p>\n\n\n\n Real-world UK costs for C-Class engine work vary by engine and condition.<\/p>\n\n\n\n Timing chain (full)<\/strong> Injectors (set)<\/strong> Turbo replacement<\/strong> Used engine (supply only)<\/strong> Figures based on UK independent workshop data (2026) and supplier pricing.<\/em><\/p>\n\n\n\n If the numbers make you wince, remember: a reconditioned engine from a reputable supplier often costs less than repairing a terminally damaged original. For C-Class owners, exploring reconditioned engines<\/a> makes sense when repair quotes exceed 50% of the car’s value. You can also compare C-Class engine replacement costs<\/a> in our dedicated guide.<\/p>\n\n\n\n Is the Mercedes C-Class OM651 engine reliable?<\/strong><\/p>\n\n\n\n Early OM651s (2008-2012) have timing chain and Delphi injector issues that can cost \u00a32,000-\u00a34,000 to fix. Later versions (2014+) with reinforced chains and Bosch injectors are significantly better. Reliability depends entirely on documented maintenance history.<\/p>\n\n\n\n What are common problems with the Mercedes M274 engine?<\/strong><\/p>\n\n\n\n Timing chain stretch between 90,000-140,000 km is the biggest concern, with repair costs reaching \u00a33,500-\u00a36,500 if the balance shaft is affected. Oil consumption (1 quart per 1,000-2,000 miles) and turbo wastegate rattle are also common.<\/p>\n\n\n\n Which C-Class diesel engine is most reliable?<\/strong><\/p>\n\n\n\n The OM654 (2016 onwards) is the most reliable C-Class diesel. It scores 85\/100 on independent reliability ratings, with only minor DPF and EGR fouling as notable issues. It avoids the OM651’s injector and chain problems entirely.<\/p>\n\n\n\n How do I check timing chain wear on a used C-Class?<\/strong><\/p>\n\n\n\n Listen during cold start<\/strong> for a metallic rattle lasting 2-4 seconds. Ask for XENTRY correlation measurements<\/strong> between crankshaft and camshaft. Any engine with chain noise or no documented chain work should be avoided or heavily discounted.<\/p>\n\n\n\n Can I replace my C-Class engine with a different engine code?<\/strong><\/p>\n\n\n\n No. The OM651, M274, and OM654 have different mounting points, ECU calibrations, and emissions systems. You must replace like-for-like. Our engine compatibility guide<\/a> explains what to check before buying.<\/p>\n\n\n\n\n
OM651 Diesel Engine: The Workhorse With Expensive Habits<\/h2>\n\n\n\n
OM651 Timing Chain and Injector Problems<\/h2>\n\n\n\n
How to Check an OM651 Before Buying<\/h2>\n\n\n\n
![\"\"](\"https:\/\/enginefinders.co.uk\/blog\/wp-content\/uploads\/2026\/09\/2-How-to-Check-an-OM651-Before-Buying-1024x576.webp\")
<\/figure>\n\n\n\nM274 Petrol Engine: Refined But Needs Care<\/h2>\n\n\n\n
M274 Timing Chain Stretch \u2014 The Big Risk<\/h2>\n\n\n\n
M274 Oil Consumption and Turbo Issues<\/h2>\n\n\n\n
OM654 Diesel Engine: The Modern C-Class Benchmark<\/h2>\n\n\n\n
![\"\"](\"https:\/\/enginefinders.co.uk\/blog\/wp-content\/uploads\/2026\/09\/3-OM654-Diesel-Engine-The-Modern-C-Class-Benchmark-1024x576.webp\")
<\/figure>\n\n\n\nOM654 Reliability \u2014 Why It’s the Best C-Class Diesel<\/h2>\n\n\n\n
OM654 Variants and What to Avoid<\/h2>\n\n\n\n
Which Mercedes C-Class Engine Should You Buy Used?<\/h2>\n\n\n\n
Mercedes C-Class Engine Replacement Costs (UK)<\/h2>\n\n\n\n
OM651: \u00a3800-\u00a31,500
M274: \u00a31,500-\u00a32,500
OM654: Rare \u2014 N\/A<\/p>\n\n\n\n
OM651: \u00a3800-\u00a32,000
M274: \u00a3600-\u00a31,200
OM654: \u00a3600-\u00a31,200<\/p>\n\n\n\n
OM651: \u00a31,000-\u00a32,000
M274: \u00a31,200-\u00a32,500
OM654: \u00a31,200-\u00a32,200<\/p>\n\n\n\n
OM651: \u00a31,500-\u00a32,500
M274: \u00a31,800-\u00a33,000
OM654: \u00a32,500-\u00a34,000<\/p>\n\n\n\nFAQS<\/h2>\n\n\n\n