Mercedes R172 OM651 EGR Cooler Removal SLK 250CDI, SLC 250d

How to Replace EGR Cooler Mercedes SLK-Class r172, SLK 250CDI, SLC 250d, OM651 engine, 2011, 2012, 2013, 2014, 2015, 2016, 2017 2018, 2019, 2020
The Mercedes-Benz OM651 is one of the most widely used and debated diesel engines produced by the brand. Introduced in 2008, it replaced the legendary OM646 and was designed to meet stricter emission standards while providing more power and better fuel economy.
​Here is a comprehensive breakdown of the OM651 in English:
​Key Specifications
• ​Type: 4-cylinder inline diesel (Common Rail).
• ​Displacement: 2.1 liters (2143 \, cm^3).
• ​Power Output: Ranges from 95 hp (in commercial vans) up to 204 hp and 500 Nm of torque in the twin-turbo (Bi-Turbo) versions.
• ​Turbocharging: Single turbo for lower-output models; sequential twin-turbo for 220 CDI and 250 CDI models.
• ​Timing Chain: Located at the rear of the engine (near the transmission/firewall).
​Pros: Why it’s popular
• ​Efficiency: It is incredibly fuel-efficient. In models like the C-Class or E-Class, it is common to achieve under 5 L/100 km (over 47 mpg) on highways.
• ​Torque: Even the lower-powered versions offer great low-end pull, making it feel more powerful than its displacement suggests.
• ​Versatility: This engine is found in everything from the compact A-Class to the luxurious S-Class, the heavy GLE SUV, and the Sprinter work vans.
​Common Issues (Weak Points)
​While it is generally a "workhorse," there are specific areas owners should be aware of:
• ​Piezo Injectors (Delphi): Early models (2008–2010) suffered from failing piezo injectors. Mercedes issued a massive recall to replace them with more reliable electromagnetic injectors.
• ​Timing Chain Stretch: The chain can stretch after 200,000 km. Because it is located at the back of the engine, replacement is labor-intensive and expensive, often requiring the engine or transmission to be dropped.
• ​Water Pump & Coolant Leaks: The plastic housing of the water pump and the coolant flange on the cylinder head are prone to cracking and leaking over time.
• ​Oil Pump Issues: Some versions feature a two-stage oil pump. If the control valve fails, the engine might run on low oil pressure without triggering a warning light immediately, which can lead to catastrophic engine failure.
• ​Plastic Intake Manifold: It is known to crack or develop leaks in the gaskets, leading to "limp mode" and loss of boost pressure.
​Which models use the OM651?
​You will find this engine in a vast range of Mercedes-Benz vehicles:
• ​C-Class: W204, W205
• ​E-Class: W212 (very common as taxis in Europe)
• ​S-Class: W221, W222 (S250 CDI)
• ​SUVs: GLK, ML/GLE (W166), GLA, GLC
• ​Vans: Sprinter (W906), Vito/V-Class (W447)
​Maintenance Tip
​To ensure longevity, change the oil every 10,000 – 12,000 km (ignore the long 25,000 km factory intervals if you want the engine to last). Always use high-quality oil meeting the MB 229.51 or 229.52 specification. If you hear a "rattling" sound for a few seconds during a cold start, have the timing chain and tensioner inspected immediately.

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