Mercedes-Benz OM651 engine (2008-on)

General description

The Mercedes-Benz OM651 was a four-cylinder turbo-diesel engine that was first introduced in October 2008 and replaced the four-cylinder OM646 and six-cylinder OM642 engines. The OM651 engine was a completely new design that was pre-assembled at Mercedes-Benz’s Untertürkheim-Stuttgart plant and completed at MDC Power GmbH in Kölleda.

The OM651 engine had a cast iron block, an alloy cylinder head, common-rail direct injection, a forged crankshaft and connecting rods, aluminium pistons, double overhead camshafts (driven via a combination of toothed cogs and a short simplex chain), four valves per cylinder (actuated via roller rocker arms), an intercooler, a compression ratio of 16.2:1 and water-cooled exhaust gas recirculation.

For smoother running, the OM651 engine also featured a Lanchester balancer which consisted of two counter-rotating shafts that turned in a cassette beneath the crankcase and were driven by crown wheels. Furthermore, friction was minimised by mounting the shafts on two needle bearings and a deep-grove ball bearing. To minimise fuel consumption, the OM651 engine had a high-pressure fuel pump with suction throttling to adjust the flow as needed and an on-demand water pump.

Injection systems

Initially, the OM651 engine was available with two injection systems –

• Engines producing up to 105 kW had magnetically-actuated solenoid injectors which provided up to four injections per combustion cycle (up to two pre-combustion injections, followed by the main injection and a post-combustion injection if required); and,
• Engines producing more than 125 kW (the 220 CDI BlueEfficiency and 250 CDI BlueEfficiency) had Delphi piezo injectors in which the injector needle was actuated directly so that five injections per combustion cycle were possible. From around mid-2012, however, the Delphi piezo injectors were replaced by solenoid injectors (see ‘Problems’, below).

Single and twin turbochargers

The OM651 engine was available with single and twin turbocharger variants –

• Engines producing up to 120 kW had a single IHI turbocharger with variable turbine geometry; and,
• Engine producing more than 125 kW (the 220 CDI BlueEfficiency and 250 CDI BlueEfficiency) had two Borg Warner turbochargers: a small high-pressure turbo and a large low-pressure turbo that operated in series. At low engine speeds, the compact high-pressure turbo operated in isolation. From medium engine speeds upwards, however, the charge-air pressure control valve of the low-pressure turbocharger opened wide enough to enable this turbocharger to take over turbine operations. To control boost pressure, both turbochargers were fitted with wastegate valves.

OM651 Problems

OM651 Delphi injector failure

The Delphi piezo injectors that were fitted for the 125 kW to 150 kW engines (i.e. the 220 CDI BlueEfficiency and 250 CDI BlueEfficiency) experienced a high failure rate, generally occurring beyond 50,000 kilometres. If the injectors failed, the engine warning light would illuminate, the vehicle would enter ‘limp home’ mode and the engine run unevenly. Initially, revised piezo injectors were introduced. Subsequently, however, Mercedes-Benz initiated a customer service action whereby the original Delphi piezo injectors were replaced with magnetic solenoid injectors, a new ECU was installed, a fuel return line was retrofitted and the engine cover was changed. From around mid-2012, Mercedes-Benz ceased using piezo injectors for these vehicles and used magnetically-actuated solenoid injectors instead.

OM651 timing chain/tensioner wear

At higher mileages, there have been reports of wear of the simplex timing chain and/or chain tensioner. Since the chain is installed on the transmission side of the engine, access is restricted and replacement is expensive.