Mercedes OM651 turbodiesel

It’s time to take a fresh look at the fresh diesel turbocharged 4-cylinder motor from Mercedes with the index OM651. It was this turbo motor that was the subject of the first review posted on our channel. Since then, we have received a lot of new data about this motor, which allowed us to give a final assessment of its reliability. And it is still capable of surprising. For example, it’s not unusual for a connecting rod to puncture the cylinder block. So, what’s wrong with the Mercedes OM651 turbo engine?

Diesel in-line “four” with the index OM651 appeared in 2008 and immediately entered the list of leaders of recall campaigns of the manufacturer. On the surface there was a problem with piezo injectors Delphi, which were installed on the powerful versions 220 and 250 CDI. Daimler’s supplier firm replaced the defective injectors with their improved versions, but this only got rid of one problem, leaving others. Unfortunately, this engine so bored some Sprinter owners and even fleets that they swapped the OM651 for older model motors: 646 and even 611. This is just the case when the new engine really turned out to be worse than its predecessors.

We will tell you about the 651st turbodiesel motor on the example of 1.8-liter internal combustion engine, which was removed from the B-Class of 2012. In general, this motor was installed since 2008 on almost all Mercedes models with longitudinal and transverse arrangement of the power unit. Regardless of the layout, its problems and weaknesses are the same. They often appear because of excessive savings on maintenance and because of hit-and-run drivers who don’t realize that Mercedes need premium treatment.

Specifications

OM 651 DE 18 LA (ed. version 180 CDI)

OM 651 DE 18 LA (200 CDI version)

OM 651 DE 22 LA (ed. versions 180 CDI and 200 CDI)

OM 651 DE 22 LA (220 CDI and 250 CDI versions)

Problems and reliability

Oil and antifreeze leaks

Technical fluid leaks are rare in Mercedes diesels – all seals are implemented at the highest level. However, rubber gaskets have a limited service life. Turbo diesel OM651 begins to “throw out” oil already at a respectable age: at least after 10 years of operation and mileage over 300 000 kilometers.

Many versions of this engine with longitudinal layout have a plastic crankcase, the gasket of which starts to leak first. Then oil “presses” from under the valve cover, and if its traces are found on the timing cover, it means that the engine “ran” about half a million kilometers.

It happens and so that leaks antifreeze on the pump – the reason here lies in a solid mileage or installation of a poor-quality pump-double. With age, coolant leaks appear from under the heat exchanger.

Turbocharger

The 651-series turbo engines are equipped with one or two turbochargers. We are considering the version with a single turbine – such superchargers Daimler has long been equipped with an electric geometry change drive. Biturbo versions of the engine are equipped with turbines without “geometry” – they have only a bypass flap.

There is nothing bad to say about the turbine – if it is timely serviced, its life will easily exceed 500 000 km, and often the turbocharger survives the “knocked” motor.

The heat exchanger

Inefficient or absent oil cooling is a common cause of wear of the 651’s liners. Before entering the oil filter, oil passes through the heat exchanger, where it cools, giving off excess heat to the cooling system. For this purpose, it has 2 circuits: for circulation of antifreeze and oil.

There are hundreds of tiny channels in the cooling circuit of the OM651 engine heat exchanger, which clog very quickly and severely. Why and with what – another question, but it occurs on almost all motors.

This results in lack of oil cooling, and when the oil is very hot, it rapidly loses viscosity, as a result of which no oil film (oil wedge) is formed on the crankshaft friction pairs.

As a result of “dry” contact of the crankshaft journals with the liners, they overheat, wear and even weld on the crankshaft. As a result, the motor knocks, increased effort is required to rotate the crankshaft, and at high speeds there is a risk of welding the crankshaft to the supports or connecting rods.

The heat exchanger on this engine is recommended to change every 100 000 km of mileage. The advice has been tested in practice by many Mercedes owners – it really works. If earlier due to knocking of liners of 3 and 4 cylinders it was necessary to change several 651 motors annually, the timely replacement of the heat exchanger solved the problem.

By the way, it is easy to check it: blow the pipes of antifreeze circulation circuit – there should be no air resistance.

Transmission timing chain

The chain timing drive of the OM651 turbo engine is installed at the rear – where the flywheel is. It is combined: the lower part is realized on gears, and the upper part – on a roller chain. The chain resource is limited, although there is no single value: it is within 200-400 thousand kilometers of mileage. To assess its condition, you can use a special “stretch gauge” (part-number 651589116300), screwed in instead of the hydraulic tensioner. If the value on the ruler of the meter exceeds 82 and more, then the stretched chain must be changed. It is changed on this motor by pulling method in just a couple of hours. Everything is simple: you buy a set with a new flared chain, clamp it to the flared old one, and then pull it through. All that is left is to rivet the newly installed chain.

When replacing the chain, the hydraulic tensioner (replacement number A651050080064) is also changed – its new version has enlarged oil channels and additional oil drain holes.

When stretching the chain on the OM651 starts to rustle quite distinctly, and at high stretching it rubs on the block head until chips appear. If it breaks, it will jam between the intake camshaft gear and the lip of the cylinder head.

Fuel system

Most of the turbo engines with the index OM651 are equipped with a fuel system Delphi, but there are also versions with a Bosch system. The easiest way to distinguish them is by the control unit of the internal combustion engine. The systems are almost identical and are based on two regulators: one on the ramp (Y74), the second on the fuel injector (Y94). The regulator valve on the ramp regulates the pressures at the time of starting, as well as when the engine and fuel are cold (when its temperature does not exceed 10 °C). Fuel is heated by compressing its volume by the fuel injector, which heats not only the fuel, but also the ramp and the pump itself.

When the engine and fuel are heated, the pressure is regulated by a metering valve in the fuel injector. It turns out that when there are problems in the fuel supply system, it is easy to understand which regulator requires checking or replacing – it is enough to trace when it happens: on a cold or hot engine.

Worn out injectors are indicated by increased engine smoke – it occurs when atomizers wear out. Delphi injectors can be repaired, but often it does not give the proper effect. It also happens that the electrical circuit in the injector itself is broken. The only way out is to buy new injectors: $300 OEM from Delphi or $400 original ones with Mercedes label.

Oil pump

The oil pump of the OM651 turbo engine is two-stage and at crankshaft rotation up to 2000 rpm it produces oil pressure up to 2 bar. With faster crankshaft rotation, the oil pressure can rise to 5 bar. By default, the engine lubrication system capacity is set to maximum.

The pressure is regulated by a spool valve, which is moved by a solenoid on command from the ECU. By means of hydraulic channels, this valve is connected to another spool valve (in the manual it is called “pilot plug”), located in the oil pump. If we omit all the technical details, then everything is simple: the plunger, when moving in the pump, opens a channel for displacement of the oil pump stator.

In general, there are no complaints about the engine lubrication system. Although there are opinions about the problems arising with plungers, springs and pressure regulation. It should be noted that in the design of the OM651 turbo engine there is no oil pressure sensor, so the performance of the system is regulated by ECU by tachometer readings. It turns out that at wear of friction pairs and strong oil drop the work of the pump with lower productivity will only increase oil starvation of the motor.

Here engineers of Daimler have addressed to the sources: they have refused the function of performance regulation. If a special plug (part number A0019905317) is screwed into the cylinder block, the pump will no longer work in the economy mode – the performance of the lubrication system then remains maximum in all modes, including when the engine is stopped.

No comments have been received from Daimler, but it appears that the economy mode is not working correctly.

To be on the safe side, it is recommended to install an oil pressure sensor, not provided by engineers, as two standard sensors are of little use.

How to maintain the OM651 engine: practical advice

Turbo engine with the index OM651 turned out to be extremely delicate and demanding to maintenance. All technical fluids (oil and antifreeze) must be changed in time. The oil pump control valve should be replaced with a plug, and the pump itself should be switched to high performance mode. But then the volume of oil circulating in the system will increase and its temperature will rise, so the cooling system must be kept perfectly clean. Accordingly, the heat exchanger must be blown out quietly.

When operating this engine on vans with manual transmission, acceleration should be carried out in lower gears – no “pulling”, jerks and overloads! Otherwise, the liners will quickly wear out and will have to be preventively replaced every 150 000 km.

It is worth noting that many Sprinters with OM651 engines come in for overhaul with deformed crankshaft bed. Often the alignment is broken, ellipse appears in the main bearings – warped supports can only be thrown away.

We have listed all the fears about turbodiesel OM651. It turns out that Mercedes inline fours are far from those – they require gentle handling and only first-class service. But if not to save on maintenance, the resource of 651 turbodiesel may well exceed 500 000 km of mileage without major repairs.