The cars of the German concern are known to absolutely all car lovers all over the world. The most attractive thing about these cars is their power units and bold German solution, which is far ahead of its time. The M51D25 engine was developed to replace the M21 engine. The displacement of this series was 2.5 liters, and it first saw the light of day in 1991.

The previous range of power units was ambiguously received by the public, and they were recognized as underpowered. But the characteristics of BMW M51D25 were characterized by greater dynamism, increased power and enviable economy. A total of 4 modifications of this power unit were produced.

The M51D25 TDS was an inline six-cylinder engine with a cylinder block made entirely of cast iron. The cylinder head was made of aluminum. The camshaft in the cylinder head was a single camshaft.

The valve train was equipped with hydrocompensators, which completely eliminated the need for service to adjust the clearances. The timing mechanism together with the fuel injector was driven by a chain transmission. The high-pressure fuel pump is electronic, regardless of the modification of the power unit. The camshaft served as a place for the installation of a vacuum pump. Along the cylinder head, the air filter housing was located along the entire length of the motor, giving the motor an unrivaled appearance.

M51D25 engine specifications

Disadvantages

The M51D25 TDS makes a rustling noise on touchdown if it overheats, and it warms up quite often. It is necessary to see if the coolant is bubbling in the reservoir. A frequent cause of such a breakdown is a crack in the cylinder head. The size of the head bolts allows you to weld the crack, but for greater reliability it is better to buy a new head.

A sufficiently warmed up engine may start badly and the operating speed may start to float. First, you need to find out where the speed sensor is located on the engine and check it, but the problem may also lie in the high-pressure fuel pump. The solution is to buy a new pump or replace the plunger pair.

The turbine serves for quite a long time – about 200 thousand, and sometimes more. After passing this number of kilometers, the supercharger begins to throw oil, and the car stops gaining speed. There is only one way out of this situation – replacement of the turbocharger.

In general, the motor does not cause problems to its owners and is characterized by a great resource, but due to the rather large age and the use of low-quality fuel, as well as maintenance of insufficient quality, various problems can occur.

Description

The first production car to be equipped with this power unit was the BMW 325td. In 1991, this motor completely displaced the M21 diesel engine from production. The cylinder block is completely made of cast iron, while their diameter is 8 centimeters, which is quite a lot. The block was equipped with a forged crankshaft, providing almost 83 millimeters of piston stroke, while the connecting rods were 13 centimeters long.

An aluminum cylinder head with a single shaft and a pair of valves per cylinder was used to close the cylinder block. The intake valves were 3.6 centimeters in diameter, with the exhaust valves being slightly smaller at 3.1 centimeters. The valve stems were 6 millimeters in diameter.

The camshaft is driven by a single-row chain. Reviews state that with proper care and normal operation, the timing chain is capable of working for more than 400,000 kilometers.

The Bosch VE pump was used as a high-pressure fuel pump. The engine was equipped with a turbine. The versions with the 25td index had a MHI TD04-11G-5 supercharger. Motors with index 25tds and vs had the same turbine, which was additionally equipped with an intercooler to cool the air flow.

Thanks to the installation of the EGR system, which allows the recirculation of exhaust gases, the engine has a reduced toxicity of the exhaust. The Bosch DDE2.1 electronic engine control unit is used for control.

The last motor was installed by the German concern in 2000, after which it was replaced by a new powerplant with the index M57.

Modifications

A total of 4 modifications of this power unit were produced, which were used for installation on various models of cars of the German concern BMW. The modifications were as follows:

• M51D25UL – power unit was produced from 1991 to 1996. The power developed by this power unit is 115 horsepower with 222 Nm of torque. Such indicators were achieved thanks to the installation of a turbine MHI TD04-11G-5, capable of inflating up to 0.9 bar.
• The M51D25OL is a motor that was produced from 1991 to 1996. It is a slightly modified version of the previous power unit, which is capable of developing up to 143 horsepower with a torque of 260 Nm. The same turbocharger is installed here, but it works in tandem with an intercooler, which increased the standard operating pressure to 1.1 bar.
• M51D25TUUL – engine, which was produced from 1996 to 1998 as a replacement for the modification with UL index. The power of the power unit at 230 Nm of torque was about 115 horsepower. But this motor was installed Garrett TB2514 turbine, and to control all this was used DDE 2.2.
• The M51D25TUOL is a power unit that completely replaced the OL modification, and was produced from 1996 to 2000. The same control unit was used to control the engine as on the TUUL. Power output was 143 horsepower with 280 Nm of torque, thanks to the MHI TD04-13T-4 turbine, equipped with an intercooler, this power unit is excellent from the ground.

Tuning

Tuning of this old power unit is a controversial matter. The safest thing to do is to leave everything stock and drive safely. But if you really want to increase the power of the power unit, you can try to find a company that will replace the software of the control unit. Chip-tuning will allow you to get an increase of about 30 horsepower. It is not worth tuning further, as it may lead to problems with the cylinder head.

This engine is installed on the following models:

BMW 3-Series (E36)

BMW 5-Series (E39)

BMW 5-Series (E34)

BMW 7-Series (E38)