BMW M51 engine

Characteristics of the M51D25 engine

Reliability of the M51 engine

The M51 engine can not be called trouble-free. Many troubles with it are related to the fuel pump, which can be rightly called capricious and not too durable. Often fans of BMW with M51 engine have diagnostic software in use and are ready to solve problems with the fuel pump on their own.

The big plus of this engine M51 is called the low cost of spare parts and the engine itself from the dismantling. Although, every year such live engines are becoming less and less.

The disassembled engine is hard to call alive: it cranks tightly, there are signs of chips in the oil. Let’s see what happened to it. But let’s start just with the fuel injector, which often spoils the impression of the whole engine.

HPVD

The high-pressure fuel distribution pump Bosch VP20 is based on purely mechanical pumps VE, but the automatic injection advance is electronic. It is controlled by a solenoid valve. The engine load, i.e. its fuel demand, is measured by a potentiometer. One of the injectors is equipped with a needle lift sensor, which is a familiar feature for mechanical fuel injectors.

This wonderful fuel injector is considered to be the weakest point of the M51 engine. Because of it all the problems and troubles, especially if there are no specialists in the neighborhood on these ancient fuel injectors.

Its simplest and harmless caprice is fuel leakage from the seals under the “head” or from under the cover of the automatic injection advance unit.

Further, all problems concerning fuel consumption, response to gas, unstable engine operation and, of course, poor start-up both cold and hot, are related to this particular fuel injector.

If the engine is uncertain and long starts, starts and immediately fails, lost a lot of traction, as well as if the fuel pressure for injection is below the norm, then it is time to replace the bypass valve – it is a fitting coming out of the “head” of this pump. There is also an option to flush this fitting. Normally, the valve inside it should move freely and without sticking, but not play.

In the head of this fuel injector there is a MUKT – a mechanism for controlling the amount of fuel. At the heart of the mechanism is an electromagnet. Over the years, the insulation on the winding of the electromagnet crumbles, there are intercircuits, which do not allow it to work normally. As a result, the magnet lacks the strength to hold the shaft. All this leads to the fact that the engine does not shake, but frankly beats. The magnet can be revitalized by replacing the winding with a new one. You need a copper wire in insulation with an outer diameter of 1 mm and the diameter of the copper core itself is 0.75 mm. It is necessary to wind 206 turns.

You can do it easier – buy an ICCT from the dismantling. And the “head” from VW 1.9 TDI pumps from the pump VP37 will do. They are almost always in better condition than the VP20 pump. There is a difference in the “tail” of the electrical connector, so you need to bite off the wire from VW and solder to them a plug from BMW. By the way, the contacts in all connectors of the fuel injector should be clean and dry – their condition also affects the quality of the motor.

And one more weak point in the VP20 pump is the plunger pair. There is only one plunger serving 6 cylinders. It has a much shorter life here than the plunger in the VP37 pump. If the plunger is badly worn, the engine will not start well when hot, i.e. when the pump itself is hot. Watering the pump with cold water helps with starting: the plunger must cool down so that the clearances are reduced and it is possible to inject fuel in sufficient quantity and under the required pressure.

In general, if the BMW M51 engine starts to work abominably, you need to comprehensively approach the diagnosis.

Insufficient fuel supply affects engine startup. The supply is adjusted in an interesting way, which we have already told about in the video with VP37 pumps. So, the fuel supply is adjusted by sliding the MCT housing. If the car has an on-board computer, then at idle when all consumers are turned on, the fuel consumption should be 0.9 l/h (or about 5 milligrams according to the scanner). If the values are higher, it indicates that the supply is insufficient – yes, that’s the confusion (“inverse proportionality” of the actual data).

In any case, the MCT head should be moved towards the passenger compartment (towards the output connections). To do this, you need to loosen the fixing bolts and shift the cover with light blows.

Poor operation of the M51 engine can be caused by the fuel system clogging and stretching of the timing chain, which affects the fuel injection timing.

The injection advance angle is checked by the voltage at the contacts of the advance valve electrical connector (lower wire). Normally it should be 7 volts, minimum 5 volts. If the voltage is less than 5 volts, the pump should be tilted towards the motor. Such adjustment is possible after loosening the fastening nuts.

The injection advance angle can also be read by diagnostic software, it should be 2.5 degrees. The most experienced knock on the pump, adjusting its position until the engine starts to run softer.

In general, it is considered that the VP20 is considered to work in more severe conditions than the more survivable VP37. Fundamentally, they are the same, but the BMW pump is more heated by heat from the engine itself, much of which is transferred from the timing chain.

Turbine

The turbine of the M51 engine is a Garrett product. There is no controlled geometry here. The bypass flap in the hot part opens, you could say, on its own when excess air pressure pushes on the actuator.

There are no special problems with this turbine. It lasts a long time, if you do not spare money on oil and air filter.

Viscosoft

The cooling fan of the M51 engine is driven by a viscoupling. If it fails and stops rotating the impeller, the motor is threatened with overheating.

There is also a reverse situation, when the visco-muff is jammed, because of which the fan rotates constantly and synchronously with the crankshaft speed. In this case, at high speeds, the impeller is able to fly to pieces, which will hit everything around. It happens that the case ends up with through holes in the hood.

Injectors

Mechanical injection injectors are completely repairable. Over time, when the atomizers wear out, atomize fuel poorly and sometimes do not close at all, you can trust the repair with the installation of new atomizers. If you can’t find atomizers from a decent manufacturer, you can buy used injectors and install them after checking for leaks and opening pressure.

A control injector equipped with a connector wire will eventually start to leak fuel. Fuel will drip through the contacts recessed in its body: the sealing around them will simply dry out. This trouble can be eliminated by improvised methods: you need to remove the plastic wrapping and apply a harsh epoxy compound around the contacts.

After removing the nozzles should be installed on new fireproof washers.

G timing chains

There are two thick single row roller chains in the timing chain drive. They last for more than 300,000 kilometers. Their stretching affects engine starting, as the injection angle becomes later. For the time being, chain stretching can be compensated for by both pump adjustment and camshaft position adjustment.

To replace the timing chain on the M51 motor, you need to either remove the cylinder head or the oil pan, as you need to remove the front cover to gain access to the lower chain.

A special tool is used to fix the shafts and chain tensioners. A template for fixing the camshaft is important. The position of the fuel injector relative to the crankshaft is also set in a certain way, for which there are marks both on the pulleys and on the lower chain.

Cylinder Head

The cylinder head of the M51 engine is weak. At a slight overheating it breaks almost immediately. Cracks appear in it and at normal temperature regime, as it is believed, because of increased loads. The cylinder heads here started cracking after 10 years of use. Cracks appear from swirl chambers. It is not uncommon for the swirl chambers themselves to burst.

If there is a crack in the cylinder-head, the engine starts to inflate the cooling system with gases during operation. And the antifreeze itself usually does not get neither into the oil nor into the cylinders. If this happens, then from the exhaust pipe there is steam with a characteristic odor.

The cracked cylinder-head is better to replace it with a used one. Some workshops offer welding cracks. Often such repairs are quite good.

Cylinder Block

The cylinder block of the M51 engine is weak. Let’s say that it is enough for a long time when operating in standard mode. But if the owner chips the engine or allows long-term operation with a blown cooling system, cracks can appear in the block: between cylinders from the upper plane. Cracks are found after removing the cylinder head when the engine gives persistent symptoms that the cooling circuit is not good at all: from steaming from the pipe to a constantly cold “heater” (due to air lock).

BMW M51 engine modifications

1. M51D25UL (1991 – 1996) – the first version of the M51 engine with a turbine MHI TD04-11G-5, which operates at a pressure of 0.9 bar. Engine power is 115 hp at 4800 rpm and torque is 222 Nm at 1900 rpm. This engine was put in BMW 325td E36 and 525d E34.
2. M51D25OL (1991 – 1996) – more powerful variation with 143 hp at 4800 rpm, torque 260 Nm at 2200 rpm. It is distinguished by MHI TD04-11G-5 turbocharger with intercooler, standard turbine pressure is 1.1 bar. This motor was used in BMW 325tds E36 and 525tds E34.
3. M51D25TUUL (1996 – 1998) – the engine that replaced the M51D25UL. It is distinguished by Garrett TB2514 turbine and DDE 2.2 control unit. Power of this motor is 115 hp at 4800 rpm, torque is 230 Nm at 1900 rpm. The motor was intended for BMW 525td E39 and 325td E36.
4. M51D25TUOL (1996 – 2000) – the motor that replaced the M51D25OL. It is distinguished by MHI TD04-13T-4 turbocharger with intercooler. The ECU used here is DDE 2.2. The power output of the motor is 143 hp at 4800 rpm and 280 Nm of torque at 2200 rpm. This motor is found under the hood of BMW 325tds E36, 525tds E39 and 725tds E38.

Tuning the BMW M51 engine

Chip tuning

Tuning such an old motor is not a thankless business and it will be more reliable to stay away from it. But if you really need to increase the power of your M51, then it is worth looking for a tuning company that will take up the flashing of this engine. In case of successful circumstances, you can get about 30 hp on top. Going deeper is not worth it, there may be problems with the cylinder head.

MOTOR RATING: 4