BMW M20B25 engine

The six-cylinder in-line engine M20B25 replaced the B20/23 engines on the assembly line. It was produced for 7 years (1986-93). The cylinder block was made of cast iron. The M20B25 was installed in the E30 325i, E34 525i and Z1 models. The BMW M20B25 motor had an injection system from Bosh Motronic. Oil consumption is 1000 grams per 1000 kilometers. Engine oil volume is 4.75 liters, replacement is required after 7-10 thousand km.  The unit resource is 300 thousand kilometers. Chip tuning allows to increase its potential from 500 thousand km.

M20B25 engine specifications

Advantages

The advantages of the motor are good life and excellent dynamics. On BMW E34 with M20B25 you can accelerate from 0 – to 100 to km/h in 9.5 seconds, and on the model E30 – one second faster. The engine is easily subject to tuning and you can quickly increase its volume to 2.7 liters enough to change the stroke of the crankshaft. In short, with such a power plant you can enjoy driving a German car.

Disadvantages

The M20B25 has great performance, but every product has its problems. This motor is not exempt from them. The main faults are as follows:

• Overheating. If the vehicle cooling system is defective, the cylinder head heats up quickly. Check the condition of the radiator and pump, check the thermostat and see if there are air plugs in the system;
• Wear of camshaft jaws. Usually such a fault appears after a mileage of 200-250 thousand. A characteristic sign is the knocking of the camshaft. It is necessary to purchase a new cylinder-head;
• A crack in the cylinder head. It is caused by overheating of the power unit. There is no cure. You should buy another cylinder head and you need to know how much force to pull it with.

It is necessary to check the condition of the timing belt and the position of the tensioner pulley. Otherwise, belt breakage may lead to bent valves and piston failure is not excluded. Among the malfunctions of the fuel system is a common problem: no spark and the gas pump does not pump. There may be several reasons. The main one is that the gas pump is defective. Or its relay is broken, fuse is out of order, electric motor is defective, contacts/terminals are oxidized/broken.

Everyone knows that to prevent the motor from overheating, you need a good cooling system. The simplest cooling agent is water. However, in severe frosts it will freeze and some metal parts of the car will rot due to its influence. Therefore, it is recommended to replace it with technical fluids. However, you need to know the required volume of antifreeze or tosol. Many motorists do not know it. This can be judged by their numerous questions. It is difficult to say exactly what the volume of coolant should be. It depends on the type of auto and the brand of the product. The range is 3-6 liters. You can be guided by the manufacturer’s recommendations.

Another serious problem is stalling when you step on the gas when the engine is warm. There are a lot of reasons for this. It is necessary to check the plugs with BB wires. If everything is all right with them, check the injectors. Next, check the throttle with ignition. Finally, the air filter may be clogged with dirt or the operation of the gasoline pump may be disturbed. Therefore, a diagnosis of the engine will be required. To make the engine work smoothly, and the car was easy to start from a place, you should know what candles are needed. Buy to replace the parts prescribed by the manufacturer. For each motor, specific plugs are specially selected. For M20B25 they must have a gap of 0.8. Bosch Super 4 or NGK ZGR5A are recommended.

Modifications

Two years (1986-88) the unit was produced with a catalytic converter. The engine power was 160 “horses”, torque – 215 Nm. Practically, at the same time (1985-87) the engine without a catalyst was produced. It had 9/11 units higher power/torque. For three years (1988-91), a unit with a short intake manifold came off the assembly line. Engine power – 168 “horses”, torque – 222 Nm.

CHIP TUNING

A little above it was already said that chip tuning extends the life of the motor. Also mentioned that it allows you to increase the engine displacement from 2.5 to 2.7 liters. This requires a crankshaft of M20B27/M21D24 engine with a piston stroke of 81 mm. The cylinder block needs to be milled by 2.2 mm. As a result, you can increase the power of the unit up to 15 “horses”. To get M20B28, the crankshaft of M51D25 engine with 82.8 mm piston stroke is required, and the cylinder block must be milled by 1.1 mm.

To get an extra 20 hp gain, you’ll need Schrick 284/272 sport-travel camshafts and reinforced rockers with springs. We add cold intake, porting of the cylinder-head, light flywheel, align the channels, strengthen the head bolts, install ~250 cc injectors and straight-through exhaust. The output is 220 horses! And this is not the limit of power. With the use of Chinese turbo kits you can raise this figure up to 350 hp. However, the costs will be considerable.

This engine is installed on the following models:

BMW 3-Series (E30)

BMW 5-Series (E34)