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The M21 is an straight-6 Diesel engine of BMW, whose 12-valve, belt-driven SOHC design in three parts, featuring a main Block with a Sump at the bottom and a Head at the top. It is directly related to the M20 engine.

The M21 was the only diesel engine fitted to the E30, with a single capacity of 2.4 litres. However, there are two versions; with and without turbo.

M21D24 non-turbo engine


The M21 was first built in 1983, but it wasn't until 1985 that it saw production use in a BMW. For its first few years, it was trial-fitted to an American car, the Lincoln Continental, before being included into the E28 and E30 engine range.

With a design heavily borrowing from the "baby six" M20 petrol engine, the diesel M21 was BMW's first foray into non-petrol fuels, and the relatively low application of the technology showed how little BMW's faith was in the power plant. For this reason, it went through very little development during its ten-year production period, and yielded only two variants with a single capacity; a 2443cc unit, with or without turbo.

FOr the E30, two distinct models were badged, available as a 4-door Saloon or Touring. The M21 powered the 324d from 1985 to 1990, and the 324td from 1987 to 1993, for the lifetime of the E30 Touring. No diesel Cabriolet was offered.

Despite the length of time for which the diesel was offered, very few units were built, relative to the entire production of the E30. The engine was fairly popular in mainland Europe and the USA, but was never officially sold in Great Britain, which is why very few of these cars are seen on the road today.


Engine Displacement Power Torque Redline Year
M21D24 2.0 L (2443 cc/149 in³) 63 kW (84 hp) @ 4600 152 N·m (112 ft·lbf) @ 2500 5150 1983
85 kW (114 hp) @ 4800 220 N·m (163 ft·lbf) @ 2400 5350 1987


The engine in all E30s is a three-part design, featuring a main Block with a Sump at the bottom and a Head at the top.


The lowest part of the M21 engine is the Sump, an aluminium dish that provides an oil bath for the crankshaft, as well as feeding the oil pump. Oil is drained through a single bolt,

Almost all M21 sumps are interchangeable between engines, except for that fitted to the 325iX.

Removing the sump on an M21 in situ is one of the most irksome chores on these engines. You can lift the engine out to get better access, but this requires dismantling a large portion of the engine ancillaries. However, you can attempt the following method:

Drive the car up onto ramps, and chock the rear wheels. Now remove the top radiator bracket, and unbolt both ends of the each rubber engine mounting block. Jack up the engine evenly using blocks of wood under each mounting arm, and remove the rubber engine mounting blocks when they become free. Lower the arms back onto small pieces of wood cut from standard 100mm fence posts placed sideways where the rubber blocks had been, making sure that the radiator moves up and down with the engine. Remove the steering column universal joint clamp bolt at the rack to allow it to slide off the rack. Disconnect one steering ball joint at the wheel hub (this may be difficult with the car on ramps as the wheel has to swivel). Unbolt the two steering rack retaining bolts, prise down one of the flanges slightly and lower the rack to hang loosely. Unbolt the earth strap and oil level sender from the nearside upper sump, under the alternator. Remove the sump bolts - including those hidden by the engine to gearbox shroud. Finally, rotate the front of the sump diagonally forward and down to allow its shallow rear section to clear the oil pump pick-up. Refitting, like always, is the reverse of removal.


The M21 block is a cast iron design with six cylinders. All M21 blocks have an 80mm bore, just like the M20B20 petrol engine.


While the block determines the maximum capacity for the engine, the crankshaft determines the stroke of the engine, and therefore the actual displacement. Mounted to the bottom of the block, the crank holds the bottom of the piston rods and converts their up/down motion into rotary force known as torque. It does this by holding the rods on a series of lobes or 'throws' which extend away from the centre of the crank by a certain distance.

The M21 crank shaft has a stroke of 81mm.

The crankshaft is arguably the most famous component of the M21, since it shares the same throw as the M20B25 engine fitted to the 325i. However, since diesel engines are built much more ruggedly, the crankshaft is made of forged steel. For that purpose it is a very desirable part for those fitting a turbo to their own engine.

The main shaft of the crank is held in place with 'caps', inside which are simple bearings called 'shells', made of steel-backed white metal. The caps and shells fit around the shaft at fixed points called 'journals'. From the journals, arms known as 'webs' extend outwards to hold the 'throws', while the throws themselves connect to the piston rod or con rod.

Both ends of the crank shaft protrude through the block. The front end carries a number of pulleys to control engine timing and to power engine ancillaries, while the rear holds a wheel to bolt to the flywheel. To prevent oil leaking through the block, an oil seal is fitted at either end of the crank inside each end main bearing. These oil seals rarely fail on the M21 engine.

M21 cranks have seven main bearings and six piston rod bearings. The main bearing bolts can be re-used, but the piston rod bolts ('big end' bolts) are stretch bolts and must be replaced.

Before fitting a used crank shaft, check it for cracks. This can be done by lifting the crank off the ground and striking each one of the 12 counter weights with a large wrench (19 mm or larger). It should ring like a bell. If it doesn’t - check the rod journal nearest the dead counter weight and look for a crack. A dead ring indicates a junk crankshaft.

Crank Bolt

The crank bolt is a 22mm bolt that holds the crank in place. This bolt is best slackened with the engine still in the car:

Remove the radiator and the lower splash panel to give access. Remove either fuse 11 (fuel pump) or the H/T lead to the coil. Place a suitable lump of 4x2 timber on the floor, where the splash panel was. 22mm socket and the biggest breaker bar that will fit in the bay, with its end resting on the block of timber. FLICK the ign switch, until the starter has freed the bolt.

Do not let anyone stand close to the car, do not attempt this with the car in gear, do not attempt this with another car close by. When you flick the ign switch, the car WILL leap into the air; it's the weight of the car coming back down again that loosens the bolt.

Piston Rods

The piston rods connect the pistons to the crank. At the top or 'small end', the piston is held in place with a gudgeon pin, while at the bottom or 'big end', rod caps bolt around the cap.

Only one length of rod was fitted to the M21 engine; 130mm


M21 Pistons
M21D24 22:1 M21D24 22:1
85-90 87-93
Ø: 80 80
KH: 42.4 46.4
MT: 3.13 2
GL: 62.4 76.3
BO: 22.0 x 54 26.0 x 69
Stroke: 81 81
Conrod Length: 130 130
Volume 2443 2443
Piston Shape: Pictures 12 813842.jpg Pictures 12 813842.jpg
Measurements (mm) Dimensions mahle.jpg
Definitions Ø: Diameter of piston KH: Distance from center of wristpin bore to top of piston top ÜH: Height of piston crown over piston top plane VT: Depth of depression in piston crown MT: Depth of depression in piston top GL: Total length of piston BO: Bore (and length) of wristpin

Crank Pulley

The crank pulley mounts to a hub at the end of the crank and is held in place with six 13mm bolts. The crank pulley is the main driving wheel for the timing belt, and does not need to be removed if changing the belt.


A single head design was used for both turbo and non-turbo versions of the M21. This aluminium core featured a design very similar to the M20 unit.

It is not uncommon for these heads to develop cracks between the valve ports, although this does not render the head useless. Modern welding techniques mean that the head can be repaired if necessary, and while the cost is currently very expensive, it is still cheaper than trying to source a new head. The problem of cracking is so prevalent that it is now very hard to find a used head that does not have at least one crack.

The head is held in place with stretch bolts, which MUST be replaced after removing.

When skimming the head, look for dimples in two corners of the head face. These are your depth gauges, and the head can be skimmed until these dimples disappear. Any further skimming weakens the head too much, rendering it scrap.

Rocker Cover

Despite the two types of engine, the rocker cover is interchangeable between the versions. It is positioned on 8 studs and fixed with M10 nuts. It has one hole for the Oil Cap, one port for the breather hose, and a final port on its top for the vacuum pump.

When removing and replacing the rocker cover, it is worth replacing the rocker cover gasket.


An overhead camshaft drives the intake and exhaust valves through rocker arms. This camshaft is belt-driven on the M21 engine. The cam is held in place using up to 7 bearings. The end of the cam protrudes from the engine to drive the distributor.

The cam has the following specifications:

Lift: 6.580mm Duration: 249° (intake) 260° (exhaust) LCA: 113.50° (intake) / 112.00° (exhaust)

The cam is also responsible for driving the vacuum pump, mounted inside the head.

Vacuum Pump

Like many diesel engines, a vacuum pump is fitted to create vacuum force to drive the brakes. This is because, unlike petrol engines, there is very little negative pressure in the engine head, since diesels do not "suck" air in the way that petrol engines do.

The M21 vacuum pump sits to the rear of the head (close to the bulkhead), and its shape is distinctive underneath the rocker cover. A single pipe exits the pump through the rocker cover, primarily to power the brake servo.


Two types of valves are fitted into an M21 head; Intake valves and Exhaust valves. Because the M21 is an interference engine, this means that the valves protrude into the cylinder when opened; for this reason, any problem with the timing belt will automatically mean damage to at least three valves. The valves are adjusted at the rocker arms, and should be set to 0.3mm when cold.

The diesel valves are a specific size, and are becoming increasingly hard to source. For reference, intake valves are 35mm intake and the exhaust are 31mm.

Rocker Arms

The valves are opened and closed by rocker arms, which are pressed on by the cam. These rocker arms perform a dual function of cam follower valve adjuster, as one end contains the eccentric and the adjustment nut. The rocker arms pivot around rocker arm shafts.

If you have broken a rocker arm, they can be easily replaced by removing the rocker cover. If, however, you are suffering from repeated broken rocker arms, it is more likely that you have bent a valve and will need to remove the head.

To remove a rocker arm, you must remove the rocker arm shaft. This is possible with the head still in place, but may require lifting of the engine to allow the shaft to exit the engine through the front.

To remove the rocker arm shaft, remove all the adjuster eccentric discs and all the clips locating the rockers. Push any rockers that are loose sideways to clear the valves, turn the engine until the remaining rockers come loose (i.e. on the heel of the cam lobe) and push them sideways as well. Rocker shaft should now slide out easily, as long as you've removed the bar that locks both shafts in place at the front of the engine. If you have already removed the timing belt, then ensure that no pistons are at TDC (put crank at 60 degrees past TDC mark) and turn the camshaft. When you put things back together, set the cam mark at 30 degrees past the mark, put the belt on, turn the engine carefully back to TDC, and check the belt isn't a tooth out.

Head Gasket

The Head gasket forms a seal between the Head and the Block, maintaining compression in the cylinder while keeping the Oil and coolant separate. Any failure in the head gasket will cause a drop in engine power, and will also lead to mixing of the oil and coolant, known as mayonnaise.

To diagnose a broken Head gasket, look for a creamy substance underneath the oil filler cap. If present, remove your dipstick and look at the oil. If it resembles milky coffee, then it is very likely your head gasket has failed.

In the event of a failed gasket, it is important to check the condition of the head; more severe damage may have been done, including cracking of the metal which will render the head useless.

Breather Hose

To balance the pressure inside the head, a rubber hose runs from the top of the rocker cover to the crank case. On turbo models, a second hose also runs to the air filter box.

Glow Plugs

Since diesels create combustion using heat and pressure rather than Ignition, there is no need for spark plugs. Instead, the latent heat of the engine in operation is enough to detonate the diesel fuel once it is compressed.

However, when starting from cold there is not enough heat in the system to initiate those first few explosions, so electric heaters known as glow plugs are fitted. These fixed-point heaters rapidly heat up the fuel in the cylinder so that it will detonate when compressed. Six glow plugs are fitted to the M21; one for each cylinder.

The glow plugs are circuit-controlled, connected to the thermostat and coolant temperature sensor. When the coolant reaches a level of 60°C (140°F) the glow plugs will shut off.

Diesel models will have extra lights fitted to their instruments to inform the driver of the state of the glow plugs. One is green, the other orange. The orange light stays on when the glow plugs are heating, and after they click off the green light should turn on to tell you that the engine is ready to be started. If the green light does not turn on you probably have a dead glow plug. Replacing the glow plugs will usually make the green light work again.



Main article: Forced Induction

Only one E30 model was fitted with a turbocharger; the 324td. However, identical components were used on the E28 524td.

The turbo is a standard forced-induction unit, whereby exhaust gases are used to drive a turbine, whose subsequent spinning forces more air through the Intake side of the engine. This increase in air flow allows more fuel to be added, thus providing more power.

The turbo itself was a variant of the standard T3 design including an internal wastegate, and was made by AiResearch (Garrett). It was had a connection to the main block for lubrication, and was connected to an intercooler to reduce the temperature of air on the intake side.

One of the most common reasons for non-functioning turbos relates to the EGR valve.


The Exhaust Gas Recirculation valve, or EGR, is a polution control device. Its purpose is to scavenge a portion of the exhaust gases and return them to the cylinders for re-use.

The EGR is sits right underneath the turbo, on the manifold. It has a pipe that runs from there to the intake pipe. Also from the EGR is a vacuum tube that runs to something called the aneroid diaphragm, which is right behind the driver's side strut tower. From there the pipework goes to a solenoid on the firewall, and from there more pipes run elsewhere, all of which are unnecessary once you remove the EGR.

The EGR is an extremely problematic system, since it blows all your exhaust right back into the intake, thus mixing soot with a little bit of oil to create a sludgey concoction that lines the intake ports and the top of the valves, ultimately reducing power. It is a very common modification to remove the EGR, blank off its port and to re-route the associated plumbing so seal the leaks.


The M21 flywheel, like all flywheels, keeps the engine spinning long after you take your foot off the throttle. It is also the flat surface to which the clutch mates, ultimately transferring power from the engine to the Drivetrain. The edge of the flywheel is toothed. Known as the ring gear, it is what allows the starter motor to bite onto the engine to start it. The flywheel bots to a pulley at the end of the crank.

Two weights of flywheel were fitted to M21 engines. The standard single mass flywheel is 12.1kg, fitted to to the 324d and early 324td models. Later turbo models were fitted with a slightly lighter dual-mass unit of 11.9kg. This latter flywheel was also fitted to diesel versions of the E34 fitted with the later M51 diesel engine. It is designed to be used in conjunction with a pulse generator, which sends a signal to the control unit for the glow plugs.

Starter Motor

The starter motor is a standard 2.2kW motor with a bendix engagement wheel. It is an all-in-one unit containing motor, solenoid and relay. When activated, the starter gear extends to meet the flywheel and then spins, driving the engine.

The starter is wired directly to the Battery + terminal and the alternator. A smaller connection, fed by a black/yellow wire, is the incoming signal from the ignition switch. On facelift vehicles there is also a black/green wire to load reduction relays.

To test your starter while mounted in the car, bridge pins 11 and 14 of your diagnostic plug. This will bypass any wiring in the cabin of the car, and should make the starter spin freely. If you want to test the car engine, turn the ignition circuits on with your key, and then bridge the same pins. Your engine should start.

It is very rare for M21 starter motors to fail. If you are having problems starting your car, it is recommended to look at the common problems of E30s before removing the starter motor.

Engine Mounts

The engine mounts are standard on all M21-equipped E30s; however, if you are taking an engine from an E28, you will need E30 engine mounts.


Main article: Diesel

Unlike all the petrol engines fitted to the E30, the M21 has an exclusive system that cannot be covered in the normal fuel section of this Wiki. However, a number of components are shared; the system still revolves around pressurised fuel, collected from a tank and delivered to injectors by way of a pump. It is only the technicalities that make the system so different.

Fuel Pump

The diesel pump was available in two flavours. The earlier system fitted to the 324d was cable-driven and directly controlled by the accelerator pedal. This was replaced by a later fly-by-wire system for the 324td that read data from a throttle pedal sensor. Both of these systems employed a belt-driven pump with a DDE ECU to control injector cycling, with the later system employing a second DDE to read the throttle data. The pumps on these two systems are clearly different, and not interchangeable.


The injectors work like all injectors; fuel is pressurised by the fuel system and delivered to the injector. When required, the injector pulses open, atomising the fuel into the cylinder. The specifics for diesel engines is that each injector connects directly to the pump, unlike petrol engines (which employ a fuel rail). For the two types of pumps, there are two types of injector.

Number 4 injector also contains a transmitter sensor, known as the needle lift sensor.

There are two main parts of the injectors that wear. The spring loses tension over its life. This tension loss causes the nozzle to open at a lower pressure than is optimal which results in bad fuel atomization and an increase in fuel delivery to the cylinder. The second piece that wears are the nozzles. The mating surfaces of the nozzles get pounded continuously which results in a less than optimum sealing surface and an enlarging of the orifice. This enlargement results in a richer than optimal condition which increases fuelling and reduces economy. Both of these wear parts wear at different rates between injectors and can cause one injector to be very rich compared to the others. This rich cylinder(s) causes the black smoke that may be present on starting, during idling and under load.


Main article: Oil

The system is lubricated by a pressurised oil system that oils each moving part of the engine. The oil is circulated from top to bottom and around, and the pressure monitored to ensure that no leaks occur. The driver is notified of this by a warning light of the dash cluster.

Oil Pressure Switch

The oil pressure switch is a basic screw-in plug that drives the Oil Pressure Light on the dash cluster. A single wire runs from the end to the main Loom. If removed, you will see that the switch is little more than a hollow bolt; inside that hole is a diaphragm. When the oil pressure is high enough, the diaphragm flexes upwards and closes a switch, which turns off the Oil Pressure Light. A drop in pressure will therefore break the switch, and make the light glow.

Oil Cooler

Both the 324d and 324td have an external oil cooler fitted and mounted under the normal engine radiator. It receives cool air through vents in the front valance. However, with modern semi-synthetic oils, the oil cooler is rather redundant, despite the increased temperature that diesel engines operate at.


Main article: Cooling

The M21 block is filled with channels for a liquid coolant. These channels are known as the water jacket, and are designed to allow coolant to flow from the the bottom to the top of the engine and out. Pressurised by a water pump, the coolant flows out to the radiator where it is cooled by air flow through the grille. In the case of high engine temperature, extra cooling is provided by a viscous fan mounted behind the radiator.

Coolant flow is regulated by the thermostat, which senses local temperature and opens and closes the supply of coolant accordingly.

The coolant is a mixture of anti-freeze and water, and should be replaced regularly. Coolant is drained at the drain plug.

Water Pump

The cooling system is pressurised by the water pump, which is belt-driven from the crank pulley.

There are two types of water pump fitted to M21 engines, depending on the age of the engine, and are distinguished by the number of pipe outlets fitted.


Engines need to be at the right temperature to run at optimum conditions, neither too hot or too cold. For that reason a thermostat is fitted to limit the flow of coolant around the system, allowing it to be warmed by the engine. When the temperature is correct, the thermostat opens to allow coolant to flow to the radiator.


Radiators, along with water pumps, are specific to pre- and facelift cars. Later models had a secondary outlet pipe to the expansion tank, and therefore you cannot swap radiators between newer and older vehicles.

The M21 radiator is an aluminium core with plastic side tanks, and is bolted to the front valance of the vehicle. A plastic 'shroud' guides air flow around the rear of the radiator, as well as channelling air blown in by the fan.

Some radiators, (especially ones that have been replaced), have a boss about 2/3 of the way up on the right hand end. This is where the electric fan switch for aircon cars is fitted. there are also spare relay and fuse sockets in the standard fusebox for the purpose.

Expansion Tank

An external tank was mounted to the inner wing, to allow hot coolant room to expand. Steam was vented from the system by a drainage tube tucked through a hole in the inner wing, while the tank itself was fitted with a cap to allow fresh coolant to be poured into the system. On Facelift cars, this tank is on the passenger wing, while pre-facelift cars have the expansion tank fitted by the battery.


The cooling fan fitted to all M21 engines is driven by the water pump, and uses a viscous clutch to regulate its speed. This is a known weak spot of the engine, with a failing clutch causing the engine temperature to rise dramatically when the car is left to idle.

To test the fan, the Zone recommended the 'newspaper test'. This involves rolling up a newspaper and poking the fan with it with the engine running. A good clutch will shred the paper, while a broken or failing clutch will let the newspaper stop the fan from spinning, The only solution is a replacement.

Rubber Hoses

All the elements of the cooling system are linked with a series of rubber hoses. The seven hoses that make up the cooling system are as follows:

  • Pump to thermostat hose: 11531275398
  • Thermostat to rad hose: 11532241078
  • Thermostat to heater and expansion tank hose: 11532241836
  • Block to radiator hose: 11532241898
  • Block to heater hose: 64211386425

Drain Plug

To properly drain the coolant from the M21, it is not advisable to simply remove bottom hose. Because of the complex design of the M21 water jacket, there will still be pockets of coolant left in the block and, if the system is being flushed, dirt and grime will still be trapped inside.

Because of this, a bolt is fitted to all M21 blocks to properly drain the water jacket. This bolt is located below the exhaust manifold to the rear of the engine, and is best undone with a 19mm ring spanner.

Common Problems

See Also