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The BMW M40 is a 4-cylinder engine, fitted to the 316 and 318 after 1987. It replaced the M10 as the standard 4-cylinder engine.
Contents |
History
The M40B16 was also fitted to the E36.
1988–1995 E30 316i
1990–1994 E36 316i
The M40B18 was also fitted to the E34 and E36
1987–1995 E30 318i
1988–1994 E34 518i
1992–1993 E36 318i
Versions
| Engine | Displacement | Power | Torque | Redline | Year |
|---|---|---|---|---|---|
| M40B16 | 1.6 L (1596 cc/97 in³) | 75 kW (101 hp) @ 5500 | 143 N·m (105 ft·lbf) @ 4250 | 6200 | 1988 |
| 73 kW (98 hp) @ 5500 | 141 N·m (104 ft·lbf) @ 4250 | 6200 | 1988 | ||
| M40B18 | 1.8 L (1796 cc/109 in³) | 85 kW (114 hp) @ 5500 | 165 N·m (122 ft·lbf) @ 4250 | 6200 | 1987 |
| 83 kW (111 hp) @ 5500 | 162 N·m (119 ft·lbf) @ 4250 | 6200 | 1987 |
Components
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.
Sump
The M40 Sump is a two part design with an upper and lower sump. The upper part bolts directly to the block, sealed with a gasket, and the lower sump then bolts to the upper.
Leaks in this area are normally due to upper bolts coming loose, despite their torque rating. Because some of these bolts are only accessible with the lower sump removed, the two parts have to be disassembled to cure the leak. When removing the lower sump, don't be surprised to find a bolt sitting in the oil that has come out completely from the upper sump.
Oil Pump
The M40 oil pump is a gear-driven unit that takes its power from the crank via an intermediate drive shaft. M40 oil pumps are usually very reliable, and only need to be replaced if you have smashed your sump somehow. To remove and replace, however, means removing the sump, which is a very involved task.
Sump Gasket
The sump is sealed to the block by a gasket, which can often develop weeping leaks. Because of the nature of the M40 sump, the engine needs to be lifted to provide access to the sump, and any pressure put on the sump (by a jack underneath, for example) is likely to damage the gasket.
There are many choices of gasket available which have their pros and cons. The original cork gaskets, while cheap, are known to crack and split in a short time, which is why the material is no longer used in modern engines. Paper gaskets offer a better compression, but should be greased on both sides prior to application. Since grease and oil don't mix well, this will require a subsequent oil change in the near future. The third option is to use a silicone sealant sparingly on the face of the sump, which will provide a strong and sturdy seal for years. Loctite 518, used sparingly, requires a 2-hour curing time but will seal the sump solid.
Block
The M40 block is a cast iron design with four cylinders bored out to make the various engine capacities from 1.6 to 1.8 litres. The block can be identified by checking the number stamped on ...
In design, all M40 engine blocks are the same. In practice, two blocks exist; the 84mm bore block which forms the heart of the 316i, and the 84mm bore block which makes the 318i. Bear in mind that although the M40 engine was also fitted to the E34, engines from that car have the dipstick located in the sump. However, an aperture exists on the block, ready to be tapped, and the sump fitted to E30s bolts straight on to relocate the dipstick to its normal position.
Crankshaft
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.
It is this distance that differentiates each of the crankshafts fitted to M40 engines. The sizes are:
It is also possible to fit the crank from the M21 engine, which has the same stroke as the 325e crank (81mm). However, the M21 crank is heavier at 24.5kg compared to the 325e crank's 23kg, due to it being a forged steel rather than cast iron unit.
The main shaft of the crank is held in place with 'caps', inside which are simple steel bearings called 'shells'. 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 M40 engine.
M40 cranks have seven main bearings and four 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.
There's no perfect length for rods, but its worth paying attention to the rod ratio; the length of a connecting rod (center to center) divided by the stroke of the crankshaft. Lower rod ratio numbers are typically associated with lower rpm torque motors, while higher rod ratio numbers tend to be high revving high horsepower motors. Shorter rods are great for low-end torque, but generate a lot of sideways force which results in engine wear. Longer rods reduce these sideways forces on crank, making for a smoother engine, while also increasing the 'dwell' of the piston stroke, or time spent at the top and bottom of the cylinder. The advantage of this is increased compression and therefore higher power, offset by the amount of wasted time the piston spends sitting at the bottom of its stroke doing nothing.
The rod lengths for M40 engines are as follows:
- M40B16 - 1XXmm
- M40B18- 1XXmm
The 135mm rod weighs 640g.
M50 rods can also be fitted between M40 cranks and pistons, providing either a longer stroke (up to 145mm) or a lighter rod, allowing the engine to rev that much more freely. The M3 or 328i piston rod fitted to the M50/S50 engines are the same dimensions as M40B25 rods at 135mm, but weigh significantly less (540-575g).
Pistons
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.
Head
Three different head castings were used over the engine's production run. The earliest was #1264200 aka the 200. These were used in all E21 320/6 and 323i and E12 520/6 engines, and later in the E28 and E30 eta engines (eta = "Power With Economy"). The next version was #1277731 aka the 731. This head was the same as the 200 but featured larger intake ports. The final version was #1705885 or 885 introduced in the 325i. Ports were further enlarged, valves were larger and the combustion chamber was redesigned to improve flow and thermodynamic efficiency.
Despite the three designs, the only differences are valve head sizes, port shapes and combustion chambers. This means that certain parts such as bearings, oil seals and even camshafts are interchangeable between heads.
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 three styles of Head, the rocker cover is interchangeable across all M40 engines. It is positioned on 8 studs and fixed with M10 nuts. It has one hole for the Oil Cap, and one port for the breather hose.
When removing and replacing the rocker cover, it is worth replacing the rocker cover gasket.
Cam
An overhead camshaft drives the intake and exhaust valves through rocker arms. This camshaft is belt-driven on the M40 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.
Despite the three types of head available for the M40 engine, the camshafts are, to some degree, interchangeable. The cam bearing dimensions are identical for models 325i, 323i and 320i. The 320i cam has about 250 degrees duration whereas the 325i cam is about 258 and has about 1mm more lift at the valve.
The M40B27 "eta" cam uses 4 out of the 7 cam bearings, the 3 unused bearing housings don't have the oil feed drilled into the head from the rocker shaft. the eta cam is extremely mild and has a lot less lift.
This comes in useful for building your own 2.7 engine, where a 325i cam does indeed make more power than a 320i cam.
Valves
Two types of valves are fitted into an M40 head; Intake valves and Exhaust valves. Because the M40 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 two valves. However, there is no need to adjust the valves on the M40 engine because it uses hydraulic tappets.
Between the heads, different sized valves are used. The 316i and 318i valves are 42mm intake and 36mm exhaust
If you wish to fit larger valves to your 320i head, then obviously new larger valve seats need to be fitted, although the valve throat has plenty of meat to reshape to the new seat and the port can be opened up to 325i sizes and a few mm beyond if you wish.
Rocker Arms
The valves are opened and closed by rocker arms, which are pressed on by the cam. 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.
Tappets
The M40 engine uses hydraulic tappets
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. This is especially common on heads fitted to the 325i.
Learn more about changing the head gasket.
Breather Hose
To balance the pressure inside the head, a rubber hose runs from the top of the rocker cover to the throttle body.
Ancilliaries
Flywheel
The M40 flywheel is a single mass wheel which 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.
On early Jetronic engines, the flywheel also has lugs which are used by the crank position sensor to detect engine speed. These earlier engines also have different ring gears, meaning a Jetronic flywheel needs a Jetronic starter motor.
Two weights of flywheel were fitted to M40 engines. The standard flywheel is 8.4kg, while cars fitted with air conditioning have a flywheel 1.3kg lighter.
Lightened flywheels will result in faster acceleration, at the cost of fuel economy when cruising. The absolute lowest weight an M40 flywheel can be lightened to is 5.8kg, although for safety reasons it is recommended to keep the weight over 6kg.
Starter Motor
The starter motor is a standard 1.4kW 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.
Two starters were fitted to the M40 engine, for pre- and facelift vehicles, and are not interchangeable; they must be matched to the appropriate flywheel.
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 M40 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 M40-equipped E30s; however, if you are taking an engine from an E34, you will need E30 engine mounts.
Oil
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 Cooler
On all models fitted with the M40B25 engine (325i and Convertible) there is an external oil cooler fitted, and mounted under the normal engine radiator. It receives cool air through vents in the front valance.
Cooling
Main article: Cooling
The M40 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 is controlled by the ECU based on readings from the temperature sensor. A secondary temperature sensor is fitted to drive the temperature gauge in the cabin.
The coolant is a mixture of anti-freeze and water, and should be replaced regularly. Coolant is drained at the drain plug.
All coolant components are interchangeable between all M40 engines of the same period. Pre-facelift components are not always interchangeable with their facelift counterparts.
Water Pump
Main article: M40 Water Pumps
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 M40 engines, depending on the age of the engine, and are distinguished by the number of pipe outlets fitted.
Thermostat
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 ECU opens the thermostat to allow coolant to flow to the radiator.
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 M40 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
For facelift vehicles, an external tank was mounted to the inner passenger 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.
Fan
The cooling fan fitted to all M40 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.
A second electrical fan was fitted to vehicles with air conditioning.
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: 11531287651
- Thermostat to rad hose: 11531722216
- Bottom rad hose: 11531718980
- Thermostat to bottom rad hose: 11531718982
- Thermostat to heater and expansion tank hose: 11531722743
- Throttle body hose: 13541719966
- Block to throttle body hose: 13541719967
- Block to heater hose: 64211380527
Drain Plug
To properly drain the coolant from the M40, it is not advisable to simply remove bottom hose. Because of the complex design of the M40 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 M40 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.
Servicing
Main article: Basic M40 Servicing
Every engine needs its fluids and filters changed regularly. The M40 engine also needs its timing belt replaced at regular intervals. Learn more about changing the timing belt.
Common Problems
Won't Rev Over 5000
Unfortunately, this is a common sign of engine wear, and indicates a worn out cam shaft. The problem can be diagnosed in two ways. If the engine sound can be described as "sounding like a skeleton wanking in a dustbin" then your cam is very likely worn.
To confirm, remove the rocker cover and baffle plate so that you have access to the cam shaft, and run your finger nail over the cam lobes. If you can feel scatches, grooves or other imperfections, the cam is in bad condition.
The only solution is to replace.