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About the BMW M70 V12

The BMW V12 engine is fitted to a few of the company's flagship products, such as the 750 (e32) and 850 (e31). The 750 is an automatic only so may require a replacement flywheel to be utilized. It is a single overhead cam engine with one cam per bank of six cylinders. Generating over 300BHP in standard form, it is controlled by a complex BOSCH engine management system called Motronic 1-3. The 850 engine has a different cam profile and so generates more power. It is a more preferable engine, but ultimately much more difficult to obtain due to the fact that the cars are sought after in BMW enthusiast circles.

A feature of the engine control system is that it is drive-by-wire. No throttle cables are utilized on the M70 circa 1990. A potentiometer connected to the accelerator pedal is weighted to give the same feel as a standard cable set up. This signal from the accelerator is fed into the primary engine management computer known as the EML. This then feeds two slave computers (Known as DME's) that control each banks ignition timing and injector sequencing. This complicated engine system results in the engine requiring two of everything. Two air flow sensors are used, two oxygen sensors, two fuel pumps, two crankshaft sensors, two throttle bodies (known as DKM's) and so on. A feature of this system is that if one of the two DME computers should fail, the engine can continue in what is called 'limp home mode'. In this mode only one bank of six cylinders run, and the EML signals the transmission to select third gear only.

The EML limits the engine rpm's and total vehicle speed to 155mph, but this can be removed by the addition of third party software. The RPM limit is only raised by 500rpm. Performance enhancements can also be made by replacing the EPROM chips in the two DME's, although with 300BHP in a light weight kit car, you need to ask yourself if its entirely necessary to either boost power output, or remove the upper speed limit unless you are deciding to use it for track days.

All of the computers are linked via a common data bus. This bus carries signals to all the other computers in other parts of the car, such as the traction control and air-conditioning systems. It allows the transmission control computer to retard engine timing during a gear change to make the transition smoother by limiting engine power output. It also allows engine condition measurements such as MPG and instantaneous fuel consumption levels. Engine fault conditions are indicated on the dashboard by the EML light. If you are looking to purchase a donor car, ensure that the light comes on when you turn the ignition key, and goes out once the engine is running. A common trick to hide engine problems when selling this type of car is to remove this bulb. If the light doesn't go out, get a fault code reader on it before deciding to buy it or not.

A useful feature of the engine is the inlet headers. As these are identical, the throttle bodies can mount to either end. A blanking plate is installed at the other end to seal the system. This is very useful in my application, as they would need to be the other side of the engine positioned over the gearbox to function correctly.


Common Faults
The M70 is a solid engine, and with regular servicing, it should last many hundreds of thousands miles. However, the BMW cars these engines are fitted to are complicated and require expensive servicing to keep running properly. Many cars, when they reach the price levels suitable for becoming a donor car, have a less than desirable recent service history, so some common faults to look for are:

• Check engine light as indicated above. Even if you are not re-using the engine management system, the EML light is a good indication of basic engine health.
• Camshaft oil feeders are secured by banjo bolts very similar to those used on fuel rails. These can come loose and the result is worn cam lobes. A simple fix is to 'wire' them so that they cannot undo. Worst case is camshaft and hydraulic tappet replacement.
• Noisy timing chain. On high mileage engines these can stretch, replacement of the chain and tensioner cures any further problems.
• Electronic throttle bodies. The motors driving them become worn, causing erratic idle and poor response. These parts are hugely expensive if you can find them at all. The good news, is that they are easy to dismantle, clean and overhaul if you are careful.
• HT leads. These break down eventually as with any engine. Problems manifest themselves as miss-fires at high RPM's and general poor running. Caps and rotor arms should be changed at the same time. The cap and rotor are identical to those used on the BMW 325, so replacements are cheap and easily available.

The blocks are manufactured using a special alloy called Nikasil. It is extremely unlikely to suffer and wear on the block in a well maintained engine, but if wear has occurred to a large extent, its going to require expensive parts. When replacing rings, the most normally required is to scour the bores clean with a scotch pad. Obviously if you are going to the extent of changing the rings, it makes sense to do the big end bearings at the same time. Some enthusiasts I have spoken to have recommended against touching the main bearings at all, although I take this advice with a pinch of salt.


Note: Thanks to Neil for pointing out an error here. It should be noted that the block is actually aluminium. It is just the cylinder bores that are treated with Nikasil, A surface hardening treatment. It is this surface that if damaged, is extremely expensive to repair.

A modification to read the fault codes from the engine management system can be performed by modifying the dashboard if you are missing a fault code reader, using what's called the 'stamp down' procedure. This procedure may not work on european sourced vehicles. Details can be found on the e32 register forum.