Due to the size of these pages, I have split them over a number
pages for quicker loading:
|» This page
||Introduction, About the BMW M70 V12
|» Page 1
|» Page 2
||Fault finding, condition checking
|» Page 3
||Stripping and cleaning
|» Page 4
||Re-assmebly, Timing chain & heads
|» Page 5
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
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
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.
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
- 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
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.