The V8 M60 engine fitted to the early E31 840i's, E32 730i's and 740i's and in the 1995-1996 E38 730i's and 740i's can suffer from flooding under certain conditions. The usual cause is starting a cold car and moving it a few feet and then stopping it again. This often causes no problems, but where the battery voltage is low and the air temperature is hot the engine floods and will fail to start again. Repeated attempts to start the engine just results in a flat battery, even jump-starting the engine fails.

The reason for this is that the DME 3.3 system injects petrol in all cylinders when the ignition is switched ON, this fuel is used to aid starting. Unfortunately, if the engine is only run for a short period the over-rich fuel mixture washes away the film of oil that lubricates the cylinder walls. The next time the engine is started a number of factors will not allow the engine to fire. These are:

The charge compression is low due to the loss of seal between piston-rings and bore
The battery voltage is reduced because the alternator failed to replenish the charge from the last start
The low battery voltage means that the crank speed is low and compression is further lost
Every time the engine is cranked an additional quantity of fuel is injected making the mixture far too rich

By the time a few starting attempts have been made the inlet manifold will be swimming in fuel, the engine has low compression and the battery will be flat. There is no chance of getting the engine going in this condition. Unplugging the Fuel-Pump-Relay does not help as there is a lot of fuel still under pressure in the fuel-rail. There are quite a few ideas around of how to get the engine going again, some of these are very complicated but the one I have found to work is detailed below. It does involve a bit of work but I guarantee that the engine will start (eventually):

1) Start charging the battery, a 10A charger will take 11 hours to fully charge the 110Ah battery
2) Remove the spark-plugs from ONE BANK only
3) Squirt a few drops of engine-oil down the bores of the bank that you removed the plugs from
4) Clean the spark-plugs that were removed
5) Replace the spark plugs and coils

You must wait for the battery to be fully charged before an attempt is made at starting the engine, do not 'have a go' before as you will end up back in the same position again. Jump-leads will NOT supply a sustained current for long enough. Once the battery is fully charged, connect jump leads to the B+ terminal in the engine bay and the ground point nut. Start the donor vehicle and have a helper keep the engine speed around 2000RPM, wait 10 minutes before doing anything else, the BMW battery must be absolutely fully charged.

Right, time to start the engine! Put your foot on the throttle and push it to the floor, do not let it go! It is absolutely imperative that the car is turned-over continuously, do not try for a few seconds and give up. So, ignition ON and start cranking the engine, DO NOT LET GO OF THAT KEY! DO NOT LIFT THE THROTTLE!

What will happen is the car will fail to fire immediately, but after a while a cylinder or two will start to fire and the cranking-speed will increase, a few seconds later a couple more chime in. At some point the engine will exceed the speed of the starter-motor, but make absolutely sure the engine is going fast enough before releasing the starter. DO NOT LIFT THE THROTTLE, at this point the car will be emitting huge clouds of black smoke, this is quite normal. Keep your foot on the throttle until the revs reach 5000RPM and then just feather it to keep the revs at that speed.

What you will find is that you will need to lift your foot slowly to keep the revs at 5000RPM. Keep the revs up for around a minute and make sure the exhaust is clear before slowly lifting the throttle, making sure all the time that the engine keeps running.

It may take quite a while for the engine to keep running and may well fail to fire on all eight cylinders for 10 minutes or so, sometimes even longer. It is important that you do not stop the engine again until all 8 cylinders are firing. Allow the engine to reach full operating temperature, disconnect the jump leads and then drive for 10 minutes or so. That's it, fully running again!

Note: The same symptoms can be due to a failed crankshaft sensor. The easy way to differentiate between the two faults is to remove a sparkplug. If the plug is wet and smells of petrol the engine is flooded. If the plug is dry then the crankshaft sensor is suspect. When a crankshaft sensor fails the fuel pump does not run and the injectors do not fire.