Case Study - E38 - Fails To Start

Date of manufacture	April 1995
Mileage	143,000
Model	730i
Engine	M60

Symptoms

I moved the car out of the drive earlier in the day, a couple of hours later I needed to move the car back and it wouldn't start. The engine turned over fine, all the right lamps were illuminated in the instrument cluster but the engine just failed to fire at all. After a few more tries the battery went flat.

A jump start did not do the trick either, the jump leads got a bit hot but that was about it. I removed a sparkplug and found that the plug was wet and smelled of petrol, it was either flooded or there were no sparks.

Repairs made

The first thing did was to remove all the plugs, clean them, and then try one of the plugs on a coil-pack to make sure that plug was firing which it was. The spark was quite weak and the fumes from the cylinders smelt of petrol. I recharged the battery for a couple of hours and tried to start again.

The engine turned over quite fast and very smoothly as though there was no compression, the engine failed to fire and the battery soon went flat again.

The fix

This was the first time I had a flooded engine on the 7-Series. It is very easy to think that something else has gone wrong with the engine as a flooded M60 (and some M62's) just do not fire at all, not a single pop. The engine turns over very quickly so it is easy to think that the battery is good and the fault is elsewhere.

Unfortunately the M60 is prone to flooding in certain circumstances, usually it requires the battery to be a bit low on power and the car to be moved a few feet and then switched off again. In this condition the engine will not start.

I have seen this happen a few times and talked to many others that have had the same problem. I have heard of at least one case on an M62B35 but have not heard of this problem on the TUB (Vanos) variants. It is a common problem and often gets misdiagnosed as 'Nikasil wear'.

The fix is to follow this procedure to the letter, trying to take shortcuts will just make things worse!

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.