BMW E31/E32/E38 V8 ENGINE CUTS OUT WILL RESTART AND STOP

If it wasn't bad enough that I had a blocked catalytic converter and was trying to get to the exhaust shop to have new cats fitted, as I was driving there the engine stopped producing any power at all - it's one of those faults that makes your heart drop to your feet, especially as the road was extremely busy with nowhere to pull over.

Fortunately the engine picked up again - and then gave up - and then picked up again for just long enough to get the car up a side road where it fizzled out completely. The engine would start again straight away, but would then go 'pop' and stop. It didn't matter how much throttle was used - the more throttle used would just make the pops and bangs louder, but the engine would stop again just the same.

There was a smell of raw fuel around the exhausts, and the stop-start behaviour appeared to be electrical rather than a lack of fuel. Fortunately I had been involved in a repair that had similar symptoms quite recently, and that was eventually traced to a non-OEM camshaft sensor (it took a long time to work that out). So, I disconnected the camshaft sensor, and that got the car going again - not great, but going enough to drive to the exhaust shop.

Disconnecting the camshaft sensor makes the engine run in 'wasted spark' mode where the plugs fire every rotation of the crankshaft, as there is no camshaft signal, the timing information is taken from the crankshaft sensor which does not have information regarding the current camshaft cycle - this means that the plug will fire at the top of the exhaust stroke as well as at the top of the compression stroke, hence the 'wasted spark' name. It also means that the injectors will fire on each crankshaft rotation rather than on the induction stroke. This wastes fuel and reduces power - but it does mean you can get home.

If the camshaft sensor completely fails while you are on the road, the engine will stop - but it will run again when re-started in wasted spark mode. However, if the sensor produces a signal when the engine is re-started and then that signal is lost, the engine will do what mine did - go one second and then fizzle out the next.

The repair is detailed here.

All done, time for a cup of tea......