Why is my BMW’s stop-start system stopping and starting randomly?

Khodrocar - The stop-start system on my 2009 BMW 320i M Sport seems to have a mind of its own. Sometimes it works as it should; more often than not, however, when I stop at a junction the engine will stop but then restart. Other times I can sit with the handbrake on, in neutral and the engine keeps running. Any ideas what the fault might be? And, having read about the timing chain issues with my car’s N43 engine, what equivalent Audi would you recommend I trade the BMW in for?

– DT

Dear DT,
Stop-start issues can often be caused by an underlying fault. For example, if your battery is getting weak, the car will keep the engine running for longer because it can sense that there isn’t enough power to run the ancillaries – things like the air-conditioning, radio and so on – while the engine is switched off.

That might be the issue here, but it doesn’t explain why the car is turning itself off while the clutch pedal is depressed.
Stop-start systems require a number of variables to be correct in order to turn off the engine to save fuel and reduce emissions. They require the speed to be zero (or as near as makes no difference), and either the handbrake or brakes to be on (in the case of an automatic car) or the clutch to be depressed (in the case of a manual car like yours).

In the latter case, they’ll use the clutch position sensor to detect whether you’ve got your foot on the clutch pedal or not. Release the clutch while the speed is zero, and hey presto, the stop-start system will cut in (and the engine will cut out). Engage the clutch again, and the engine starts once more. Simple.

If your car’s clutch position sensor is faulty, though, the car might be thinking the clutch is depressed when it isn’t – and vice versa. This won’t be an issue while you’re moving, because the speed won’t be zero, so not all of the conditions are met. But when you’re stationary, it might be that the car doesn’t realise you’ve put your foot on the clutch.

If you decide to trade in your BMW, the equivalent would be an Audi A4 with a 1.8-litre petrol turbo engine. But beware, because these can also suffer with stretched timing chains – and I suspect you’ll find the front-wheel-drive Audi less interesting to drive than your rear-driven BMW.