Why does my car stall occasionally?

Stalling always seems to happen at the worst possible time – making a right turn across a busy road or setting off when the light changes to green, ahead of a long line of traffic in peak hour.

Inexperienced drivers who attempt to launch a manual car without stalling have long been a source of despair for any driver in a vehicle behind. They let the clutch out too fast, without enough revs, or they slot into the wrong gear, or they forget to throw the clutch as the vehicle slows to a halt. Any of those 'junior moments' while driving could result in a stall.

Thankfully, few learners – even those who claim to be motoring enthusiasts – bother to obtain a manual driving licence these days.

A recent technical innovation is the idle-stop system, an automated feature invented to stall an engine while waiting at a stop sign, red light or pedestrian crossing to save fuel. When the driver lifts his or her right foot off the brake pedal and applies it to the accelerator, the idle-stop system will immediately restart the engine.

The system intentionally stalls the engine to save fuel, but our innate fear of the humiliation we experience whenever we stall a vehicle ensures that many drivers don't welcome the idle-stop system. They prefer to disable it and leave the engine running constantly, even if it means filling the tank sooner.

But a vehicle can stall for reasons other than a learner behind the wheel or a too-clever-by-half automatic system designed to conserved fuel. In ye olden days of manual chokes, contact points and even gravity-fed fuel delivery and manual ignition advance, there were so many ways for a vehicle to stall.

The introduction of long-life spark plugs, fuel injection and electronic ignition has made stalling almost a thing of the past. Once a common complaint on the nation's roads, stalling has become mercifully rare. Cars still occasionally stall unexpectedly though. Sometimes they restart quickly – with a quick crank of the starter motor, but at other times no amount of coaxing will bring the internal combustion engine back to life.

An engine needs spark, fuel and air to run. But everything must be delivered in the right proportions and at the right time. Get the wrong ratio of fuel to air, for instance, and the mixture will be too rich or too lean. The right ratio is 14.7:1 (air to fuel) for petrol engines.

More air and the engine runs 'lean', less air and the engine runs rich – which it should do for cold starts, but not once the engine has warmed up. Lean running will make the engine hard to start in the mornings.

A mixture that's too rich will eventually 'foul' the spark plugs, resulting in a fuel residue building up between the electrodes, preventing electrical current running through the plug from sparking or arcing across the gap and igniting the fuel. The engine becomes progressively harder to start and keep running.

In older cars, tuning the carburettors was often a common undertaking during a regular service. Modern, fuel-injected cars shouldn't require as much servicing to maintain the right air-fuel ratio, but sometimes an oxygen sensor in the exhaust or a mass airflow (MAF) sensor in the engine's induction system can fail. And the injectors that spray fuel directly into the engine or via the inlet ports can become silted up and clogged by fuel. This can result in poor spray, poor combustion and fuel starvation.

Fuel starvation can also result from blockages in the fuel line. Replace the fuel filter in the line, or unblock the fuel line with an air hose or replace the fuel pump if that device has failed.

Of course a common cause of fuel starvation is forgetting to fill the fuel tank and ignoring the low-fuel warning light for 100km – or some other similarly conservative distance. That will stall the engine too.

Lack of spark can lead to poor running and stalling. Long-life spark plugs are commonly fitted as original equipment in modern cars, but eventually even these plugs need replacing. Older style plugs should be replaced typically within 20,000km. Leave them in an engine much longer and the spark will become sporadic at best.

The same is true of the supporting cast in the car's electrical system, plug leads and the coil. Ignition cannot take place without these parts working to specification. Over time the leads can bend and fray, breaking the flow of electrical current to the coil (which converts low voltage to high voltage), or from the coil to the spark plugs. These must be replaced at intervals just like the spark plugs – albeit not as frequently.

Spark plugs must be properly 'gapped'. Mechanics use a feeler gauge to set the gap between the plug's two electrodes so that electricity jumping across the gap creates a spark or arc to ignite the fuel. Get the gap wrong and the electricity won't provide the right kick for the fuel to combust.

In extreme cases, a battery that will no longer hold charge will eventually run flat and the car will stall then, but there are usually at least a couple of tell-tales – dimming headlights and an alternator warning light in the dash.

Air is the third cause of poor running and stalling. A leak in an inlet manifold gasket, for example, will result in too much air entering the engine, which will consequently run lean. The airflow into the engine is monitored for the precise ratio and a gap or hole in an air filter or a gasket or other component will throw out the Powertrain Control Module's reckoning, making the car hard to start or keep running. A clogged air filter can reduce performance too.

The exhaust system can also be a problem. Back pressure (the pressure of pumping exhaust gases through pipes, catalytic converters, mufflers and resonators) is a factor taken into account to keep the engine running. Remove the car's exhaust system and not only will it run loud, as likely as not it won't idle. Take your foot off the accelerator and the engine will stall. Fitting an aftermarket exhaust system is not just a bolt-on process. It calls for a mechanic to re-tune the engine to suit the lower back pressure from the new exhaust.

Sometimes, the back pressure can be too high and the car's induction and exhaust systems become 'constipated' if the catalytic converter reaches the end of its useful life, as an example.

There are also numerous other mechanical problems that could result in rough running and stalling. These include cracks in the cylinder head, bent or burnt valves, the wrong fuel used (diesel instead of petrol, for instance) or oil travelling past the piston rings and fouling the spark plugs.

In a manual car, press the clutch pedal to the floor, check that the gear shift is in first or reverse (whichever gear applies) and press the starter button or turn the key to start. You may find that manual cars with idle-stop systems will restart the engine automatically as soon as you press the clutch pedal.

Automatic cars do not stall unless they are mechanically faulty in some way or another. Getting an automatic car moving again is not necessarily a simple process.

If the induction system is a carburettor rather than a fuel injection system, check whether you've pulled out the manual choke (if the car is fitted with one). If the carburettor has an automatic choke the engine should rev higher than normal idle speed for a cold start. If it doesn't, the driver may have to rev the engine a little higher before selecting Drive. Take care not to let the car gallop away. To keep the engine running in the event the automatic choke isn't operating you only need about 1500rpm to move off.

If the engine is running rough, due to fuel or ignition problems, a bit of a rev in neutral may clear the problem long enough for the engine to produce enough torque for a standing start.

Is the engine dying while the car is cornering? This could be a design fault in some carburettor-fed cars suffering fuel starvation when lateral g forces reduce the fuel flow to the carburettor, such as some earlier Nissans. Provided the car is carrying enough momentum through the corner it should keep moving and performance will return once the car is travelling in a straight line.

Take the vehicle to a recognised repairer, one with professional staff trained to track down that misfire or other symptom of rough running.

If it's an ignition problem (spark), the mechanic can replace the spark plugs first, followed by the plug leads. The coil shouldn't need replacing unless the car is very old.

An air filter can become clogged over time, so replacing that often helps too.

Replacing fuel injectors is risky for anyone other than a trained mechanic. Modern injectors run at very high pressure and removing faulty units without relieving the pressure first is dangerous.

A normal tune-up should take no more than an hour or so, and it won't cost mega-bucks.