Why did my car turn off at a stoplight?

This blog will explain the factors that can cause a car to stall at a stoplight and answer the following question that is also provided: what to do if your car turns off at a stoplight? 

Why did my car turn off at a stoplight?

Your car can turn off at a stoplight due to the following reasons:

• Eco start/stop
• Low engine speed
• Fuel system problems
• Crankshaft position sensor 
• Alternator or generator malfunction

Let’s take a detailed look at each of the reasons:

Eco start/ stop 

The eco start/stop is a function that allows the internal combustion engine to switch off automatically when the vehicle comes to a slow stop. The stop can be at a stoplight or at an interaction. As the driver lets go of the brake pedal and press the accelerator pedal, the engine starts automatically.

Therefore, if the car turns=off at a stoplight it is important to check if the Eco start/stop system is active or not. When the Eco start/stop system is active, the ECO symbol will be displayed in the instrument cluster. Simply release the brake and press the accelerator for the engine to start again.

The Eco start/stop system does a function check after the engine is started and the system will only remain active if there is no fault code present in the onboard electrical system management control unit. 

However, if there is a fault in the starter motor of the vehicle; the Eco start/ stop will not be able to start the engine. The vehicle has to be towed to the authorized dealership. The engine will not start until the fault in the starter circuit is rectified.

The starter motor is used for cranking the engine. If the mileage of the vehicle is too high, the solenoid on the starter motor of the contact brushed of the motor gets worn out. It can either be repaired if child parts are available or the complete motor must be replaced.

Low engine speed.

When the car stops at a stoplight, the engine must be able to maintain the engine speed in the Ideal RPM range. For gasoline-powered cars, the idle speed is slightly less compared to diesel engines as the weight and the inertial of the moving parts in the engine are less. 

When the engine speed falls below the idle RPM range it will turn off because it cannot overcome the load from the drive train components. This problem is very common in cars with manual transmissions. When the clutch is released and the friction plate starts to bite, the engine turns off if the engine speed is not increased to match the increase in the load.

Many car manufacturers provide an intelligent manual transmission that can increase the engine speed in order to match the specified torque during the start-off. Another cause for low engine speed is lean air flow mixture.

If there is a problem with the air mass sensor or if unmetered air going into the intake manifold will cause a lean mixture in the cylinder. The engine control unit cannot increase the fuel quantity and the engine speed will drop till it turns off.

Finding the leak in the air intake system can be time-consuming but if the fault is in the air-mass sensor, it can be an expensive repair. In diesel engines, the car can turn off if the rate of exhaust gas recirculation is too high.

Fuel system problem

The fuel system of an internal combustion engine consists of low and high-pressure fuel pumps and fuel injectors depending on the type of injection and number of cylinders in the engine. The pressure of the fuel is really important for the engine in order to keep idle speed.

The pressure in the low fuel pressure system is up to 7 bars and the pressure in the high-pressure fuel system is up to 250 bars. Diesel engines run at higher pressure compared to the petrol engine and the pressure in some of the latest engines can go up to 2500 bars.

If there is a drop in pressure either in the low or high-pressure system will cause the car to turn off at a stoplight. It can also prevent the engine from starting again. Fault codes are stored in the engine control unit along with the fault frame freeze data that helps in identifying the cause of the pressure drop.

Another common problem is incorrect fueling or poor quality of fuel in the car. Diesel is a self-igniting fuel and it needs a high compression pressure to ignite while gasoline engines use spark plugs to ignite the mixture.

Accidentally refueling the gasoline engine with diesel and vice versa will damage the components in the fuel system. Draining the fuel will not be sufficient as the damage to the components like fuel pumps and injector is irreversible. The complete fuel system must be renewed and the mixed fuel is disposed of according to the safety norms.

Crankshaft position sensor

If the car turns off at a stoplight and it did not show any warning lights before turning off then it can be a faulty crankshaft position sensor. If the driver tries to start the engine again, it continues to turn and does not start.

The engine control unit uses the signal from the crankshaft position sensor to determine the position of the crankshaft. This data is used to determine the ignition timings and the start of the fuel injection. As the ECU cannot detect the ignition TDC or top dead center of the first cylinder, the engine cannot be started.

The cost of the crankshaft position is not high but if the car does not start even after replacing the sensor could mean that the engine control unit is faulty. The power supply for the crankshaft position sensor is directly from the ECU and an internal fault can easily cause the CKP to stop detecting the crankshaft position and display the engine speed.

What to do if your car turns off at a stoplight?

If your car turns off at a stop light follow the steps given below:

• Turn off the ignition completely and switch it back ON
• Try to start the engine and observe for any warning light in the instrument cluster.
• If the car does not start after 2-3 attempts, do not crank the engine as it can drain the main battery.
• Keep the ignition ON and shift the transmission into Neutral, push the vehicle to the side of the road and wait for roadside assistance to arrive.

Conclusion

This blog explained the causes of a car stalling at a stoplight. If the car cannot be started again then follow the towing instruction given in the owner’s manual to avoid any damage to the drivetrain components.