Why Does My Car Jerk When I Accelerate?

If your car jerks, hesitates or stumbles when you accelerate, it is often a sign that the engine is misfiring under load.

On modern petrol engines, one of the common causes is an ignition coil breaking down when the engine is under load. The fault may not be obvious when the engine is idling because the ignition system has a much easier job at idle than it does when the engine is producing power.

Why does a car jerk when accelerating?

When you accelerate, the engine has to produce more power. Cylinder pressures increase and the ignition system needs to generate enough voltage to produce a reliable spark across the spark-plug gap.

If the ignition system cannot deliver that spark consistently, the affected cylinder can misfire.

You may feel this as:

• A sudden jerk or hesitation
• A stumble when you press the accelerator
• Intermittent loss of power
• The engine running roughly under acceleration
• A noticeable hesitation when climbing a hill or accelerating hard

Sometimes the vehicle will drive normally when cruising or idling but develop the problem as soon as you ask the engine to work harder.

A common cause: an ignition coil breaking down

Ignition coils have to generate very high voltages to fire the spark plugs.

A coil can appear to work perfectly at idle but break down when the voltage demand increases under load.

There are different ways this can occur.

Primary-side failure

The primary side of the ignition coil is the low-voltage side controlled by the vehicle’s ignition system.

A fault in the primary circuit can prevent the coil from building or releasing the magnetic energy required to produce a strong secondary spark.

This can result in an intermittent or weak ignition event and therefore a misfire.

Secondary-side failure

The secondary side is where the ignition coil produces the high voltage required to fire the spark plug.

If the insulation on the secondary side breaks down, the high voltage can escape through an unintended path rather than reaching the spark-plug electrode.

In some cases the voltage can arc to earth through the coil, boot or another nearby component.

The result is the same: the spark doesn’t occur correctly inside the cylinder and the engine can misfire.

This type of failure can be particularly noticeable under acceleration because the ignition system is working harder and the voltage required to fire the plug increases.

Can worn spark plugs cause the problem?

They can contribute to ignition-system problems.

Spark plugs are a service item and their condition changes as they accumulate kilometres. The electrodes can wear and the spark-plug gap can increase over time.

As the gap and condition of the plug change, the ignition system may need to generate more voltage to produce the spark.

If spark plugs have been left in service beyond their recommended replacement interval, this can place additional demands on the ignition system and may contribute to ignition-coil problems.

This is one reason manufacturers specify spark-plug replacement intervals as part of the vehicle’s service schedule.

A regular service isn’t simply about changing the oil. Service items such as spark plugs are specified at particular intervals because their condition can affect the operation of the engine and ignition system.

What if the check engine light isn’t on?

A misfire does not necessarily mean that the check engine light will immediately appear.

An intermittent fault may only occur under particular conditions, such as:

• Hard acceleration
• High engine load
• Uphill driving
• Higher engine speeds
• A cold or hot engine

Modern vehicles can monitor engine misfires, but the conditions and severity of the fault determine what the vehicle’s computer records and whether a warning light is triggered.

If your car consistently jerks under acceleration, it is worth having the problem investigated even if there is no warning light.

How is the problem diagnosed?

A proper diagnosis involves more than simply replacing parts until the problem disappears.

A technician can use the vehicle’s diagnostic system along with testing equipment and inspection to determine what is happening.

Depending on the vehicle and symptoms, this may include checking:

• Stored diagnostic fault codes
• Misfire information
• Spark plugs
• Ignition coils
• Coil boots and insulation
• Ignition waveforms
• Engine operating data

The objective is to establish why the engine is misfiring, rather than simply guessing which component might be responsible.

Don’t ignore a car that is misfiring

If your car is repeatedly jerking or hesitating when you accelerate, don’t assume it is something you simply have to live with.

A persistent misfire can affect drivability and, depending on the cause and severity, can potentially cause further damage if the vehicle continues to be driven with the fault.

The sooner the cause is identified, the better.

Motorfix

Motorfix Automotive Service & Repair can diagnose engine performance and ignition problems and explain what we’ve found before recommending repairs.

If your car is jerking, hesitating or misfiring when you accelerate, contact Motorfix to arrange a diagnostic inspection.