Why Does My Car Jerk When Accelerating?

Updated on July 2, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

A car that jerks can be a bit difficult to diagnose sometimes.
A car that jerks can be a bit difficult to diagnose sometimes. | Source

If your car jerks when accelerating, there could be one or more components throwing your engine off balance:

  • faulty mass air flow sensor
  • air intake leakage
  • faulty engine temperature sensor
  • bad throttle position sensor
  • faulty oxygen sensor
  • intermittent electrical problems in the ignition or fuel system

These and other potential faulty sensors or circuits can have a negative effect on the air-fuel mixture and cause your car to jerk.

Don't confuse this engine condition with a hesitation problem. During hesitation, your car seems to momentarily lose power. When your car jerks, technically known as stumbling, the engine misses during acceleration. You may feel the car vibrating or going through a 'spasm', as engine smooth operation stumbles. This is because fuel flow or ignition spark is momentarily cut.

Diagnosing the problem can take you a bit of work as you troubleshoot components to find the culprit. However, faulty sensors, especially those that can influence the air-fuel mixture or ignition system, can cause the car computer to trigger a diagnostic trouble code (DTC) when they step out of their normal parameters. So, even if you don't see the check engine light come on, scan your car computer for possible pending codes that can guide your diagnostic.

If your computer doesn't have any codes stored, check the following sections for a component you may suspect or want to test. Or, if you've found a DTC for a specific component and you want to test it, look for it in the following sections.

These sections describe some of the most common components or faults that can cause your car to jerk during acceleration, and show a test procedure or give you a link to a post that describes the diagnostic procedure for the component. Sometimes, though, you may need to consult your vehicle repair manual.

Be Careful With Your Car's Computer!

Before disconnecting or connecting a sensor, turn the ignition key to the Off position to prevent induced-voltage damage to the computer.

Index
1. Engine Coolant Temperature (ECT) Sensor
2. Intake Air Temperature (IAT) Sensor
3. Manifold Absolute Pressure (MAP) Sensor
4. Mass Airflow (MAF) Sensor
5. Oxygen Sensor (O2S)
6. Throttle Position Sensor
7. Accelerator Pedal Position (APP) Sensor
8. EGR Valve Position Sensor
9. Vehicle Speed Sensor (VSS)
10. Crankshaft Position (CKP) and Camshaft Position (CMP) Sensors
11. Fuel Pressure Regulator
12. Fuel Injectors
13. Ignition System
The ECT sensor provides input to the computer to optimize air-fuel ratio.
The ECT sensor provides input to the computer to optimize air-fuel ratio. | Source

1. The Engine Coolant Temperature (ECT) Sensor

The car computer uses the ECT to get the engine's average temperature, which helps regulate other processes, including air-fuel ratio.

You can find the ECT sensor near the thermostat housing. Follow the upper radiator hose to the engine, which connects to the thermostat.

Depending on the fault, a bad ECT sensor or circuit may:

  • cause other emission devices to work improperly
  • enrich or lean the air-fuel mixture
  • make it hard to start the engine

Although the ECT sensor may fail, it's more common to find problems in the circuit, like loose or corroded connectors or damaged wires.

Head over to this other post, if you need to test the ECT sensor.

An air leak in the air cleaner assembly may cause jerking.
An air leak in the air cleaner assembly may cause jerking. | Source

2. The Intake Air Temperature (IAT) Sensor

The car computer also uses this sensor to calculate fuel and spark. For example, it will deliver more fuel to the engine if incoming air is cold.

You may find an IAT sensor in the intake manifold, if your vehicle comes equipped with a manifold absolute pressure (MAP) sensor; otherwise, it may be part of the mass air flow (MAF) sensor.

Just like a bad ECT sensor, an IAT sensor is a thermistor and may cause problems with the air-fuel ratio and the ignition system. Basically, an IAT sensor's resistance goes down when incoming air temperature goes up and vice versa.

You can use an ohmmeter to test the sensor under different ambient temperatures. Place the sensor in a container with water. As you heat the water, measure the device's resistance and compare your results to specs in your repair manual.

Consult your vehicle repair manual to locate the MAP sensor in your vehicle.
Consult your vehicle repair manual to locate the MAP sensor in your vehicle. | Source

3. The Manifold Absolute Pressure (MAP) Sensor

The MAP sensor measures air pressure or vacuum in the air intake manifold. It receives a reference voltage signal from the computer and returns a voltage signal to the computer. The returned voltage signal varies with changes in intake manifold vacuum.

The computer uses the MAP sensor signal to compute the amount of fuel the cylinders need and decide when to ignite the air-fuel mixture in the cylinders. With low vacuum in the intake (open throttle) more fuel is injected; as vacuum increases (close throttle), less fuel is needed.

Thus, a faulty MAP sensor can cause problems with the air-fuel mixture.

You can find the sensor mounted on the intake manifold or somewhere high in the engine compartment. When checking the MAP sensor, closely inspect the vacuum hose, electrical connector and wiring. If necessary, check this other post for MAP sensor symptoms and testing.

Also, keep in mind that a common issue, specially with some vehicle models, is a torn air duct or hose between the MAF and the throttle body. A leaking duct will introduce unmetered air to the intake manifold, leaning the air-fuel mixture considerably and causing the vehicle to stumble or stall. When suspecting a bad MAF sensor, closely check the air ducts or hoses for possible damage.

A dirty MAF sensor can lead to stumbling.
A dirty MAF sensor can lead to stumbling. | Source

4. The Mass Airflow (MAF) Sensor

The MAF sensor measures the air flow rate going into the engine. This measurement is used by the computer to calculate throttle opening and air volume to control fuel injection and ignition spark and other systems.

You can find this sensor mounted in the air cleaner duct assembly, between the throttle plate and the air filter; or inside the air cleaner assembly.

The most common of MAF sensor is the hot-wire type. However, even testing of a hot-wire sensor varies among vehicle models. Always check your vehicle repair manual.

First, make a visual inspection of the sensor and cleaner assembly:

  • The MAF sensor housing should be free of debris to operate properly.
  • Check the air inlet assembly for blockage, proper installation and proper sealing of connecting points, including the air filter, housing, and ducts.
  • MAF sensor screen should be clean.
  • Throttle plate bore should be free of dirt and carbon buildup.

Testing a MAF sensor with a digital multimeter (DMM):

  1. Engage the parking brake and set the transmission to Neutral (manual) or Park (automatic).
  2. Connect the meter red lead to the reference voltage wire and the black lead to battery ground (-).
  3. Turn the ignition key On but don't start the engine.
  4. Compare your reading to specs in your vehicle repair manual.
  5. Now connect your meter red lead to the sensor's signal wire and the black lead to battery ground.
  6. Start the engine and check the readout. Depending on your specific application, you may get around 2.5 volts.
  7. Lightly tap the sensor with the handle of a screwdriver. If voltage fluctuates or the engine misfires, replace the MAF sensor.
  8. Increase engine rpm by depressing the accelerator pedal.
  9. You should see an increase in voltage; otherwise the sensor is not responding. This could be because of a dirty hot wire. Also, if response is sluggish, check the self-cleaning circuit relays. Consult your vehicle repair manual.

Oxygen sensors are reliable devices, but may fault after a few years of service.
Oxygen sensors are reliable devices, but may fault after a few years of service. | Source

5. The Oxygen Sensor

The car computer uses an oxygen sensor (O2 sensor) to monitor oxygen content in the exhaust stream going into the catalytic converter, to prevent damage.

Modern vehicle models use a sensor before the catalytic converter to monitor air-fuel ratio, and another one after the catalytic converter to monitor catalytic efficiency. Your vehicle may have more than two of these sensors, depending on the number of catalytic converters and configuration.

Oxygen sensors produce a voltage signal that correspond to the amount of oxygen in the exhaust gases. For example, the more oxygen content (the leaner the mixture) the lower the voltage signal; the more fuel content (the richer the mixture) the higher the voltage signal. The voltage signal will range from 0.1 to 0.9 volts.

O2 sensors need to operate at a high temperature of 750 F (400 C) and may incorporate a heating element to speed up heating and to keep the sensor at the best operating temperature.

The most common O2 sensor is the Zirconium type (Zr02). They come with platinum electrodes and a heating element. However, platinum electrodes are sensitive to contamination or corrosion, which interferes with the voltage signal, and are not very sensitive to small changes in exhaust-stream oxygen content.

The Zr02 sensor is now being replaced by the Air-Fuel Ratio (A/F) sensor. Unlike the common O2 sensor, the A/F sensor produces a higher voltage signal with a lean mixture, and a lower voltage signal with a rich mixture. And they are easy to confuse with their regular counterparts, since they look pretty much alike. Furthermore, the computer doesn't monitor a voltage signal from this sensor; instead, it monitors the changes in electric current through a special circuit, which produces a voltage signal that correspond to the amount of oxygen in the exhaust stream.

To complicate things even more, some vehicles on the road are using Titanium Dioxide (TiO2) sensors. These sensors act like a resistor instead, and work on a 5 voltage reference signal. When oxygen content increases, it sends a signal above 2.5 volts; when oxygen contents decreases, it sends a signal below 2.5 volts.

Testing an Oxygen Sensor

Oxygen sensors are reliable, but some have service life of only a few years. Car manufacturers may provide a service schedule to replace these sensors. So make sure to check maintenance specifications for your particular model and replace the sensor, if necessary.

Even if your computer stores a diagnostic trouble code (DTC) pointing to an oxygen sensor fault, it's a good idea to check systems or components that might be affecting sensor operation:

  • Air cleaner assembly leaks
  • Faulty or contaminated MAF sensor
  • Leaking fuel injector
  • Faulty fuel pressure
  • Vacuum leak
  • EGR system leak

If these other systems seem to be operating properly, test your oxygen sensor.

First, make sure you know what type of oxygen sensor is installed on your vehicle. If you are testing a regular type, you can use this guide to help you test the oxygen sensor. If you are testing a newer type sensor, consult your vehicle repair manual for the correct procedure and specifications.

Locate and test the TPS in your vehicle, attached to the throttle body.
Locate and test the TPS in your vehicle, attached to the throttle body. | Source

6. The Throttle Position Sensor (TPS)

The car computer monitors the throttle opening rate using the throttle position sensor (TPS). The TPS is a potentiometer. One end of a sensor internal wiper arm is connected to the throttle shaft, and the other end travels along a resistor, which increases or decreases a reference signal. The output signal is used by the computer to interpret throttle plate position to increase or decrease fuel injection.

You'll find the TPS mounted on the throttle body. On some models, you may also find an idle contact switch or wide-open throttle (WOT) switch to monitor those positions. Most TPS sensors use three wires for the reference signal (5V), ground, and output signal. The output signal may range from 0.6 volts to 4.7 volts.

TP sensors in electronic throttle control use an extra wiper arm as a backup.

If the check engine light comes on, you may get trouble codes P0120-P0124, P0220-P0229.

Head over to this post to test a common throttle position sensor. If necessary, consult your vehicle repair manual. The following video shows you how a faulty TPS can cause engine stumbling.

Testing an APP sensor is similar to testing an TPS.
Testing an APP sensor is similar to testing an TPS. | Source

7. Accelerator Pedal Position (APP) Sensor

APP sensors are used with electronic throttle control systems. Just like the throttle position sensor for the electronic throttle configuration, this sensor uses two potentiometers (two wiper arms).

The car computer uses the information from the APP sensor to calculate power torque demand. Then opens or closes the throttle plate and adjusts fuel injection to the cylinders accordingly. You can test the APP as you would other variable resistor sensors (see the previous section). Consult your vehicle repair manual to identify wires and specifications.

If failed, the EGR system may cause engine performance issues.
If failed, the EGR system may cause engine performance issues. | Source

8. EGR Valve Position Sensor

The computer also uses a position sensor to monitor EGR valve opening. This affects the air-fuel mixture adjustment as well. Exhaust gases introduced to the cylinders by the EGR valve reduce oxygen content and the fuel needed, which helps maintain low hydrocarbon levels (HC-unburned or partially burned fuel molecules in the exhaust stream). They also reduce combustion chamber temperatures to control spark knock and NOx (nitrogen oxides) emissions.

Most EGR position sensors are mounted on top of the valve and work similarly to a throttle position sensor. Usually, these sensors have three wires, one for the reference signal (usually 5 volts), voltage signal to the computer, and ground.

Often, you can diagnose a EGR position sensor using a simple procedure:

  1. Engage the Parking brake and set your transmission to Neutral (manual) or Park (automatic).
  2. Connect (back probe) your DMM red lead to the sensor signal wire.
  3. Connect the DMM black lead to battery ground.
  4. Turn the ignition switch to the On position, but don't start the engine.
  5. You should get about 0.8 volts.
  6. Connect a hand-held vacuum pump to the EGR valve and gradually apply 20 in.-Hg of vacuum.
  7. Voltage should increase smoothly to about 4.5 volts; otherwise, replace the sensor.

If the valve opens too soon, you can experience a jerk when accelerating. Carbon build up in intake passages may also cause the same type of problems.

When experiencing a stumble during acceleration or misfire, temporarily disconnect the EGR valve and test the vehicle on the road. If the stumble or misfire disappears, most likely the problem is in the EGR system. If you need to remove buildup from EGR passages, make sure to check and clean the intake manifold as well.

EGR issues may trigger the check engine light. You may get a trouble code P0400-P0408.

A faulty vehicle speed sensor can also cause an engine to stumble.
A faulty vehicle speed sensor can also cause an engine to stumble. | Source

9. Vehicle Speed Sensor (VSS)

A VSS measures vehicle speed by generating a waveform at a frequency proportional to the speed. Frequency increases with vehicle speed.

The signal from the sensor helps the computer control fuel injection, ignition system, torque converter clutch lock-up timing, EGR flow, transmission shift timing, and other processes.

Often, you'll find the VSS mounted on the transmission, in the speedometer cable opening to measure the output shaft's rotational speed. Most sensors use a magnetic pulse generator.

Usually, car shops use a scan tool or lab scope to diagnose problems with the VS sensor. However, it's possible to use an ohmmeter to make specific measurements. Consult your vehicle repair manual for the procedure and specifications. A typical resistance across a sensor's terminals will produce 800 to 1,400 ohms of resistance.

To test a hall-effect type sensor, consult your vehicle repair manual.

Speed-position sensors like this CKP sensor can cause trouble as well.
Speed-position sensors like this CKP sensor can cause trouble as well. | Source

10. Crankshaft Position (CKP) and Camshaft Position (CMP) Sensors

Position-speed sensors like the CKP and CMP sensors help the computer monitor the speed and position of a component. In this case, the position of the crankshaft and camshaft components. This help the computer control ignition and fuel delivery, and variable valve timing on some models.

The CKP and CMP sensors read the rotation of a toothed wheel mounted on the crankshaft and camshaft respectively. The sensors generate an AC voltage frequency signal that helps the computer monitor rotational speed.

When checking these sensors, always make a visual inspection of connectors and wires. Loose and corroded connectors and wires will interfere with the sensors' signals.

Although you can find hall effect type, most of these sensors are magnetic pulse generators. So you may use an ohmmeter to check these sensors. Head over to this other post to check a crankshaft position sensor using a multimeter.

A fuel-pressure-regulator diaphragm may become sticky and cause the engine to jerk.
A fuel-pressure-regulator diaphragm may become sticky and cause the engine to jerk. | Source

11. Fuel Pressure Regulator

On some older models and a few newer models, a bad fuel pressure regulator may also cause a vehicle to jerk during acceleration. This is usually due to a sticking diaphragm inside the regulator. Although rare, keep this in mind if you have an old 1980s model.

The diaphragm in the pressure regulator may also start leaking. If fuel enters the vacuum line that operates the regulator, extra fuel will be fed to the intake manifold, which may cause a rich mixture misfire and stumble, specially on cold starts.

12. Fuel Injectors

Dirty fuel injectors may cause a variety of performance issues, including jerking or stumbling. Debris or buildup in the injector passages may intermittently limit fuel flow.

This condition may be hard to diagnose since you can't see fuel flow through the injectors. Although not as necessary today as it used to be, adding a high-quality fuel injector cleaner to the fuel tank may help sometimes.

Your computer may set a P0171 (lean condition) or a P0300 (random misfire) trouble code. Check for a dirty MAF sensor, vacuum leak, low fuel pressure (often a clogging fuel filter or faulty fuel pressure regulator), or dirty fuel injector.

Poor ignition system maintenance can lead to engine stumbling.
Poor ignition system maintenance can lead to engine stumbling. | Source

13. Ignition System

Ignition system issues are more prone to cause jerking or stumbling complaints. Many car owners forget the need to replace spark plugs, spark plug wires, and (where applicable) distributor cap and rotor.

Most car manufacturers suggest inspecting and replacing spark plugs and wires at regular intervals. Ignition coils tend to last much longer, but they can also cause trouble. So keep them in mind.

Modern vehicles may use a coil pack or one ignition coil per cylinder. These ignition coils may also cause trouble from time to time. While checking spark plugs and wires, test ignition coils as necessary.

Consult your vehicle repair manual for the recommended service schedule and troubleshooting procedures for your particular model, if necessary.

14. Establishing Your Diagnostic Strategy

When your car jerks when accelerating, often, you can diagnose the problem without much trouble. Most sensors involved in this type of issue trigger a fault code. Scanning your computer memory for diagnostic codes will speed up your diagnostic and repair. However, always test components before replacing them. Sometimes, a circuit or a different component can have a bad effect on a different part that can trigger a fault code.

Also, you may find that there's no code stored in memory, or the component you believed to be faulty is not actually bad. Corrosion, loose connectors and damaged wires are common faults. So don't forget the circuits during your testing procedures. Test circuits for shorts, opens, and high resistance.

Another thing to keep in mind is that particular vehicle models may come with other sensors not common in other models that help the computer regulate the air-fuel ratio. Here, we only touched on the most common sensors. But your particular model may come with a cylinder head temperature (CHT) sensor, engine oil sensor, fuel rail sensor and so on, which are not common to all models. So make sure to consult the repair manual for your particular vehicle make and model, if necessary.

If you don't have this manual yet, get a relatively inexpensive copy from Amazon. Haynes manuals come with step-by-step procedures for many troubleshooting, replacement and maintenance projects. So you'll recoup your investment in a short time.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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    © 2019 Dan Ferrell

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