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Why Does My Car Jerk When Accelerating?

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

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

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

If your car jerks when accelerating, one or more components could be faulty:

  • 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 affect the air-fuel mixture and cause your car to jerk.

Don't confuse this engine condition with a hesitation problem. As it hesitates, your car seems to lose power momentarily. When your car jerks, known as stumbling, the engine misses as you accelerate. You may feel the car vibrating or going through a 'spasm', as the engine stumbles. This is because the computer, due to a potential fault, momentarily cuts fuel flow or ignition spark momentarily.

Finding the problem can take you a bit of work as you troubleshoot components to find the culprit. However, faulty sensors, especially those that affect 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 do not 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. On the other hand, 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.

When diagnosing your vehicle, you may want to have the vehicle repair manual for your model. 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.

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.

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:

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  • 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.

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 when cold air enters the engine.

You may find an IAT sensor in the intake manifold, on vehicles equipped with a manifold absolute pressure (MAP) sensor; otherwise, it may be part of the mass airflow (MAF) sensor.

Just as a bad ECT sensor, a faulty IAT sensor may cause problems with the air-fuel ratio and the ignition system. Also a thermistor, 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 with the 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.

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. With low vacuum in the intake (open throttle), fuel volume increases; as vacuum increases (close throttle), fuel volume decreases.

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 torn air duct or hose between the MAF and the throttle body will disrupt performance. 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.

4. The Mass Airflow (MAF) Sensor

The MAF sensor measures the airflow rate going into the engine. The computer uses this measurement 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 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:

  • Debris inside the MAF sensor housing may interfere with proper operation.
  • Check the air inlet assembly for blockage, proper installation, and proper sealing of connecting points, including the air filter, housing, and ducts.
  • Remove dust and debris from the MAF sensor screen.
  • Get rid of dirt and carbon buildup from the throttle plate bore.

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 with the 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, check the sensor for a dirty hot wire. Also, a faulty self-cleaning circuit relay may lead to a sluggish sensor. 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.

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 corresponds 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, contamination or corrosion can interfere with the voltage signal on platinum electrodes, and don't sense small changes in exhaust-stream oxygen content.

Manufacturers are replacing Zr0₂ sensors with Air-Fuel Ratio (A/F) sensors. 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.

In addition, you can easily confuse the new A/F sensors with their regular counterparts, since they look 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 corresponds to the amount of oxygen in the exhaust stream.

To complicate things even more, some vehicles on the road come equipped with 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

Despite their reliability, some oxygen sensors have a 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, you still may want to check systems or components that could affect 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 appear to be operating properly, test your oxygen sensor.

First, ensure you know what type of oxygen sensor your vehicle uses. With a regular type, use this guide to help you test the oxygen sensor. With a newer sensor configuration, 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.

6. The Throttle Position Sensor (TPS)

The car computer monitors the throttle-opening rate using the throttle position sensor (TPS -- a potentiometer).

One end of a sensor internal wiper arm connects to the throttle shaft, and the other end travels along a resistor, which increases or decreases a reference signal voltage.

The computer uses this signal 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.

7. Accelerator Pedal Position (APP) Sensor

Electronic throttle control systems use APP sensors. 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.

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 lie 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 five volts), voltage signal to the computer, and ground.

Often, you can diagnose an 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 buildup 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, check the EGR system. If you need to remove build-up 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.

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.

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 the crankshaft and camshaft components. This helps 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 generate a magnetic pulse. So you can 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.

11. Fuel Pressure Regulator

On some older models, and a few newer ones, a bad fuel pressure regulator may also cause a vehicle to jerk during acceleration. Usually, you can blame it on a sticking, internal diaphragm. 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 flow to the intake manifold, which may cause a rich mixture. This may cause a misfire, and the engine will stumble, especially 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.

You may have a hard time diagnosing this condition since you can't see the fuel flowing through the injectors. Although not as necessary today, adding a high-quality fuel injector cleaner, or fuel treatment, 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.

If necessary, check for any Technical Service Bulletins (TSB) from your manufacturer for your specific vehicle model. Design problems or specific components used in certain models may eventually lead to misfires, jerking, or bucking issues. Check the Recall section in the National Highway Traffic Administration (NHTSA) site for TSBs for your vehicle.

Poor ignition system maintenance can lead to engine stumbling.

Poor ignition system maintenance can lead to engine stumbling.

13. Ignition System

Ignition system issues can lead to jerking or stumbling complaints as well. Many car owners forget the need to replace spark plugs, spark plug wires, and (where applicable) distributor cap and rotor, which may lead to misfires.

Some manufacturers recommend a specific type of spark plugs for specific engine models. Installing a different type of plug can lead to misfires and jerks. If necessary, consult your vehicle repair manual, if necessary.

Most car manufacturers suggest inspecting and replacing spark plugs and wires at regular intervals. Inspect the spark plugs and boots for signs of carbon tracks. If you find any, replace the plugs and wires or coil on plugs (COP) as necessary. 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 accelerate your diagnostic and repair.

Preferable, you should fix the problem as soon as possible to prevent expensive repairs. However, always test components before replacing them. Sometimes, a circuit or a different component can badly affect a different part that can trigger a fault code.

In addition, you may find no code stored in memory, or the component you blamed the fault on turns out to be good after all. Instead, corrosion, loose connectors, or a damaged wire can lurk behind the problem.

So don't forget the circuits during your testing procedures. Test them for shorts, opens, and high resistance. A voltage-drop test can quickly help you find circuit problems that otherwise may prove difficult to discover.

Some particular vehicles may come with other sensors not common in other models to help the computer regulate the air-fuel ratio, for example, a cylinder head temperature (CHT) sensor, engine oil sensor, fuel rail sensor and so on.

Consult the repair manual for your particular vehicle make and model, if necessary. Here, we only touched on the most common sensors.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2019 Dan Ferrell

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