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Why does my car hesitate when I accelerate?

Troubleshooting a Hesitating Engine

Woman driver scared shocked before crash

Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles.

Updated on 05/02/18

You know how frustrating it can be when your car hesitates when you step on the gas. Instead of get-up-and-go, you get hurry up and wait. But you can troubleshoot this common problem, maybe even fix it yourself. At the very least, you’ll be able to give your mechanic a head start and save yourself the extra money it will cost for a diagnosis.

It’s a No Go

It’s Monday morning, and you’re on your way to work in your trusty car and truck. You’re running a little late, but sitting in pole position at the last light before hitting your employee parking lot. So when the stoplight turns from red to green, you punch the gas pedal, hoping to sail through the intersection a little, um, faster than usual.

But instead of moving when you hit the gas pedal, your car hesitates. Juuust long enough for every car behind you to honk, flip you the bird, or angrily swerve around you. They’re sailing right on through. So why are you sitting there, bogged down, pumping the gas until your little engine that could finally muster enough power to come up to speed?

Causes of Engine Hesitation

The feeling is unmistakable. Your engine either seems to bog down when you hit the gas pedal, or it takes a second or two to respond. It’s not quite a stutter, not quite a stall, and it doesn’t matter if the engine is hot, cold, or low on gas.

There are a number of possible causes, but you’re going to have to take a good look at your engine to try and diagnose the problem. You can do this by shutting off the engine, putting on some old clothes and gloves, and making sure nothing is dangling from your body that could get caught in a part of a cooling fan that suddenly decides to start running, even thought the engine is off.

An engine that hesitates when accelerating is either sucking too much air, not getting enough fuel, or misfiring.

Here’s what you might discover—and what you can do to fix the problem:

  1. Dirty air filter.
    The Fix:Replace the air filter.
  2. The spark plugs may be dirty or worn.
    The Fix:Replace spark plugs.
  3. The ignition wires may be bad.
    The Fix:Inspect and replace ignition wires.
  4. Ignition system problems.
    The Fix: Check distributor cap or rotor. Ignition module may be bad.
  5. You may have water in the gasoline.
    The Fix:Drain the gas tank and flush with fresh gas and refill. (Generally not a DIY job)
  6. If you have a carburetor, you may have a bad accelerator pump or power circuit.
    The Fix: Replace accelerator pump or clean orreplace the carburetort.
  7. The fuel filter may be clogged.
    The Fix:Replace the fuel filter.
  8. Your catalytic converter may be clogged.
    The Fix: Replace catalytic converter.

On-board Diagnostics

In a newer model car, the vehicle’s computer diagnostic system is likely to tell you what’s going on by flashing the Check Engine light on your dash. Then you just need to learn what the codes mean.

How To Diagnose & Repair an Engine Hesitation Problem

throttle hesitation

Hesitation is when your engine misfires, stumbles or lacks power when you accelerate or step on the throttle. The problem often means the air/fuel mixture is not being properly enriched or is going lean, or the ignition system is weak and is misfiring when the engine comes under load or the air/fuel mixture goes lean.

When you step down on the accelerator and the throttle opens, the engine sucks in more air. The computer should respond by adding more fuel.

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If the engine has a speed-density type of fuel injection system (no airflow sensor), the computer uses inputs from the throttle position sensor, manifold absolute pressure sensor, air temperature sensor and engine rpm to estimate airflow and how much fuel the engine needs. NOTE: Speed-density systems are much less sensitive to vacuum leaks than EFI systems that use an airflow sensor.

If the engine has an airflow sensor (vane airflow or mass airflow), it looks primarily at the airflow signal from the airflow sensor, but also takes into account what the throttle position sensor and MAP sensor (if equipped) are telling it. NOTE: Airflow EFI systems are very sensitive to vacuum leaks, and air leaks downstream of the airflow sensor.

Consequently, if the inputs from any of these sensors is inaccurate or missing, the engine computer may not add enough fuel, allowing the fuel mixture to go lean causing a misfire that produces a hesitation or stumble when accelerating or opening the throttle.

The amount of fuel added by the computer when the throttle opens may also be insufficient if the fuel injectors are dirty or fuel pressure is low. The oxygen sensors in the exhaust monitor the air/fuel mixture so the computer can adjust fuel trim as needed to maintain the proper air/fuel ratio. Fuel trim adjustments can compensate for dirty injectors and/or low fuel pressure to a certain extent, but occur too slowly to offset a throttle hesitation problem.

Vacuum leaks will typically cause the fuel trim to run rich as the computer tries to compensate for the extra air being sucked into the engine through the leak.

Possible Causes of Engine Hesitation or Stumble:

* Dirty fuel injectors (cleaning the injectors often fixes this).
* Bad MAP (manifold absolute pressure) sensor
* Bad TPS (throttle position) sensor
* Bad or dirty MAF (mass airflow) sensor
* Low fuel pressure (leaky fuel pressure regulator, weak fuel pump or low system voltage or charging voltage that causes the fuel pump to run slow)
* Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
* Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfirerather than lean misfire. The causes of ignition misfire may include:

* Dirty or worn spark plugs
* Bad plug wires
* Weak ignition coil
* Wet plug wires

using scan tool to diagnose hesitation

A scan tool that can display sensor data and fuel trim values can help you diagnose a hesitation problem.


Diagnose may require checking the engine computer with a scan tool for any fault codes (including misfire codes), checking sensor response with a scan tool by looking at the various sensor PIDS (sensor values displayed on the scan tool), and testing sensors with a DVOM or scope if the values are out of range or the sensor is not responding normally. Additional diagnostic checks may include searching for vacuum leaks, inspecting/cleaning the EGR valve, measuring fuel pressure and volume, removing and inspecting the spark plugs, etc.

You can use a scan tool to check the fuel trim readings to see if the engine is running lean. Lean mixtures that are caused by vacuum leaks will have the most noticeable effect at idle. At part and full throttle, there is so much air entering the engine that a little extra air from a vacuum leak has a negligible effect.

Plug your scan tool into the diagnostic connector and start the engine, then look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range is typically plus or minus 8. If the numbers are +10 or higher for STFT and LTFT, the engine is running LEAN. If you rev the engine to 1500 to 2000 rpm and hold it for a minute or so, and the STFT value drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. If the STFT value does not change much, the lean fuel condition is more likely a fuel delivery problem (weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator) than a vacuum leak.

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For more information about using fuel trim to diagnose a lean fuel condition, read this article on Fuel Trim by Wells Manufacturing (PDF file, requires Adobe Acrobat to read).


A code P0171 or P0174 (or both) indicate the engine is running lean. This means there is too much air and/or not enough fuel.

If you have a scan tool that can display Short Term Fuel Trim (STFT), you can confirm the engine is running lean by looking at the STFT at idle. If STFT is greater than about 10 to 12, the engine is running LEAN. Increase engine speed to 1600 to 2000 rpm and hold for a minute or so, then recheck the STFT value. If it has dropped 3 or 4 points or more, the lean problem is due to a vacuum leak (vacuum leaks have more of an impact on the idle fuel mixture than the cruise mixture). If the STFT valve is still the same, the problem is probably sensor-related (dirty or bad MAF sensor, or bad MAP sensor), or is due to low fuel pressure. The next steps would be to use the scan tool to look at the airflow and MAP sensor readings, and to check fuel pressure.

A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Check for vacuum leaks)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)

* TIP: On many Fords, a P0171 and/or P0174 Lean Code may sometimes appear because of a bad Differential Pressure Sensor (DPFE). This sensor monitors EGR flow, and is located on the engine near the EGR valve. There are two hoses that connect the sensor to the tube that runs from the exhaust manifold to the EGR valve. The sensor misreads EGR flow and the computer increases EGR which has a leaning effect on the fuel mixture. The fix is to replace the DPFE sensor.

* TIP: For information about other causes of P0171 and P0174 lean codes on Ford Windstar, Click Here


Repairs will depend on what is causing the hesitation.

If the cause is fuel related, it may require cleaning the fuel injectors or fixing a vacuum leak. On an older vehicle with a carburetor, it may require rebuilding or replacing the carburetor.

If the cause is ignition related, it may require replacing the spark plugs and plug wires.

If the cause is an air leak or vacuum leak in the intake manifold, it may require replacing gaskets or leaky vacuum hoses.

Car Hesitates When Accelerating – Find Out Why This Happens In Your Car!

Car Hesitates When Accelerating

Drivers need to determine the main reasons why their car hesitates when accelerating. By realizing the most common symptoms associated with hesitation, stuttering, or trouble speeding up on highway roads, car owners can prevent these issues from worsening over time.

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10 Symptoms of Car Hesitates When Accelerating

Noticing the symptoms of your car hesitates when accelerating can be frustrating for drivers.

These symptoms can occur on a variety of cars, with the most common being on high mileage cars or older cars that have many years of driving. A driver might not necessarily notice these symptoms every day of driving their car, but they will build up over time and become more obvious during strenuous circumstances.

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If you find that your car hesitates when accelerating when the extra load is placed on the engine, this can be a sign that something is wrong with your vehicle.

There are many common causes of why your car hesitates when accelerating when you push the gas, with some of the causes being minor and some being more serious that requires immediate attention.

Clogged Mass Airflow Sensor

When trying to answer why your car hesitates when accelerating, the most common reason might be the mass airflow sensor is clogged or damaged. The mass airflow sensor, also known as the MAF sensor , is connected to the air cleaner.

Since the airflow sensor works to measure the air flowing into the engine, a clogged sensor can lead to incorrect data being sent to the engine control unit to calculate the necessary air and fuel mixture ratio.

  • The average price for replacing your mass airflow sensor is around $300 on average, with the labor around $60 and the parts costing drivers typically around $240. The labor is not that extensive, so most mechanics should have this procedure done in under one hour.

Low Fluid Levels

If you have an automatic transmission and notice the transmission is slipping, the most common cause of your car lacking acceleration is low transmission fluid. The transmission fluid is necessary to lubricate the parts and keep your car running correctly.

If you are asking yourself what causes ‘the car hesitates when accelerating,’ this means that the car engine revs, but no power is transferred to the wheels to move the car. If the lack of power is caused by low transmission fluid, it will only get worse as the transmission overheats and friction builds during use.

Checking Transmission Fluid

To figure out why the car hesitates when accelerating, you need to learn how to check your transmission fluid level . In many cars, you can do this easily by using a dipstick in the engine bay. The dipstick will look like the oil version but will be red. The transmission dipstick is located near the engine bay by the firewall.

Start by making sure the transmission fluid is warm and ready to be checked. Insert the dipstick into the transmission fluid – when you pull out the dipstick, note the color and the fluid condition. If you have trouble noting any characteristics about the fluid, hold the transmission fluid to bright light and look for any contaminants. Any debris or shavings can indicate internal transmission damage.

Now, you must decide whether or not to change the transmission fluid. If you need to change the transmission fluid, look at the transmission filter and make sure it is free of debris. If the filter is cleaned, you can go ahead with the change to prevent your car from lacking acceleration when you press on the gas.

  • It costs around $100 to have your fluid changed by a dealer, service center, local mechanic, or independent garage. The typical price range is between $80 and $250 and depends on the other problems in your vehicle. The average cost is around $100 for both manual and automatic transmissions.
  • Along with changing the transmission fluid, you should look into replacing the fuel filter and the transmission fluid pan every time you change your fluid.

Burnt Transmission Fluid

Sometimes, the lack of acceleration while pressing on the gas pedal can be due to the transmission fluid burning instead of being at a sub-par level. If the fluid is not the right color, like black instead of red, or you notice a burning smell, this is likely the culprit. Burnt fluid can occur when the transmission overheats and there is too much friction. In this case, you need to swap out your transmission fluid to answer the question of why your car hesitates when accelerating.

  • A transmission fluid flush is typically between $127 and $250, which is approximately twice as much as a fluid change due to the extra fluid required to completely refurbish and change out the old fluid that is causing damage to your transmission. The average price comes to $150 for a fluid flush.
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Faulty Clutch

Both automatic and manual transmission use clutches . Although clutch problems are more common in manual cars, automatic cars can still have clutch issues. These clutch problems can cause you to ask yourself why your car hesitates when accelerating. In an automatic car, the transmission and the torque converter have clutch plates that can become worn out and damaged due to insufficient transmission fluid.

  • The average price of a clutch replacement is usually between $703 and $755, depending on the specific parts required for your vehicle. The labor pricing is between $526 and $664, making up most of the overall procedure’s price.

Spark Plug Misfire

A spark plug misfire can directly cause a lack of acceleration. Your car’s lack of power is another noticeable sign that you have faulty spark plugs in your car that can misfire. The lack of acceleration means your engine is unable to ignite and will be unable to increase power when the driver presses on the gas pedal.

  • Spark plugs are usually quite inexpensive when looking into how to fix why your car hesitates when accelerating – the price is usually between $16 and $100, with the labor costing between $40 and $150 for around 30 minutes to 2 hours of work.

Worn Gears

The gear set is a common reason you might ask yourself why your car hesitates when accelerating. The gear set in your vehicle needs clutches and bands to change the gears. The automatic transmission bands are usually the key issue in slipping and a lack of acceleration. If the fluid is not the issue, transmission slipping can be a sign of a worn band . The worn-out band and damaged band can allow parts of the transmission to continue to rotate for a few seconds after you change gears.

    • Fixing the signs of worn gears or a damaged band might sometimes require drivers to replace the entire transmission. In this case, they have to decide what is best for their specific vehicle and their car’s condition.
    • The average transmission repair costs between $1500 and $5,000 for most domestic cars in the United States, while major transmission repairs can cost as much as $2,000 in labor alone.

    Oxygen Sensor Malfunction

    An oxygen sensor monitors the exhaust emission within your car so that it can adjust the proper ratio of the air and fuel mixture as necessary to keep the engine running at an optimal performance level. If the car has too little fuel in terms of the air and fuel ratio, the engine will not combust at the proper time internally. Not to mention, the ignition timing will be off, leading to a lack of power and acceleration.

    If the oxygen sensor is damaged, it will not send the information about the fuel to the engine control unit in your vehicle. The engine of your car will then have no idea how to adjust the ratio, resulting in a lean or too rich fuel mixture. A maladjusted fuel mixture can lead you to ask the question of why your car hesitates when accelerating.

    • Replacing your o2 sensortypically ranges between $20 and $94 depending on the brand of the sensor and the severity of the other damage in the car. The labor of a mechanic will usually range between $113 and $478 in total for parts and labor.

    TPS Malfunction

    The throttle position sensor, also known as the TPS, is in charge of monitoring the air intake within your engine. Usually housed in the valve that regulates the fluid from the shaft, the position sensor monitors the throttle’s position, sending this data to the engine control unit.

    One of the most important sensors in your vehicle sends signals to the transmission control module and is called the throttle position sensor. If the throttle position sensor cannot send the proper data to the transmission, this lets you know as the river that your transmission control module is bad. To determine if the position sensor is faulty, you might need to perform a throttle position sensor test.

    Performing this test in your car can help you figure out why your car hesitates when accelerating. Some common signs of a throttle position sensor malfunctioning are jerking while accelerating, trouble changing gears, car stalling during use, engine surging, warning lights on the dashboard coming on, and a decrease in fuel efficiency.

    • Replacing your throttle position sensor is usually between $171 and $219, with the labor between $59 and $74 or between 45 minutes and 1 hour of work. The parts are usually between $112 and $145 for this procedure.

    Faulty Solenoids

    Along with the reasons above as to why your car might be lacking acceleration and power, there are other reasons for your transmission and engine problems. The gear changes in the automatic transmission are sparked by the transmission solenoids, functioning to allow the automatic transmission fluid to move and the clutches and bands to disengage from the system.

    The problem with having the automatic transmission slipping when accelerating and lacking power is that it can be hard to determine what is going on inside the automatic transmission. Without the tools at a mechanic’s shop or tearing the transmission apart to look for worn or damaged parts, this diagnostic process can be lengthy. The computer-controlled transmissions have error codes that you can determine from diagnostic tools, similar to how your engine might notice the check engine light.

    As soon as you figure out that you might have faulty solenoids, you can get one step closer to determining why your car hesitates when accelerating.

    • Depending on the make, model, and year of your car, replacing a single faulty transmission solenoid is around $250 for most vehicles. Unfortunately, replacing an entire pack to fix your car’s faulty symptoms can cost around $400 on average.

    Missing Gears

    Missing gears can occur from a low amount of transmission fluid, resulting from water getting inside the transmission, a lack of routine maintenance, fluid contamination, or a transmission fluid leak. If you do not fix this issue, it can lead to your engine overheating, excessive friction, and serious damage that can cause you to ask yourself why your car hesitates when accelerating.

    • The cost to have your fluid changed by a dealer ranges from $80 to $250, with the average price being around $100 for manual and automatic cars.
    • Along with changing your fluid for around $100, you should replace the transmission filter and the pan.
    • You will pay between $250 and $340 to have your transmission filter changed, with the labor between $!00 and $125 for 1-2 hours of work, and the parts ranging from $150 to $215.

    The Bottom Line

    By figuring out the most common symptoms as to why your car hesitates when accelerating, you can prevent these issues from leading to expensive repair prices. Knowing the main reasons why your car hesitates when accelerating can help you find out what part you may need to replace within your vehicle, like the solenoids, throttle position sensor, spark plugs, or worn-out gears!

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