Pushing the gas pedal and feeling your engine stutter instead of immediate power, and you seem to lag? Don’t panic just yet. Sluggish acceleration is sometimes an easy fix.
Keep reading to discover the most common causes for hesitation while under acceleration before your mechanic starts changing perfectly good parts.
But, don’t ignore the issue since inconsistent and unpredictable acceleration can be a bigger issue than you realize.
What is Engine Hesitation?
When your car experiences engine hesitation, it can briefly lose power or “stumble” when you push down on the accelerator. Unlike stalling completely, hesitation is a momentary lack of response, almost like the car “hiccupped” when you tried to accelerate.
The most common symptom of hesitation is a lag or delay when pressing the gas pedal and your car responding. The engine may feel like it “misses” or loses power for a moment.
Hesitation can occur from a complete stop, while already moving, or when trying to pass a vehicle. If you’ve ever driven a manual transmission car and it was in too high of a gear, you know what it feels like.
Some drivers may also feel that the car feels jerky or surges slightly at steady acceleration. In cold weather, you may experience hesitation more until your engine warms up.
Common Causes of Hesitation When Accelerating
#1. Dirty/Faulty Mass Airflow Sensor (MAF).
The mass airflow sensor helps the car’s computer determine the amount of fuel needed for the engine’s air intake. Basically, a MAF sensor measures air entering the engine and sends data to the car’s computer, so the right amount of fuel can be sent as well (the optimal air/fuel ratio).
If the MAF sensor is dirty or completely fails, the sensor sends back incorrect readings of the air entering the engine, which will result in either too much fuel or too little fuel being sent to the engine from its fuel injectors.
The good news is that cleaning a mass airflow sensor generally solves the issue and it is easy to remove and clean, which you can do yourself with a special mass airflow cleaner (I personally like CRC cleaner).
If cleaning the sensor doesn’t work, then it likely needs to be replaced. Signs of a bad MAF sensor include rough idling, stalling a short time after starting the engine, high idle speed (or low idle speed), and the check engine light will typically illuminate when the sensor fails (and generally has fault codes that are specific to MAF sensor issues).
Cost to Fix:
If the MAF sensor is just dirty or is clogged and simply needs a good cleaning, you should expect to pay $10-20 for a mass airflow sensor cleaner spray, and maybe a few minutes of your time.
If you need to replace the sensor, expect to pay somewhere in the neighborhood of $100-$400 total. The cost of the mass airflow sensor itself is typically $30-$300 (and labor if you take it to a mechanic is probably another $70-$100).
Certain European and luxury vehicles have a higher parts cost, so you may need to expect costs to be closer to $500 total or more.
Make sure to verify that the MAF sensor is the issue, either by diagnosis from a repair shop or by scanning with an OBD2 scanner. You can go to many local auto parts stores are offer a free scan.
#2. Clogged Fuel Filter.
The job of the fuel filter is to act as a barrier, keeping rust, dirt, and debris away from your engine’s fuel system. Over time, those contaminants build up and restrict the flow of fuel.
When your fuel filter is clogged, your engine will struggle to get enough fuel during acceleration, which will result in hesitation.
Nowadays, most vehicles can go between 40,000-80,000 miles between fuel filter replacements (some can go longer). This is great news compared to older vehicles, which typically had a replacement schedule of every 20,000-40,000 miles. Your best bet is to stick to whatever the manufacturer recommends for replacement intervals.
The main symptom of a clogged filter presents itself and usually over time. You may first notice worse performance when accelerating up hills, and then start to notice hesitation during normal acceleration. If you leave it long enough, you will experience stalling and/or difficulty starting your car.
If you’re not sure when the fuel filter was last changed, it could very well be the source of the hesitation.
Cost to Fix:
Fuel filter replacement is one of the cheaper repairs and generally would only cost $70 to $160 total (just check the total with your chosen shop before consenting!). The fuel filter itself usually runs $10 to $50, so it is possible to consider that the labor for the change could cost more than the part!
Some vehicles make the fuel filter easy for DIYers, so you could consider changing it yourself. Most vehicles nowadays, do have the fuel filter as part of the fuel pump assembly, which can make it difficult and expensive to replace.
#3. Worn Spark Plugs.
Spark plugs generate the electrical spark that ignites the air-fuel mixture in your engine’s cylinders. When spark plugs get old or become fouled/fouling with carbon deposits, they can’t produce a strong enough spark to provide proper combustion.
The weak spark is usually quite obvious during acceleration when your engine needs it. Besides hesitation, old or worn spark plugs often show other symptoms such as a rough idle, bad gas mileage, and difficulty starting (especially when it’s cold outside).
The problem usually develops gradually, making it easy to overlook until it degrades performance significantly.
Good quality, modern spark plugs last from 50,000 to 80,000 miles or more, but they still wear down.
They may still work, but just poorly. The condition of your spark plugs may also provide evidence of other engine problems because they show different appearances depending on running conditions.
Cost to Fix:
Most four-cylinder engines typically cost $200 to $300 to fully replace the spark plugs. The plugs themselves range from $5-$20 each, depending on the type and quality of plug.
Labor costs usually run $100-$250, but a V6 or V8 engine, which has more plugs and can be harder to get to, can double the cost.
I have seen dealership quotes up to $900. If that is the case, an independent mechanic may be substantially cheaper for the same work.
For most vehicles, for many people, a simple, clean spark plug replacement can be a part of many DIY jobs. But there is always the possibility that a plug may break off during the removal step, and may even fall into the cylinder. If that happens, it can turn an easy job into a very difficult job.
#4. Bad Fuel Injectors.
Fuel injectors provide a very small and very precise mist of fuel into the cylinders of your engine at a very precise pattern and timing. Over time, fuel injectors can become clogged due to carbon deposits or simply wear out to the point where they no longer provide the mist required.
What is the result? The atomized fuel is not mixed with the air properly, which produces hesitation when accelerating.
Clogged fuel injectors usually occur due to age and mileage, or due to lower quality fuel or lack of maintenance on the car, which typically creates a slow change. You will often feel your car hesitate slightly or even run roughly before it gets severe enough to cause problems.
If you use high-quality fuel (with fuel injector cleaners in it) and pay attention to your fuel injectors from time to time, you will have a huge difference in the life of your injectors and your overall engine operation.
Many drivers have even used cleaning services (not necessarily professional, sometimes DIY) with significant gains, before having to replace fuel injectors!
Cost to Fix:
Professional fuel injector cleaning services to clean the injectors range between: $50-$150 (to clean the injectors). If your injectors are clogged beyond being flushed, the cost to replace your injectors differs significantly, but one price is: $50-$150 per injector plus labor ($200-$400 for the entire job).
Most mechanics typically recommend replacing all of the injectors (at least in four-cylinder engines) after the failure of one injector. The normal cost will be about $600-$1,200 for parts/labor to replace all injectors.
If you have a six- or eight-cylinder engine, expect to pay substantially more for repairs/changes. This job it would be wise to get multiple estimates!
#5 Bad Fuel Pump.
The fuel pump does just that: pumps fuel from your tank to your engine, which requires a certain amount of fuel at a certain pressure at a certain time. The fuel pump’s location can either be inside or outside of your fuel tank, and it will be running as long as your engine is running.
When a fuel pump begins to fail, it is unable to keep the right fuel pressure your engine needs to accelerate properly.
Typically, you will notice problems first when you accelerate up a hill or when you try to pass another vehicle at highway speeds.
In some cases, you may be warned by your fuel pump giving off some sort of weird noise from the back of your vehicle, or your engine may backfire or sputter intermittently, even while driving normally.
In serious situations, your car may stall completely, particularly when climbing steep hills and/or when it’s hot, as the fuel pump will work harder under these more difficult situations.
Cost to Fix:
Replacing a fuel pump on average costs between $250 and $1,200, with some vehicles costing up to $1500 (or more). The part will cost between $100 and $600, with labor between $150 and $600.
Replacing just the fuel pump relay can cost between $50 to $150 total, but it’s often an easy DIY replacement, and the part costs $25 or less.
#6. Throttle Position Sensor (TPS).
The throttle position sensor reads how far down you press the gas pedal and the throttle plate’s position in the throttle body. This information is sent to your car’s computer. The computer uses this information to adjust engine timing and the air-fuel mixture accordingly.
When the TPS fails, the car’s computer receives incorrect input on how much power you’re requesting. If your TPS is not working properly, you can have hesitation, surges in power, and/or uneven acceleration.
Most modern vehicles use redundancy for safety, and have either multiple position sensors or two different sensor types.
Common indicators of a bad TPS are a sudden change in engine speed while maintaining a steady gas pedal position, an inability to maintain constant cruising speed, or an unexpected change in idle speed when pressing the gas pedal slightly.
If there is a TPS problem, your check engine light will usually be lit, often with your car entering “limp mode” (a safety feature built to prevent engine damage). Unlike some other issues that can slowly degrade, TPS problems often come on suddenly.
Cost to Fix:
On average, replacing a TPS will cost $150 to $350 total. The sensor normally costs $75 to $250.
Labor typically costs $75 to $150. When the TPS is integrated as a component in the throttle body assembly, replacement costs can be higher at $500-$800 total.
Related Issues/Symptoms
There could be other engine problems causing similar symptoms or happening at the same time. Knowing about them may assist you in pinpointing the actual problem and possibly avoiding misdiagnosis.
Vacuum Leaks
Some small cracks or loose connections in vacuum lines may create air leaks, throwing your engine’s air-fuel mixture out of whack. Vacuum leaks can create hesitation, especially with light acceleration, but may be resolved when heavy throttle is applied.
Carbon Buildup
Newer direct-injection engines can build up excessive carbon deposits on the intake valves. This would accumulate slowly and result in consistent rough running, hesitation, and eventually a loss of power.
Transmission Problems
It’s very possible, what you are feeling as hesitation is actually a transmission problem! If you only notice the hesitation during gear changes, or you feel a significant delay between pressing the gas and actually feeling acceleration, you may want to get your transmission fluid checked.
Temperature Dependent Problems
You could possibly find it helpful to think of hesitation as being connected to temperature or conditions.
For instance, if you are only noticing hesitation when the engine is warm from temperature or under certain weather conditions, this could further limit the possibilities.
Here is an example of possible hesitation related to temperature or conditions:
- Hesitation when cold (weather) most usually indicates problems with the fuel delivery system or the spark plug.
- Hesitation when hot (weather) usually indicates problems with the fuel pump or vapor lock.
- Hesitation under high humidity may indicate weakness in the ignition/spark system.