A car that is hesitating to accelerate is a very soft and subtle symptom, but also one that can cause damage to other components given enough time. Luckily, almost all potential causes of engine hesitation are relatively inexpensive to fix or replace, so there is not much reason to delay addressing the issue.
If your car’s engine hesitates during accelerating, a dirty or faulty MAF (mass air flow) sensor is one of the most common causes. Other, also common causes include a faulty throttle position sensor, a weak fuel pump, and a clogged fuel filter.
And lastly, you could be looking at worn-out spark plugs, a failing ignition coil, or a major vacuum leak, but these will also present other symptoms that we will cover later on.
However, pinpointing the exact culprit that can cause your car to hesitate during acceleration is a bit difficult and time-consuming, especially if you don’t get any fault codes, which often happens with hesitation. But not to worry, as we will also explain how to test many of these components so you can avoid wasting money on replacing the wrong parts.
Nonetheless, we’ll explain how to properly diagnosed a car hesitation or stumble issue, so you can avoid wasting money on replacing parts that aren’t the root cause of the issue.
In This Article Show
Best Way To Diagnose Engine Hesitation
If you notice your car hesitates when you try to accelerate, you should first scan the fault codes. Even if the check engine light isn’t on, soft codes will still be saved in the ECU (engine control unit) memory. Now, to scan the fault codes, you need an OBD2 scanner or an OBD2 Bluetooth adapter.
You can find both on Amazon for $20 to $30, and they are well worth your money. Alternatively, you can visit your local Auto Zone shop, and they will scan the fault codes for free. Lastly, you will find fault codes for each potential cause under its own subheading, however, note that each of those won’t necessarily trigger a fault code.
What’s Causing Your Car to Hesitate on Acceleration?
Here, we will quickly explain what happens in an engine when it hesitates, and the main reason is a lean air/fuel mixture. In other words, too much air going into the engine or insufficient fuel, depending on the cause.
That usually happens when you accelerate because the ECU then needs to adjust the air/fuel ratio. And with any of these faulty components, that process is delayed, and for a brief moment, the engine gets a lean air/fuel mixture and struggles to accelerate.
The second probable reason is misfiring in one or more cylinders. When a misfire happens, the affected cylinder isn’t working because the fuel isn’t ignited and becomes dead weight. Naturally, in that case, the engine loses a significant amount of horsepower and struggles to accelerate.
However, with a misfiring engine, you will also notice the car is shaking or vibrating, making it easier to narrow down the cause. And lastly, misfires are most noticeable when the engine is under load, such as when accelerating, but are apparent when the engine is idling.
7 Reasons Why Your Car’s Engine Hesitates During Acceleration
Now, we will dive deeper into all the potential causes, how you can test them, their other symptoms, fault codes, the cost, and how to replace them. And we will start with the most common cause of engine hesitation, the MAF sensor.
1. Dirty or Faulty MAF Sensor
The MAF sensor sits right after the air filter and measures the amount of air entering the engine. The ECU receives that information plus other parameters from additional sensors and adjusts the air/fuel ratio accordingly.
However, when the MAF sensor starts to fail and sends wrong information to the ECU, or it gets dirty, the car will present symptoms like reduced power, rough idle, and poor fuel economy.
But other than that, the most common symptom is the car hesitating to accelerate for a couple of seconds when you first press the gas pedal. You might also notice hesitation when driving uphill or when towing a heavy load. Now, if the MAF sensor fails, you will get a clear and undisputable fault code, unlike with a dirty MAF sensor.
So, the first course of action is to scan the fault codes and replace the MAF sensor if you get any of the following:
However, getting the following fault codes means the air/fuel mixture is lean for an unknown reason. And in that case, clean the MAF sensor.
How To Test a MAF Sensor
Testing a MAF sensor is pretty simple. All you need to do is unplug the sensor and take your car for a drive. After that, if you notice the car isn’t hesitating anymore, you have found the problem. In case you didn’t get any of the P0100 to P0104 fault codes, the first course of action is to clean it. And if that doesn’t work, replace it.
How To Clean or Replace a MAF Sensor:
If you need to clean the MAF sensor, use only a designated MAF sensor cleaner spray. Other carburetor, throttle body, and intake cleaners will damage the MAF hot wire.
- Open the hood and locate the MAF sensor. You can look for the air filter box and follow the intake hose until you see the sensor.
- Unplug the MAF sensor connector. The connectors are different between cars, but a small flat screwdriver might help you.
- Unfasten the two hose clamps holding the sensor and remove it.
- Once removed, look inside the sensor, and you will see the thin wire right in the middle. Now, take the MAF cleaner and spray it on the wire with three to four quick squirts.
- Let the MAF dry for a couple of minutes, and install it in reverse order. But make sure to install the sensor in the correct orientation. You will see a small arrow on it, which indicates airflow, so make sure the arrow is pointing toward the engine instead of the air filter box.
2. Faulty TPS (Throttle Position Sensor)
The throttle position sensor is essentially a potentiometer attached to the throttle valve. And as you press the gas pedal, the throttle valve opens, moves the potentiometer, and it sends a voltage signal to the ECU.
Now, when the TPS starts to fail, its reaction time will slow, leading to a delay between you pushing the gas pedal and accelerating the car. Also, in the case of the TPS sensor, the car will hesitate every time you open the throttle regardless of driving conditions, be it uphill, downhill, under load, etc.
Moreover, the car could also stall while driving or idling, and newer cars could even experience surging while cruising.
Now there are two different types of TPS sensors. The first is found on older cars, and it’s essentially a standalone unit. However, the TPS is integrated into the electronic throttle body in modern cars. In that case, you need to replace the throttle body, which is more expensive and complicated.
For example, the standalone TPS costs around $10 to $30, while an electronic throttle body costs $100 and $200. Also, the labor for replacing the TPS is between $55 and $69, according to repairpal, while the labor for a standalone TPS is between $87 and $109.
Now, you could replace both yourself, but I would recommend leaving that to the professionals if you don’t have much experience, although I will briefly explain how it’s done.
But before that, you should first check the fault codes to make sure the TPS is faulty, and if you get any of the following, replace it.
How To Replace an Electronic Throttle Body
The biggest issue with replacing the throttle body is that it’s more often than not buried under other components. For example, in some cases, you even have to remove the intake manifold, which is a major job. So, the best thing you can do is check how it’s done on your specific car. But here is what the job essentially looks like.
If you are confident enough to try replacing the throttle body alone, here is a video to help you.
- Locate the throttle body and remove any parts that are in the way, such as intake hoses, plastic covers, etc.
- Unfasten the intake hose on the throttle body and remove it.
- Unplug the throttle body connector.
- Remove any small vacuum hoses that could be attached to the throttle body.
- Now, remove the four bolts holding the throttle body to the intake manifold and remove it.
- Replace the throttle body with a new one and install everything in reverse order.
How To Replace a Standalone TPS
Also, here is Scotty Kilmer demonstrating the whole process.
- The TPS sits on the throttle body, on the opposite side of where the throttle cable attachment is.
- First note, mark or take a photo of how the old TPS is adjusted. The TPS has adjustable bolt holes, and it’s crucial to install the new one in the same position the old one was.
- Unplug the TPS connector.
- Unfasten the two bolts holding it and remove the TPS.
- Install a new TPS sensor and adjust it like the old one.
3. Worn-Out spark plugs
Bad sparkplugs are somewhat difficult to diagnose because they don’t have any specific fault codes other than the standard misfire ones. Still, suppose you notice other symptoms like rough idle, difficulty starting the engine, and especially the car vibrating or shaking when trying to accelerate or while cruising. In that case, chances are the sparkplugs are worn out.
So, if you notice these symptoms, you don’t get any fault codes, and you have done more than 30,000 miles since you last replaced them, then sparkplugs are a good place to start. First of all, they should be replaced every 30,000 miles, plus they are fairly inexpensive, and you can replace them at home.
Lastly, the most common fault code for spark plugs is a P0300 which means a random misfire.
How To Replace Sparkplugs
- First, you need a correct-size spark plug socket. These are deeper than the standard sockets but are usually included in socket wrench sets.
- Then, open the hood and remove any plastic covers that might be over the spark plugs.
- Now, unplug the first sparkplug lead, remove the old sparkplug and replace it with a new one. After that, plug the sparkplug lead back on.
- Now, repeat the process on all sparkplugs, replacing them one at a time to avoid mixing up the sparkplug leads.
Even though it’s one of the easiest automotive DIY jobs, a video always helps if it’s your first time doing it.
4. Failing Ignition Coil (Coil-on-Plug)
The coil-on-plug ignition coils sit on top of the sparkplugs, replacing the old sparkplug leads and the ignition distributor. The job of these ignition coils is to fire the sparkplugs when the ECU commands and to increase the input voltage.
Now, these ignition coils almost always fail intermittently, and they stop working only when the engine is under load, such as when you accelerate, drive uphill, or tow a heavy load.
When that happens, you will also notice a flashing check engine light, the car shaking and vibrating, and the car hesitating to accelerate.
Since these coils are expensive, you should first scan the fault codes to see which one is failing so you replace the correct one. Here are the fault codes you will get.
Here each code corresponds to a specific cylinder. For example, the P0301 cylinder number one misfire, the P0302 cylinder number two, and so on. You should check your owner’s manual to see how the cylinders are numbered in your car.
How To Replace Ignition Coils
- Open the hood and remove any plastic covers that might be over the spark plugs.
- Count the cylinders to see which coil you need to replace.
- Unplug the ignition coil.
- Unfasten the three or four bolts holding the coil and remove it.
- Install a new coil and assemble everything in reverse order.
Here is a neat video of the whole process to help you out.
5. Weak Fuel Pump
Even though, in most cases, the fuel pump fails suddenly and without any warning signs, it can still happen that it weakens first, especially if it’s older. When that happens, the fuel pressure will drop, leading to a lean air/fuel mixture and, consequently, hesitation. Furthermore, the car could stall during hard acceleration, shut down when idling, and even fail to start.
Unfortunately, there is no easy way to test the fuel pump when it presents these symptoms. To do that, you need special tools and, in most cases, a lot of experience. So if you suspect the fuel pump is the problem, it’s best to take your car to a repair shop.
Other than that, you are looking at anywhere between $100 and $250 to replace it for both parts and labor. And that’s for smaller cars. If you have a truck, or any other vehicle where the fuel tank needs to be removed, the cost will be closer to $300 to $400.
6. Clogged Fuel Filter
The fuel tank in your car and the underground fuel tanks at a gas station contain small debris and other contaminants. That’s why all cars have a fuel filter, to keep those contaminants away from the engine. However, the fuel filter will naturally clog over time, decreasing fuel flow and fuel pressure.
In turn, your car will present the same symptoms it does with a weak fuel pump. Namely, hesitation, stalling, and difficulty starting. Unfortunately, the fuel filter can’t really be tested other than performing a fuel pressure test, for which you need special tools. But even then, you can’t narrow it down to the fuel pump.
However, you should replace the fuel filter every 20,000 miles; plus, all repair shops leave a sticker with the mileage it was last replaced. That sticker can be on the door sills, somewhere in the engine bay, or in your service book. So, the best thing you can do here is look for that sticker or try to remember when it was the last time you replaced it.
Luckily, fuel filters usually cost between $15 and $30, plus about $30 to $50 for labor. In theory, you could replace the fuel filter at home, but since it’s under the car, it’s not DIY-friendly. Plus, the labor isn’t that expensive, and in the end, it makes much more sense to have a repair shop do it.
7. Vacuum Leaks
Vacuum leaks refer to any air that goes into the engine that’s not accounted for by the MAF sensor. That can be through cracked vacuum hoses, cracked intake boots, the oil filler cap, and a whole slew of other components.
Now, with a vacuum leak, the engine will intermittently run lean, and you will notice that the engine speed fluctuates while idling, the RPMs increase when changing gears, and you might even have some hissing noises in the engine bay. And naturally, the engine will have a decrease in performance, fuel economy, and poor throttle response.
If you notice any of the above symptoms, there is no need to look further; you can be certain it’s causing a vacuum leak. However, if you only notice hesitation, scan the fault codes, and if you get a P0171 or a P0174, it’s a good idea to perform a smoke test.
To do that, you can either take your car to a repair shop and have them do it, which will cost anywhere between $60 and $150, according to carfromjapan.com. Alternatively, you can buy a smoke tester or make your own, which is what I have done.
But instead of me explaining how to do it, here is a video where I learned how to make it. You will also see how to perform the test.
Frequently Asked Questions
Is hesitating acceleration a serious problem for my car?
No, car hesitating isn’t a serious problem in terms of long-term damage, although some can occur if you keep driving for thousands of miles. Furthermore, the most common causes are easy and cheap to fix.
Car Hesitates When Accelerating No Check Engine Light
If your car hesitates but has no check engine light, all the above causes are still possible. That’s everything from a faulty MAF sensor, faulty ignition coil, and a weak fuel pump to a vacuum leak and everything in between we have covered.
In the end, diagnosing a hesitating car is the most difficult part of the job. So, to make the process as easy as possible, it’s best to scan your car for fault codes and come back to this page to see what the code means and how to fix the problem.
However, if you don’t get any fault codes, starting the diagnosis process with the MAF sensor is best since it’s the easiest one to test. And after that, move further down this article until you find the exact culprit.