Car Won't Accelerate Past 40: Reasons & Fixes

When a car suddenly stops accelerating after 40 mph, it is hardly a common issue. Unfortunately, there are a lot of possible reasons why this can happen to your vehicle. Diagnosing the root of this problem can take time and certain expertise.

If you press the gas pedal further and further, but the car refuses to go faster than 40 mph, it can be a good idea to stop in a safe spot and begin to troubleshoot potential problems. The reasons range from simple to fix to really dangerous issues that require professional interference.

Key Takeaways:

• Preventive care is pivotal for the vehicle's longevity and optimal performance. Routine check-ups can identify budding problems, helping car owners save time, money, and potential risks in the future.
• Properly diagnosing the exact cause requires keen observation, specific diagnostic tools, and, sometimes, expertise.
• There are numerous reasons why a car might not accelerate past 40 mph. These reasons can range from simple factors like a mat obstructing the gas pedal, low fuel levels, or a partially engaged handbrake to more complex mechanical issues such as malfunctioning spark plugs, a clogged fuel filter, or issues with the throttle position sensor.

Usually, it’s a clogged fuel filter that causes your car to lose power after 40 mpg. Over time, the fuel filter can get dirty and lose its ability to let the filtered fuel flow to the tank. This can lead to a sudden drop in acceleration.

To determine the actual reason, you will have to gather as much information as possible. Here, we collected a checklist of sorts to help you identify the problem and find the best solution to it.

Why Won’t My Car Go Over 40 mph?

This is one of the most complicated issues to diagnose in your vehicle. All because there are at least 15 different reasons why your car won’t go over 40 mpg. But even after you accurately pinpoint the issue, the work is far from being done.

Often, this is not just some random problem but an effect of a more serious underlying issue. The best thing you can do after dealing with this is to get your vehicle thoroughly checked by an experienced car mechanic.

1. Rubber Mat Stuck Under the Gas Pedal

Get ready for the most embarrassing reason why your car can stop accelerating past 40 mpg, which you will never share with anyone. A rubber mat under your feet can be the culprit, and I suggest checking if it’s not a piece of rubber that blocks the pedal from going any further as your first step.

Fix the mat position and it should be okay after that. This kind of weird thing happens to all of us.

2. Low Fuel

You should get notified before the fuel level becomes an issue, so this is rare. But low fuel levels can cause your car to lose power after 40 mpg or somewhere close to that mark. This happens because the fuel pump can pick up air or might not be entirely submerged, which can lead to inconsistent fuel delivery and overheating of the pump.

Low fuel can also cause another issue. If there is sediment at the bottom, the pump can pick it up and clog the system in a similar way, preventing fuel from getting further.

Another potential problem is maintaining pressure when there is too little fuel left. If the fuel level is critically low, the fuel pump will struggle to sustain the right pressure. It leads to erratic performance and power loss.

Some vehicles actually have a safety feature that allows your car to avoid problems when you are low on fuel. It is called a “limp mode”. When the fuel level is critically low, this mode protects the engine from potential damage.

3. Handbrake Not Fully Disengaged

If the handbrake is partially engaged, it creates drag that can lead to overheating and other problems. As a result, the engine has to work harder to overcome the drag that leads to a drop in performance.

This not only affects the acceleration but can also cause significant wear and potential damage to the brakes. So, checking the position of your handbrake should be on your checklist, just in case, whenever you start the vehicle. Unfortunately, the warning light might not come on when the brake is only partially engaged.

4. Clogged or Dirty Fuel Filter

A clogged fuel filter restricts the normal fuel flow through the system. This can result in insufficient engine power and cause your car to not go over 40 mpg. It can also trigger issues like hesitation during acceleration, poor fuel economy, engine misfires, and even potential engine stalling.

To diagnose a clogged fuel filter, you should listen to unusually loud noise from the pump and look for signs of rust, dirt, or leaks. You can also use an OBD-II scanner to read the codes that might point to fuel delivery problems.

The best solution is often to replace the fuel filter and observe if the issues are resolved. It costs around $215 at a dealership service center or even less if you can do it yourself.

5. Malfunctioning Fuel Pump

You can diagnose a faulty fuel pump by sudden power surges and the engine sputters at high speeds before it causes your car to stop accelerating after 40 mpg. A fuel pump's main function is to transport fuel from the tank to the engine. When malfunctioning, it might not supply enough fuel, especially during acceleration.

A simple fuel pressure test can determine if the pump is working correctly by measuring fuel line pressure. If the results are below the advised level, the pump could actually be at fault. You must address a suspected faulty fuel pump immediately because driving can cause even more issues.

6. Faulty Oxygen Sensor

Finally, we have finished with the fuel pump, but there is one more thing inside the fuel system that can cause your car to lose power. A bad oxygen sensor can potentially cause a car not to accelerate past 40 mph because it can send incorrect data, leading the engine to run either too rich (too much fuel) or too lean (not enough fuel).

You will not miss this issue because the first symptom will be a check engine light on the dashboard. Sometimes, rough idling or stalling and Codes related to the oxygen sensor, such as P0130 - P0167 can lead you to the issue. Proceed to replace the sensor to fix this issue and enjoy the ride.

7. Air Intake System Blockage

Similarly, a blockage in the air intake system can cause a car not to accelerate past 40 mph. Your engine needs a very specific air-to-fuel ratio to function and once there is not enough air, you slow down and lose performance.

When there is a serious leak or a block, you can even hear hissing. But usually, it’s the drop in performance and a visual inspection that lead you to the root of this problem. In severe cases, there can be a check engine light on the dashboard as well.

If your MAF sensor is dirty but not damaged, it can be cleaned. But it is better to let the professionals do it. However, if the sensor is damaged, just replace it for good. And do not forget to inspect the intake hoses and tubes for any cracks, holes, or other damage.

8. Malfunctioning Spark Plugs

Surprisingly enough, even the spark plugs can make your vehicle lose power. The reason is simple, faulty spark plugs do not produce enough combustion. This means the engine isn't producing power as efficiently as it should.

Bad spark plugs can also cause poor acceleration, misfires, rough idling, and other related issues. If the spark plugs have gone bad, the best course of action is to replace them. When replacing spark plugs, ensure you use the recommended type and gap size for your vehicle, or let the shop do it for you.

Regular maintenance can save you from this and some other issues on the list. Keep that in mind, prevention is always better than dealing with these issues.

9. Worn Out or Damaged Clutch

Clutches get worn out and damaged over the course of their service. Unfortunately for you, this means they can be the reason why your car won’t accelerate. When the clutch slips, it prevents the engine's power from effectively reaching the wheels.

The damning clue of a slipping clutch is that the engine RPM increases significantly when you press the gas pedal, but the car doesn't accelerate proportionally. This does not happen with other problems on our list. Another clue is a weak response from the pedal, like it suddenly lost all the resistance.

I would not recommend dealing with the clutch in your garage. Instead, consult a professional mechanic. Only they can thoroughly inspect the car and do the necessary repairs correctly. You never really know if the problem will come back too soon if you don’t address the true cause.

10. Transmission Control Module Issues

The TCM is often to blame in vehicles with automatic transmissions. If the TCM gets damaged or malfunctions, it can get stuck in a lower gear, leaving your car unable to accelerate any further.

But it can also be a response from the TCM to a bigger issue. In some cases, if the TCM detects a severe problem, it may put the vehicle in "limp mode," where it restricts power or speed to prevent further damage.

Asking a mechanic to check what’s wrong is a good idea. A TCM can malfunction due to many reasons, and it can get confusing really fast. Anything from wiring issues to a software update gone awry can be the reason.

11. Low Transmission Fluid

The TCM can react to a low transmission fluid level and limit your acceleration after 40 mpg to prevent overheating and friction damage. This fluid acts as a lubricant, ensuring the transmission's internal components operate smoothly. Without it, your car is in serious danger.

Low fluid levels can cause these components to overheat or not engage properly, which can lead to hard shifting, slippage, or the car being stuck in a particular gear. So look for signs like unusual whining or clunking noises or dashboard warnings about overheating. A delay in shifting gears can be an early sign before the real problems start.

An easy fix is to refill the fluid. Low transmission fluid can hinder a car's acceleration. To check the fluid level, use the transmission dipstick if available. Healthy fluid is clear and reddish. If the fluid appears brown, smells burnt, or contains particles, it likely needs replacement. A low fluid level may suggest a leak or neglect.

12. Malfunctioning Throttle Position Sensor

Normally, the TPS monitors the throttle position and sends this data to the car's computer. But when it starts malfunctioning, the data it sends is false. This discrepancy can cause poor acceleration.

Thankfully, a diagnostic tool can detect error codes related to the TPS. But let the local shop handle this one. They will find the problem and replace the faulty TPS and reset the car's computer if necessary.

13. Exhaust System Blockage

A blockage in the exhaust can limit your performance just as much as a faulty air pump. A damaged catalytic converter can, among other things, impact your acceleration. The most telling symptom is a lack of power even when the gas pedal is fully pressed.

To find the problem and resolve it, you need to inspect the exhaust system. Once you do, clean any blocked or replace any damaged components.

14. Malfunctioning Mass Airflow Sensor

The MAF is tasked with measuring the amount of air entering the engine, which is crucial for maintaining the correct air-fuel ratio. If something happens to the MAF, it can lead to poor engine performance in general. And the loss of acceleration is only the top of the iceberg.

Using an onboard diagnostic tool (OBD) you can detect related error codes. This will narrow down your options a lot. You can clean the sensor as a first step, but if the issue persists, consider replacing it with a new one.

15. Brake System Issues Causing Drag on the Car

It’s not just the handbrake, but your brakes can also let you down like that and create drag. If the brakes are dragging, it means they aren't fully released even when the brake pedal is not pressed.

Besides the noise from below the car, you might notice a burning smell or even smoke coming from the wheels. First, you must check brake calipers, pads, and rotors. Replacing or lubricating parts like the brake caliper might even quickly solve the issue.

But if the brake system is hydraulic, ensure there's no air in the brake lines by bleeding the brakes. This is also a possible cause.

As you can see, timely maintenance and care can prevent almost anything of the above. By attending to your vehicle's needs, you can address minor issues before they escalate into major ones.