19 Reasons Why Your Car Won’t Accelerate Past 40

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Do you feel like your highway drive could be more active? You notice that your car won’t accelerate past 40; it’s a signal to investigate. As a driver, it’s crucial to acknowledge and swiftly resolve this car acceleration problem.

Spotting your car won’t accelerate past 40 mph might stem from issues like a trapped mat near the gas pedal, insufficient fuel, a partially engaged handbrake, a congested fuel filter, a problematic fuel pump, or a faulty oxygen sensor – among various possible reasons.

Before moving on to the causes, let’s find out what to do if the car suddenly stops accelerating on the road.

What Do I Do If My Car Suddenly Stops Accelerating on the Road?

Activate your emergency signals if you’re unable to speed up, informing nearby drivers.

Then, locate a secure spot to pull over, such as the road’s shoulder, public parking areas, or designated turn-outs, ensuring safety for all.

If your vehicle’s speed is significantly below the minimum limit, towing or roadside help might be necessary for safety reasons.

19 Reasons why a car won’t accelerate past 40 and possible fixes

1 – The Quick Fixes

Occasionally, it’s a straightforward matter you can recognize and resolve on your own. Professional assistance might be necessary for certain problems, but typically, they don’t signal a major engine issue.

Mat Stuck Under Gas Pedal

At times, floor mats might shift and get lodged beneath the gas pedal. It often occurs when mats aren’t correctly secured or don’t fit the car’s dimensions.

It can hinder the gas pedal’s movement, restricting full acceleration.

How to Fix:

The solution here is relatively straightforward.

Check the region near the accelerator pedal. If the floor mat is causing the blockage, adjust or remove it properly. Ensure it doesn’t hinder the pedal’s motion.

Think about buying non-slip mats or fasteners to keep the mat secure and avoid potential problems later on.


High altitudes often bring about reduced air density, which can lead to cars struggling on steep inclines, as the decreased nitrogen and oxygen content in the air results in roughly a 20% power reduction for the vehicle.

How to Fix:

Maintain cleanliness of air filters and fuel injectors to lessen the burden on your car.

Brake System Issues:

When the emergency brake is partially or fully activated, it pushes the brake pads against the rear tires, resulting in friction that impacts acceleration. Additional indications of problems with the braking system include an odor of burning or uneven tire wear patterns.

To identify concerns with the brake system, examine the brakes for signs of deterioration, leaks, or components showing wear. Equally important is a thorough assessment of the brake calipers to verify their effective release.

 How to Fix:

Resolving this car acceleration problem involves releasing the emergency brake. We advise consulting a professional to verify the condition of rear brake pads, bearings, and wheel drums post potential friction-induced warping. You should fix or substitute any affected parts.


Do you know what occurs when the car’s fuel tank is close to depletion? It might hold the solution when your car wont accelerate past 40.

The engine faces difficulty in acquiring sufficient energy for acceleration when the fuel tank is nearly empty, while subpar fuel containing sediment and grime can obstruct filters and lines.

How to Fix:

Simply refill the tank with the right amount and proper fuel needed.

2 – Dirty Car Parts

Over time, dirt buildup may impact how swiftly your vehicle accelerates. We suggest routine maintenance and repair, as a multitude of components collaborate to uphold optimal car functioning.

Yet, excessive grime on a single part can impede performance. The subsequent components necessitate frequent cleansing or substitution:

Dirty Spark Plugs:

damaged spark plugs

Spark plugs initiate the combustion process within the engine’s chamber, generating the power needed for vehicle movement.

As time passes, spark plugs can accumulate grime or experience wear, resulting in inefficient combustion and decreased engine potency, causing limited acceleration. Your car can even die when you put it in gear if the spark plugs become corroded.

How to Fix:

Begin by allowing the engine to cool down and disconnecting the battery terminals. Locate the spark plugs connected to the ignition coils. Separate the coil from the problematic plug, then use a socket and wrench to extract the old plug. Verify the gap on the new plug, hand-tighten it, and then apply the manufacturer’s specified torque. Finally, reconnect the plug to the auxiliary components.

Problems with the Fuel Injector:

The component responsible for delivering the precise fuel quantity into the combustion chamber is the fuel injector. When it’s obstructed or malfunctions, the engine misfires, limiting speed.

Detecting fuel injector issues involves noting engine misfires and pronounced vibrations when you start your car and even during idling. However, resolving it might require professional intervention due to its integrated nature within the engine assembly.

How to Fix:

Make sure to get it fixed by a professional.

Clogged Fuel Filter:

The fuel filter carries out its role in safeguarding the engine against particles, grime, and muck. Accumulation of dirt has the potential to harm the fuel tank when the filter becomes blocked, resulting in the clean fuel being unable to reach the engine and subsequently hindering optimal acceleration.

How to Fix:

Find the fuel filter in your vehicle, examine it, and swap it out if it’s soiled. Periodically, remember to inspect and change the filter more frequently for the best engine operation. You can even change the fuel filter by watching the following video.

Clogged Air Filters:

Similar to how the fuel filter delivers pure fuel to power the engine, the air filter ensures clean air circulation. This combination is directed to the combustion chambers to facilitate the combustion process. When a blockage occurs, the balanced mixture is disrupted, and that’s why your car won’t accelerate past 40.

How to Fix:

Locate your car’s air filter. Inspect and replace it if it is dirty.

Frequently consider checking and replacing the filter more often to ensure optimal engine performance if you drive in dusty or dirty conditions.

Malfunctioning Mass Air Flow Sensor:

A faulty mass air flow sensor can lead to difficulties in achieving the desired speed since this component detects incoming air volume, directing the ECM to regulate the fuel supply. Indications of its dysfunction involve weakened acceleration, stalling, or uneven surges.

To identify such an issue, assess the sensor’s voltage signal accuracy to the ECM through multimeter or scanner testing.

How to Fix:

Replace the malfunctioning sensor with a new one.

Malfunction In Oxygen Sensor:

The oxygen sensor plays a role in observing the engine’s exhaust emission levels, essentially assessing the fuel-to-air ratio across the entire engine.

When this sensor malfunctions, the car’s computer won’t obtain the fuel and air data, leading to inconsistent acceleration, regardless of pedal input.

How to Fix:

Swap out the faulty sensor with a suitable original replacement endorsed by the manufacturer. It should be done after a thorough expert diagnosis and adhering to proper installation protocols. This will result in better engine performance and a more seamless acceleration experience.

  1. Technical Issues

Vehicles depend on various mobile components for optimal performance. A breakdown becomes unavoidable if these components aren’t up-kept, fixed, or renewed following usage. Regular car maintenance is crucial to steer clear of the subsequent technical issues:

Engaged Limp Mode:

Contemporary vehicles feature a limp mode, referred to as limp home mode, which is a safety function triggered when the car’s computer identifies a significant problem that could harm the engine or transmission, often caused by sensor problems, insufficient fluid, gasket issues, or internal transmission problems.

This mode restricts the car’s speed and engine power, averting additional damage and impeding acceleration.

How to Fix:

On occasion, the limp mode might activate unintentionally. For example, the limp mode activates even without inspecting the engine lights or alternate alerts. If this happens, you can guide your vehicle to the roadside and restart.

Nevertheless, our experts advise you to seek help promptly if the dashboard shows numerous warning lights.

Worn Out or Damaged Clutch:

An aging or impaired clutch might be the reason your car won’t accelerate past 40 since it facilitates power transfer from the engine to the wheels. Indications encompass tough gear shifts, a burnt odor, and a loosely responsive pedal.

Detecting clutch problems involves test-driving to identify slippage or gear-shifting issues.

How to Fix:

A replacement becomes necessary when the clutch shows signs of wear or harm. This task demands specialized tools and expertise, so we suggest seeking a mechanic’s assistance for the job.

Transmission Control Module Issues:

Car acceleration problems might arise from a fault in the transmission control module (TCM), which manages the gearbox. Indications encompass gear-shifting issues, stalling, or uneven surges.

To detect TCM problems, make use of an OBD-II scanner for trouble codes; if TCM-related codes appear, a replacement is necessary.

How to Fix:

Swapping out a TCM is quite intricate and usually demands specialized equipment; hence, having an expert mechanic handle this task is a good idea. It should

Low Transmission Fluid:

Insufficient transmission fluid might lead to challenges in acceleration for vehicles since this fluid plays an essential role in lubricating and cooling the transmission system. Indicators of low transmission fluid include problems with gear shifting, slippage, and unusual grinding sounds. To identify low transmission fluid, examine the dipstick while the car is in operation and during the warm-up phase.

How to Fix:

When fluid levels are insufficient, add the proper transmission fluid type. Remember, consistently low levels might indicate leaks, warranting a mechanic’s check for transmission problems and leaks. It will hopefully solve the issue if your car won’t accelerate past 40.

Misaligned Timing Chain or Belt:

chain belt

If the timing chain in your vehicle isn’t aligned properly, it can lead to improper opening of exhaust and intake valves within the engine, often signaled by an uneven idle known as “lopey idle.” This irregular idle shows engine RPMs cycle in erratic upward and downward patterns while idling.

The primary reason behind a misaligned timing chain is frequently an incorrect belt tension—either excessively loose or tight—resulting in a belt position that’s off by one or more teeth in terms of timing.

How to Fix:

Think about changing the timing belt. If you’ve recently swapped the belt, make sure it’s set with the right tension to prevent incorrect timing for opening the exhaust and intake valves, which are the root cause of why the car won’t accelerate past 40.

Malfunctioning Throttle Position Sensor (TPS):

If the throttle position sensor isn’t working properly, it can lead to difficulties in acceleration. This sensor detects the throttle position and communicates with the engine control module (ECM) to regulate the fuel and air mixture. Symptoms of a malfunctioning throttle include sluggish acceleration, stalling, or erratic surges.

How to Fix:

Examine the sensor’s voltage signal using a multimeter or scanner to identify issues with a problematic throttle position sensor. If the sensor isn’t working correctly, replacement is necessary.

Failed Catalytic Converter:

The catalytic converter can also cause the problem of why your car won’t accelerate past 40 mph, as it might create back pressure affecting the engine. This pressure issue can lead to acceleration difficulties.

In more severe cases, it could even result in an unforeseen and hazardous fire.

Therefore, if you detect a sulfur or rotten egg odor, we suggest you stop the car, power it down, and refrain from restarting until the problem is resolved.

How to Fix:

Identify the problem using an OBD-II scanner, opt for a reliable replacement converter, and deal with any hidden engine concerns. Routine upkeep and warranty considerations are valuable, and aftermarket choices must comply with emission regulations. 

Ultimately, conduct a post-replacement emissions system check for the best results. 

Malfunctioning Fuel Pump:

The gas tank delivers fuel to the engine, facilitated by the fuel pump. If the pump isn’t working correctly, the engine might not get enough fuel, and the car won’t accelerate past 40.

Indications of a faulty fuel pump involve:

  1. Engine stalling or sputtering.
  1. Challenges initiating engine ignition.
  2. A noticeable whining sound emanating from the fuel tank.

To identify a problematic fuel pump, an auto technician might utilize a fuel pressure gauge to assess pressure at the engine. Low pressure could suggest a failing pump. Sometimes, a blocked fuel filter might also induce low pressure, underscoring the need for a professional inspection to pinpoint the precise issue.

How to Fix:

When the fuel pump isn’t working right, a replacement is necessary. This intricate task usually requires gas tank removal for fuel pump access.

Compression problems:

Engines that use combustion depend on compressing the mix of air and fuel for operation. Increased compression leads to a powerful detonation, consequently impacting acceleration.

When gases escape the engine, it can lead to reduced compression. Inadequate compression may arise due to the accumulation of carbon around valve seats, cylinder linings showing wear, head gasket failure, fractures in piston rings, as well as worn valves and valve springs.

How to Fix:

You might want to consider conducting an engine compression test, examining piston rings, valves, and head gaskets, looking into timing components, and assessing fuel/ignition systems. Seeking advice from a skilled mechanic can provide you with precise identification and suggested actions if your car won’t accelerate past 40.

Is It Dangerous To Drive Too Slowly?

Driving at a moderate pace is generally acceptable, but avoid obstructing traffic by going too slow; hence, don’t overlook this concern for an extended period. Legal firms assert that driving too slowly carries risks similar to speeding, as it compels other drivers to adjust, increasing accident possibilities.

This is particularly accurate when other drivers must adapt to your pace, like altering lanes or adjusting speed.

Due to its high risk, this is regarded as a non-criminal traffic offense in various regions. Nevertheless, the penalties and citations for such actions can be considerably more substantial!

Traffic violations can also result in higher insurance premiums, states the Insurance Information Institute. In extreme cases, you might face a greater accident risk because of significant vehicle issues.

Car won’t Accelerate Past 40 – FAQs


What Can I Do If My Car Won’t Accelerate Past 40?

Given that the majority of drivers lack the ability to recognize and resolve vehicle problems, asking for assistance from a professional is the optimal choice. Using a travel or roadside assistance membership card can provide cost savings.

When your car is not accelerating, various elements contribute to the issue. Thus, our team advises you to entrust your vehicle to a trustworthy mechanic. It will help avert potential accidents or penalties for sluggish driving. Only a seasoned mechanic can conduct a thorough diagnosis and suggest an appropriate solution.

What is the price of repairing a car that does not Accelerate?

Repairing a vehicle with decreased engine performance varies in cost based on factors like location, damage extent, car brand, insurance, etc. Take your time to find a reputable local mechanic, compare rates, and inquire about discounts. Prioritize car maintenance and consult a mechanic for unusual noises to prevent costly repairs.

Why my vehicle won’t accelerate past a certain speed?

When your vehicle doesn’t go faster than a certain speed, it may signal reduced compression and issues with fuel delivery, the ignition system, or the transmission. Timely resolution of these matters is essential to regain your vehicle’s regular acceleration performance.

Why is my vehicle not going quicker than 40mph?

If your car doesn’t accelerate past 40mph, factors like engine wear, valve or piston ring issues, timing problems, or fuel and ignition system glitches might be responsible. Our team recommends performing diagnostics and consulting a professional to pinpoint and fix the speed-limiting problem.

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