Reasons why cars won’t accelerate over 40 MPH

You know there is a problem when your car won’t accelerate over 40 mph. Most drivers do not notice these signs unless when trying to accelerate into fast-moving traffic or drive up a steep hill.

If your car hesitates or won’t accelerate over a certain speed, it clearly indicates that it has a wide range of issues, such as a slipping automatic transmission or faulty sensor.

But the most frequent reasons why a car won’t accelerate over 40 mph are related to engine problems that often lead the engine to run into limp mode, a safety feature that protects it, and not letting the car accelerate over a certain speed no matter how much you press the gas pedal.

Unfortunately, determining the exact cause why your car won’t accelerate can be challenging even for professionals.

What Should You Do If Your Car Stop Accelerating on the Road?

If you fail to pick up speed when driving uphill or merging onto a busy highway, your vehicle is a road hazard, and you should get it fixed.

For safety reasons, put on the hazard lights to alert other drivers and proceed with caution. Typically, you should find a safe place to get off the road.

You can drive to a public parking lot, and turn out off-the-road or shoulder of the road if you are not endangering yourself or other motorists. Call a tow or roadside assistance immediately to avoid causing further damage to your car.

Reasons Why Cars Won’t Accelerate over 40 mph

Sadly, there are several reasons why cars won’t accelerate as they should, especially high-mileage cars. To determine the problem’s root cause, you or your mechanic need to troubleshoot the car.

Remember to check the repair database or manual to determine the correct approach to fix the problem. Ideally, poor acceleration is caused by one or more of the following issues:

1. Dirty Car Parts

Accumulated dirt or debris can affect a car’s acceleration. Always consider taking your car for regular maintenance and repair. To keep your car running perfectly, numerous parts work together.

However, if one of the parts accumulates too much dirt, it can hinder your car’s performance. Here are some of the car parts that require regular cleaning or replacement:

Dirty or clogged air filter – air is vital in fuel combustion. If the air filter is clogged or dirty, the engine fails to breathe properly because it does not get the right air-fuel mixture.

Dirty or clogged fuel filter – since engines require clean fuel to run optimally, fuel filters remove the dirt and sludge accumulating in the fuel tank.

Eventually, fuel filters are filled with debris and sediments, limiting the amount of fuel reaching the engine’s combustion chamber. Inadequate fuel translates to acceleration issues.

Dirty fuel injector – anything that hinders the fuel or air from reaching the combustion chamber affects acceleration. Dirty fuel injectors cause an irregular fuel spray or misfires, disrupting the combustion process.

Dirty spark plugs – misfiring issues can arise from dirty spark plugs or ignition coils. Misfires cause severe acceleration issues in your car.

2. Technical Issues

Cars rely on several moving parts to perform well. A breakdown is inevitable if any moving parts are not maintained, repaired, or replaced after wear and tear. Car owners need to service their cars regularly to avoid the following technical problems:

Timing belt – a worn-out or damaged timing belt affects acceleration. If you replaced the timing belt recently, it could affect acceleration if it’s misaligned or misplaced.

Failed clutch – acceleration issues arise when the clutch fails to engage the transmission fluid properly with the engine. If your car cannot accelerate over 40 mph, failed crutch or low transmission fluids could be the reason.

Malfunction of Throttle Position Sensor (TPS) – TPS detects the throttle valve opening angle and sends this information to the ECU to help determine how the system should adjust.

If the TPS malfunctions, it is difficult for the system to accelerate or decelerate accordingly.

Compression problems – combustion engines rely on compression of the air-fuel mixture to run. A higher compression results in a forceful explosion and hence high acceleration.

If gases leak out of the engine, it can cause low compression. Low compression can result from carbon build-up around valve seats, worn cylinder linings, blown head gaskets, cracked piston rings, worn valves, and worn valve springs.

Is Limp Mode the Main Reason Cars Won’t Accelerate Over 40 mph?

Also known as limp home mode, the limp mode is a preservation feature that’s designed to help drivers get to a mechanic, home, or off the road safely if something goes wrong.

It also protects the engine by turning off some features that might harm him until the issue is fixed.

Therefore, low fluid level, failed clutch, low compression, or faulty sensors can trigger the limp mode. Since numerous things can trigger limp mode, it is arguably the main reason why cars won’t accelerate beyond 40 mph.

If your car has problems accelerating and the engine light comes on, consider contacting a mobile mechanic to come to your location.

How can I Fix a Car That Won’t Accelerate Beyond 40 mph?

Since most drivers cannot identify and fix automotive issues, hiring an expert is the best option. With a travel or roadside assistance card, you can get a discount.

If your car doesn’t accelerate, numerous factors come into play. Therefore, take your car to a reliable mechanic to avoid accidents or fines for driving too slowly. An experienced mechanic will properly diagnose and recommend a fix.

How Much Does It Cost to Fix a Car That Won’t Accelerate?

The cost of fixing a car that’s experiencing reduced engine power ranges depending on factors like location, the extent of damage, car make and model, insurance policy, etc.

Take your time to research the best-certified mechanic in your area, compare prices and check for discounts to get the best deal.

Prevention is better than cure. Therefore, service your car regularly and consult a mechanic if you hear a strange sound or noise from your car.


Your car might experience acceleration problems; this should be a good start for a diagnosis. Some of the issues causing acceleration problems are minor, and you can fix them yourself.

However, if you have no automobile expertise, avoid taking risks and contact a mechanic or towing company. Finally, consider researching online and watching tutorial videos to familiarize yourself with why your car won’t accelerate beyond 40 mph.

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