Any problem with acceleration in your car is a bad sign; it means there is a problem with either the air-fuel mixture or the ignition.
What is the next step in diagnosing the problem if your car won’t accelerate? In this article, I will discuss the most common reasons why your car won’t accelerate. To start off, I will summarize the possible causes list.
Why Won’t My Car Accelerate?
Cars generally won’t accelerate due to bad throttle bodies or bad air-fuel mixtures. Bad air-fuel mixtures occur due to bad MAF sensors, clogged fuel filters, clogged air filters, bad fuel pumps, and sometimes even bad ignition.
While these are far from all of the possible reasons, it’s a good start. Below is a more detailed list of the 8 most common causes why your car won’t accelerate:
#1. Dirty/Damaged Mass Air Flow Sensor.
The most common reason for a car that doesn’t accelerate is lean air-fuel Mixture. Mass Airflow sensors are used to measure the amount of air coming into the engine, and the measuring of air is translated into how much fuel is injected into the engine, by the engine control unit.
Typically, by this time, this sensor will have dirt on it, if you have an open-air filter or if you haven’t changed the air filter in a long time.
If you have a dirty or broken MAF sensor, it will under-measure the amount of air going into the engine and you will then likely have some trouble with a lean mixture.
Fortunately, MAF sensors can often cleaned and be put back into service. You can remove the sensor and clean it with electric cleaner.
#2. Clogged Fuel Filter.
The fuel filter is located after the fuel pump which is critical making sure that no dirt gets into the fuel system putting dirt and debris into sensitive parts, like the fuel injectors.
If you haven’t changed the fuel filter in a while, the fuel filter could be clogged and restricting the flow of fuel and creating low fuel pressure. Low fuel pressure causes a lean mixture in the engine and often leads to acceleration issues.
The fuel filter is often inexpensive and easy to replace if you haven’t done it already, it is worthwhile to replace it.
#3 Clogged Air Filter.
The air filter ensures that no dirt, or other particles gets into the engine and destroys sensitive pieces and sensors. The air filter should be replaced on a schedule, depending on the model and engine of your vehicle.
The engine needs a breath of air to function properly and if the air filter is clogged and restricts airflow it could choke the engine, restricting the engine from not being able to build speed.
#4. Failed Throttle body valve.
The throttle body is the flap that opens when you press the accelerator pedal to allow more air into the engine.
If the throttle body valve has issues there could be limited horsepower, because the throttle body valves may not open.
Throttle Body issues can often illuminate the check engine light.
#5. Ignition Issues.
It’s not just the air-fuel mixture that can hinder your acceleration; a strong spark is also needed to make a strong engine. If there are any issues with pieces of the ignition system such as bad spark plugs or burnt-out ignition coils it could cause issues with acceleration.
When ignition issues are present, you might notice it by a misfire. If your car sounds a little funny and has a very rough acceleration, then something could be happening like misfires.
Misfires are where ignition has failed, and unburnt fuel is going out the exhaust. Misfires often happen due to burnt out spark plugs or burnt ignition coils.
You could also check the ignition timing if it is adjustable on your car. Usually, late ignition timing causes a car to be very weak.
#6. Bad Fuel Pump.
The fuel pump is delivering fuel to the engine. When the engine is running, the fuel pump builds pressure in the fuel rail to make sure there is fuel in the fuel rail when it is needed.
If the fuel pump goes bad a loss of acceleration can occur due to low fuel pressure. If there are any trouble codes saved in the engine control module regarding low fuel pressure, then the fuel pump might be the source of your poor acceleration.
#7. Bad Camshaft Timing.
Has somebody replaced the timing belt or chain in your car recently and the car was fast until it was replaced? When someone is replacing a timing belt or chain, if they cannot figure it out and install it incorrectly, your car will most commonly be very slow and not have much power.
If the timing belt has jumped off for no reason you need to consider looking into it as soon as possible, because an engine job timing belt or chain can destroy the entire engine.
#8. Limp Mode.
Limp mode is activated when the engine control module sees a very wrong parameter from a sensor. This can be often be high boost from the turbo or something similar.
Limp mode often limits power output and engine RPM. If you have an automatic transmission, limp mode may even limit the transmission to not shift over third gear.
Limp mode is a pretty common reason as to why your car isn’t accelerating, and if you notice this symptom along with a check engine light, you should go check for trouble codes.
How do you diagnose a car that will not accelerate?
Diagnosing a car that does not accelerate is quite easy if you have the tools and knowledge. Here is how a professional would diagnose the condition. You may need some special tools to actually follow this guide.
- Connect an ODB2 scanner and check for trouble codes and associated trouble codes. Continue the diagnosis with the trouble codes you find. There are plenty of trouble shooting articles on this website. These articles lead you through your specific trouble codes and will help you make more sense of the issue at hand. If you did not have any trouble codes, you could continue.
- Check the live data in your diagnostic tool while a friend accelerates the vehicle. When the car accelerates, check the turbo pressure and air flow. Check all other values and continue troubleshooting of the fault sensor if you see any strange values.
- Make sure that the MAF airflow sensor is clean. You can use an electric cleaner to clean the sensor.
- Check the air filter. Change the filter, if the filter is dirty.
- Check the fuel pressure with a fuel pressure meter. Your fuel filter or fuel pump could be replaced or changed, if the pressure is low.
- Check for leaks with an EVAP smoke machine. Check to see if your PCV valve is functioning. Repair leaks and try again.
- Check the condition of the spark plugs and ignition coils. Replace the spark plugs or ignition coils if they are faulty or are in bad condition.
- Check the cam shaft timing. Replace or correct cam shaft timing, if it is faulty.