Car Engine Runs But Will Not Accelerate

If your car engine runs ok when you start it, but your car won’t accelerate then you’ve got an obvious engine problem that needs looking into. Car acceleration problems are very common and can have numerous causes. So what causes this type of problem and how do you fix it?

If your car engine runs but will not accelerate then there is a problem with one or more engine components. The problem could be caused by a lack of fuel supply to the engine, a bad mass air flow sensor, a faulty throttle or throttle position sensor, a blocked air filter or fuel filter, or a problem with sticky brakes.

If your car won’t accelerate at all or is stuck idling then you should try to restart it to see if it’s an intermittent problem. If there is very little power from the engine but it still accelerates slowly then there may be a lack of air getting into the engine.

Either way, it’s advisable to get the problem checked out as soon as possible to avoid possible damage to the engine.

What Causes A Car To Run But Not Accelerate? (5 Common Reasons)

A car engine that runs ok but won’t accelerate usually has a problem with the adequate intake of air or fuel.

Lack of air or fuel to an engine can have many causes, some simple to fix such as blocked filters or bad mass air flow sensor, and others not so simple such as worn or burnt valves, a faulty throttle body, blocked fuel injectors, or a bad fuel pump.

Sometimes the problem can be an electrical one, caused by a faulty sensor such as the throttle position sensor or the throttle itself, especially if it’s a fly-by-wire throttle that is commonly used in modern cars.

Here are a few of the most common reasons why your car is running but won’t accelerate, in more detail.

Reason 1. Bad Mass Air Flow Sensor

One of the most common causes of lack of acceleration power can be a bad mass air flow sensor. The MAF sits between the air filter and the intake manifold and the air filter and keeps track of the amount of air that is being sucked into the engine.

A bad mass air flow sensor will not be able to accurately keep track of the air supply into the engine and this can lead to the engine running rich as the ECU will find it hard to keep the correct fuel-to-air ratio needed for the engine to run smoothly.

ECUs can generally adjust the fuel supply by plus or minus 12% as necessary to keep an engine from running too rich or lean. However, a bad mass air flow sensor may confuse an ECU into supplying more fuel than is necessary leading to problems with the combustion of fuel.

This can result in engine misfires and a lack of power when trying to accelerate. Mass airflow sensors usually fail because of dirt or contaminants in the air that damage the delicate sensor surface within the MAF housing.

Reason 2. Faulty Throttle Position Sensor

A malfunctioning throttle position sensor is another common cause of lack of acceleration. The throttle position sensor is connected to the throttle body and keeps track of the aperture of the throttle flap.

The opening and closing of the throttle body actuator (flap) controls the amount of air that is getting into the engine cylinders. The throttle position sensor sends actuator data back to the ECU and this helps to determine the necessary amount of fuel that is supplied by the fuel injectors.

If there is a problem with the throttle position sensor then delivery of fuel to the engine is no longer accurate and this can lead to problems with acceleration power and fuel economy.

The ECU can also use data from the mass airflow and manifold pressure sensors to calculate the amount of fuel needed when it learns of a drop in vacuum in the intake manifold due to acceleration. This will lead to a hesitation in acceleration and will cause a jerky, rough increase in power if the car accelerated at all.

Reason 3. Blocked Air or Fuel Filters

Another common cause of lack of acceleration when an engine is running is dirty or blocked filters.

The air filter is located before the mass air flow sensor and is there to remove large dirt particles from the air before it enters the engine. Air filters become blocked over time and should be changed at least once a year.

If an air filter becomes too dirty it can restrict the air getting into the engine significantly. This lack of air may not be apparent when idling but may cause a serious lack of power when trying to accelerate due to the shortage of air available to mix with the increased fuel being pumped into the cylinders.

A blocked fuel filter is just as likely to cause a problem with acceleration even though the engine may start and idle just fine. Fuel filters can become blocked or damaged by dirt or water in the fuel and this can affect their permeability and therefore the amount of fuel getting to the injectors.

A bad fuel filter can sometimes stop a car from moving at all if it gets sufficiently blocked. The fuel filter can affect the fuel pressure within the fuel lines and if the ECU detects a fall in fuel pressure when trying to accelerate it can prevent the vehicle from moving to protect the engine from damage. This may result in the engine cutting out or a flashing check engine light on the dashboard.

Reason 4: Blocked Fuel Injectors

Blocked or damaged fuel injectors can also stop a car from accelerating when the throttle is pushed. Most modern engines use direct injection that sprays fuel directly into the engine cylinders by the fuel injector.

This process is controlled by the ECU and each injector needs to be able to supply an accurate, consistent spray of fuel for the engine to accelerate smoothly.

Blocked fuel injectors are very common, especially in older cars as they often become blocked or worn out due to wear and tear.

This is caused by the burning of the hydrocarbons that are present in the fuel that the injector is spraying into the engine. When you shut off your car engine, the fuel injectors tend to stay hot for a long time as they absorb some of the heat from the engine.

This can cause the remaining fuel in the injector to chemically crystallize into hard carbon deposits that can block the fuel injector tip and result in an inaccurate spray of fuel when the engine is running.

Reason 5: Lack of Engine compression (damage to the cylinder head)

One of the more serious causes of lack of acceleration, when the car is running, is the lack of internal compression within the engine cylinders.

This is usually caused by a crack in the engine cylinder head or the engine block that prevents the engine from building sufficient pressure within the engine during the compression of the fuel-air mixture.

A lack of cylinder compression hinders the proper ignition and burning of fuel and reduces the amount of force that is created to drive the piston. This will cause a significant drop in power output and will severely hinder the ability of the car to accelerate.

A leaking head gasket or cracked engine block can also lead to oil and coolant getting into the engine cylinder. This is often caused by an engine overheating and will result in engine misfires and other problems that will affect the ability of the engine to generate enough power to accelerate.

What Next – How To Fix A Car That Runs But Won’t Accelerate?

To begin, you need to first determine the extent of the problem and the exact symptoms that you are experiencing. Start by trying to answer some questions such as: Is the car engine starting ok, or is it hesitating? Does the engine idle smoothly or is it rough or shaky? Does the engine cut out by itself or when you try to accelerate? Will the car accelerate at all or is it stuck idling?

To diagnose and possibly fix any acceleration problems, you should first run a diagnostic readout of the ECU to see if there are any engine error codes stored that could help to pinpoint the cause of the problems.

  1. Check the fuel filter. If there is an error code that points to a problem with fuel delivery or fuel pressure at the fuel rail, then start by checking and if necessary, replacing the fuel filter. A blocked fuel filter will affect the ability to accelerate. If a new fuel filter does not resolve the problem then there may be a problem with the fuel pump or a blockage in the fuel lines, fuel rail, or fuel injectors. This will be harder to resolve as they operate at very high pressures and you may need a specialist to perform some tests.
  2. Engine Misfires. If there are issues with the internal compression then this can lead to engine misfires. It’s not uncommon to see error codes such as P0301 to P0306. Check the spark plugs and the air filter to make sure they are working properly. Bad ignition coils and blocked fuel injectors can also cause these error codes.
  3. Clean or change the MAF sensor. The mass airflow sensor is attached to the air filter box and can be easily removed for inspection and cleaning. If possible temporarily fit a new sensor to see if it fixes the lack of acceleration.
  4. Remove and check the throttle body and idle air control valve. The throttle actuator can sometimes stop responding properly when the gas pedal is pressed. It can become clogged or sticky and it may be possible to release it by removing it and cleaning it with some throttle body cleaner. The same can be done with the idle air control valve if the engine is cutting out when idling or if it idles but won’t close when trying to accelerate.

FAQ – Car Engine Runs But Will Not Accelerate

1. When I accelerate my RPM goes up but car doesn’t accelerate?

If your car revs but doesn’t actually accelerate this can have numerous causes such as low transmission fluid, problems with the transmission actuators, sticky brake calipers, a jammed parking brake, a worn clutch plate or damage to the dual mass flywheel or clutch fork.

2. Can Bad spark plugs cause your car not to accelerate?

Yes, bad spark plugs are a common cause of lack or acceleration. Spark plugs are an integral part of the combustion process and if they are not delivering a strong, consistent spark when the engine is running then this will cause engine misfires and an overall lack of power.

3. Can limp mode go away by itself?

Yes, sometimes restarting the engine will clear the check engine light and limp mode if the problem was caused by an intermittent issue. For example, a bad engine sensor may intermittently fail or give out-of-range readings under specific engine conditions such as acceleration or when idling.
Limp mode can also be caused by partial blockages in the fuel system such as a partially blocked fuel filter or fuel injectors. These may trigger limp mode when the engine is run hard but it may disappear when you restart the engine and not reappear until the engine is driven hard again.

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