(Updated on September 15, 2020)
Gasoline-powered vehicles use a complex system to get moving. Problems with fuel or air delivery, exhaust emission, or anything in between can be the reason your car is sluggish or won’t accelerate.
A combustion engine requires a specific ratio of air to fuel to be in each cylinder at the right time, followed by a carefully-timed spark. The exhaust gases also need to be released properly.
These variables can change depending on what the car is doing during each moment, and the environment you’re driving in. A car at idle, for example, has different air/fuel requirements than a car accelerating rapidly to merge onto the freeway. Cold temperatures require more fuel because the air is more dense.
The electronic control module (ECM) or electronic control unit (ECU) is the brain of the car. It uses input from many sensors to control spark timing and determine how much fuel is needed. It also has the tough job of keeping track of the emissions system to ensure the vehicle is running as environmentally clean as it was designed to.
There are many possible causes of acceleration problems, so troubleshooting may be tricky. Ten main categories are discussed below – start troubleshooting with the issues that are easiest and cheapest to repair/replace and go from there. You will likely need the help of a mechanic for some of these systems.
Top 10 Reasons Your Car is Not Accelerating
1) Emergency Brake is Engaged/Out of Fuel
These may seem like obvious causes of slow acceleration, but it’s important to check that the emergency brake is off and the vehicle has enough gas. One or both of these issues will absolutely cause the vehicle to have slow acceleration or even to stall.
While this is rarely going to be the case, it’s worth mentioning in the number one spot before further troubleshooting is done.
2) Fuel Problems
Anything stopping the fuel from reaching the combustion chamber in the proper quantities can cause acceleration issues.
The fuel filter is an often overlooked component, but a clogged or dirty fuel filter is a common culprit. Over time, fuel filters become filled with sediment and debris from the fuel and should be replaced regularly.
Clogged or dirty fuel injectors can cause misfires or an irregular spray which disrupts the combustion process. Sometimes you can clean these by putting a fuel additive into the gas tank.
A bad fuel pump can also provide inadequate fuel flow. If this is the case, it will need to be replaced. Fuel pumps may weaken over time if you frequently drive around with an empty gas tank. Fuel pumps are cooled by the fuel they are submerged in, so if there’s not enough fuel to cool the pump it may shorten its life.
Having a bad tank of gas, using the incorrect octane, or contaminants in the fuel (such as dirt or water) can also cause slow acceleration. If this is the case, draining the tank and filling it with fresh fuel should solve the problem.
3) Spark or Spark Timing Problems
Faulty ignition coils, distributors, or spark plugs can cause misfires as well. If the spark is missing from a cylinder, there will be no explosion (and of course, having explosions at precisely the right times is what ultimately powers the vehicle). A spark at the wrong time will do no good, and may even harm the engine in some cases.
Ignition coils convert the low voltage from the battery to high voltage needed for the spark to jump the gap on a spark plug. The distributor passes this voltage from the ignition coil(s) to the spark plugs themselves. Most modern cars no longer use a distributor, so this may not even be a possible problem for you.
4) Mechanical Problems
Since there are many moving parts in a combustion engine, there are a lot of pieces that can break or fail.
A misaligned timing belt or chain can cause the engine’s intake and exhaust valves to open at the wrong times. You may notice a lopey idle if this is the case. If the timing belt was recently replaced, it may be off a tooth or the tension could be set wrong. If the timing belt is too loose, the timing may not be correct.
A slipping clutch or low clutch fluid can lead to issues as the gearbox is unable to correctly engage with the engine. A slipping clutch will cause the engine speed (RPM) to increase quickly while your ground speed stays roughly constant.
If the exhaust gas recirculation (EGR) valve is stuck open then too much exhaust can enter the system, which throws off the sensors. This also reduces the amount of oxygen in the cylinder, which means less power. Note that an EGR valve stuck closed should not affect power, unless it causes the ECU to place the vehicle in limp mode.
A clogged or faulty catalytic converter can lead to too much back-pressure on the engine. A clogged catalytic converter is very dangerous and may even catch fire! In this case, it’s best to park the vehicle until it can be repaired.
5) Electrical Problems
If any of the sensors don’t work right, the vehicle’s ECU/ECM can become confused and respond to incorrect data, leading to issues such as sluggish acceleration.
Different types of vehicles use different types of sensors. These may include the mass air flow sensor, MAP sensor, oxygen sensor, throttle position sensor, coolant temperature sensor, crankshaft sensor, camshaft sensor, and others.
Since the timing of combustion events and mixture of gases need to be exact for the vehicle to run efficiently and as expected, having the wrong data about what’s going on in and around the vehicle can result in problems with acceleration or even with starting the car.
6) Airflow Problems
Since air is a basic requirement for combustion, restricting airflow in any way will result in acceleration problems. A clogged or dirty engine air filter can cause this. Manufacturers recommend the air filter to be replaced at regular intervals – check your service manual and keep a record of when this is done.
Keep in mind that the cabin air filter has a different function, as it is part of the HVAC system and not the engine. However, it may simply be called an “air filter” if you’re searching online. Make sure to purchase the appropriate one when replacing the filter.
The throttle body is the tube containing the butterfly valve (also known as the throttle plate). This valve changes how much air is allowed into the intake manifold, and is connected to inputs from your right foot. If this is stuck in one position or debris builds up the vehicle won’t work as expected. Rough idle is likely if this is what’s going on in your vehicle.
7) Limp Mode
Some vehicles will enter “limp mode” when a fault is detected. This fault could be an errant sensor, excessive engine knock, or a problem with the emission control system. Typically, limp mode is accompanied with a check engine light and greatly reduced power.
It is designed to keep the engine safe while allowing you to move the vehicle to the nearest repair shop. If your vehicle enters limp mode, bring it in to a mechanic as soon as possible so they may diagnose and correct the fault.
8) Leaky or Clogged Vacuum Hoses
Engine vacuum is a measurement of air pressure inside the intake manifold relative to atmospheric pressure. When the throttle is completely closed (idling, foot off the gas), the engine vacuum is at its highest.
In this state, the cylinders are trying to pull more air than the throttle plate will allow into the intake manifold, and therefore the intake manifold pressure is much lower than ambient atmospheric pressure.
When the throttle is fully open, pressure in the intake manifold is roughly equivalent to ambient atmospheric pressure. There is little to no vacuum in this state. On a turbocharged (or supercharged) vehicle, it may enter positive pressure as the turbo or supercharger pushes compressed air into the manifold. This pressure is greater than ambient atmospheric pressure.
Vacuum hoses control the behavior of many components around the vehicle, including the brake booster, fuel pressure regulator, and EGR valve. If there is a leak in any of these hoses, the linked component may behave erratically.
Furthermore, air leaking in (or out) is not measured by the MAF or MAP sensors, and will throw off the vehicle’s air fuel ratio. On turbocharged vehicles, this may manifest as a boost leak which lowers the amount of air entering the combustion chamber well below the expected volume.
9) Low Compression
The ratio between the cylinder volume when the piston is at the lowest point and the volume when the piston is at the highest point is known as the compression ratio. Compression is crucial for the function and efficiency of a combustion engine as higher compression means a more forceful explosion.
Anything that causes gases to leak out of the engine can cause low compression, which will lead to power problems.
Potential causes of low compression include, but are not limited to: worn/cracked piston rings, worn valve springs, worn valves, carbon buildup around valve seats, and worn cylinder linings, valve clearances out of spec, and blown head gaskets.
All of these things will cause combustion gases to leak outside of the combustion chamber, lowering compression and reducing the potency of the explosion in that particular cylinder.
10) High Altitude
When driving at high altitudes, you may notice your car struggling or lagging, especially when driving uphill. Oxygen is a crucial part of the mixture in the combustion chamber and there is less oxygen at higher elevations.
Naturally aspirated cars are more likely to have this problem than forced-induction (turbocharged or supercharged) vehicles as the forcing of air into the engine can somewhat compensate for having less oxygen in the environment.
The sensors should detect that there is less oxygen and suck in more air to make up for it.