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Here's Why Your Car Is Struggling to Accelerate

By Mia Bevacqua, May 14, 2018

Your car used to zip through traffic — but now, it can barely keep up with the pack. Where did all that horsepower go? If your vehicle is struggling to accelerate, there are dozens of possible reasons why. Before you can rejuvenate your engine, let’s look at how it works. Or you can jump ahead to see problems that can stifle it

What your engine needs to make power

Your engine needs the proper mixture of air and fuel, spark, compression and exhaust to run right. Anything that disrupts this combustion process can result in misfiring, hesitation, stumbling and a loss of power. 

Since the late 1980s, engine performance has been computer-controlled. The engine control module (ECM) receives input from various sensors. It then uses this information to determine control of outputs like the fuel injectors, exhaust gas recirculation (EGR) valve and ignition coil packs

In this way, the ECM directly controls fuel delivery, emissions equipment operation and spark timing. It may also play a part in engine compression (through variable valve timing) and air delivery (through electronic throttle control).

Reasons why your vehicle struggles to accelerate

Poor acceleration is typically attributed to one or more of the following:

Note: The following information pertains to standard gasoline engine. Diesel engines, turbocharged engines and supercharged engines may differ.

Air delivery problems

Throttle body problems: A dirty or restricted mechanical throttle body can inhibit airflow, causing the engine to run poorly. Cars made since the mid-2000s have an electronic throttle body that’s controlled by the car’s computer. A problem with an electronic throttle body or its circuitry can cause the engine to struggle.

Solution: With a mechanical throttle body, cleaning may be all that’s needed to fix the problem. With an electronic throttle body, the circuit needs to be checked. If it’s good but your problem still exists, you will need a new throttle body or actuator.

Variable length intake manifold: Some vehicles made in the past couple decades have a variable length intake manifold. This design uses butterfly valves mounted in the intake runners to vary air delivery to the engine. A problem with this system can cause poor acceleration.

Solution: Determine the cause of the problem and repair the concern. Common failure areas include the intake manifold, actuator and linkage.

Get it diagnosed by a professional
 

Fuel delivery problems

Injector issues: A failing or clogged fuel injector can cause an engine to misfire. Because fuel injection systems use one injector per cylinder, your engine may not completely shut down if only one injector isn’t working. Instead, you will notice a lack of performance, hesitation on acceleration, and shaking or vibration felt through the steering wheel and floorboards.

Solution: Because modern fuel injectors are computer-controlled, your mechanic will check to ensure each injector’s circuit is working. If they’re all working, you likely have a faulty or clogged injector. In some cases the injector can be cleaned to restore performance; in others, a replacement will be needed.

Lack of fuel pressure: Poor fuel pressure can stem from several sources, the most common being the fuel pump. A weak fuel pump can starve the engine for fuel, causing it to struggle. Other causes of low fuel pressure include a restricted fuel filter or fuel line, or a faulty fuel pressure regulator. 

Solution: Because of the risks associated with gasoline, it’s best to let a professional handle these repairs. If you have low fuel pressure, you’ll typically need a new fuel pump.

Sensor issues

There are two basic engine fuel management systems in use today: speed density and mass air flow.

In a speed density system, the car’s computer determines fuel injector control based on input from the throttle position sensor (TPS), manifold absolute pressure (MAP) sensor, coolant temperature sensor (CTS) and intake air temperature (IAT) sensor. A problem with any of these sensors, or their circuits, can cause poor acceleration.

In a mass air flow system, the computer determines fuel injector control based on input from the mass air flow (MAF) sensor as well as a mix of the other sensors mentioned above. A problem with the MAF sensor or its circuit can cause the car to accelerate poorly. 

Keep in mind that all your sensors provide information to the computer. A sensor can not directly do anything but provide that information. If the computer decides that sensor information requires something to change, the computer uses an actuator — such as the idle air control (IAC) valve or fuel injectors — to make the change. 

Fortunately, most sensor problems will trigger the check engine light and store a diagnostic trouble code, which can help point you in the right direction to repair the problem.

Solution: If you have an OBD-II code scanner you might be able to figure out which sensor is causing the problem. But most of these repairs should be left to a professional mechanic.

Vacuum leaks

A vacuum leak allows unwanted air to enter the engine. This causes the engine to run lean, which means it’s getting too much air and not enough fuel. This will make your car run poorly, and may even cause it to stall, if the leak is large enough. Common vacuum leak sources include hoses and lines, the positive crankcase ventilation (PCV) valve and the intake manifold.

Solution: A technician can locate the source of the vacuum leak and repair as needed.

» MORE: Get an estimate for your car repair

Ignition problems

Almost all vehicles made within the last decade or so have a coil-on-plug ignition system. With this design, there is a high-voltage ignition coil that sits atop each spark plug and fires it when it receives a signal from the ignition timing device or computer.

The car’s computer makes decisions regarding spark control based on input from the crankshaft position sensor, camshaft position sensor and knock sensor. A problem with any of the secondary ignition components or their circuity can result in a misfire under load. This is often perceived as a hesitation or lack of power.

Solution: Determine the cause of the ignition problem. In many cases, the remedy will be either replacing the crankshaft position sensor, camshaft position sensor or ignition switch. These tests are best left to your mechanic.

Emissions equipment issues

Cars made since about 1974 have emissions equipment that can fail and cause stalling. Some common offenders include the exhaust gas recirculation (EGR) valve and catalytic converter

If the EGR valve is stuck open, it will allow too much exhaust gas to enter the engine. This can result in hesitation and poor performance. A clogged catalytic converter can cause performance problems by putting too much backpressure on the engine.

There are many other components that can have the same effect. Fortunately, the car’s computer monitors emissions system operation. If it detects a problem with one or more components, it will turn on the check engine light and store a trouble code.

Solution: If you have an OBD-II code scanner, you can try to pinpoint the problem area. Catalytic converter problems will need to be handled by a mechanic. If your EGR valve is stuck, you might be able to access it and fix it, depending on your make and model of car. Some require disassembly and special tools, though — in this case, a mechanic is likely your best option.

Engine problems

Engine mechanical problems: Internal engine problems such as worn piston rings or sticking valves can cause low compression. This can result in an engine that runs poorly and won’t accelerate properly. 

Solution: In most cases, the engine will need to be rebuilt or replaced. This is an expensive repair, and may not be worth it, depending on the age and value of your vehicle. Talk with your mechanic, do the math and decide if you would be better off getting a new car.

Variable valve timing issues: Many vehicles made in the last couple decades have variable valve timing. These systems are operated by the car’s computer. Some vehicles may also have a system that controls valve lift. A problem with either of these systems can result in a lack of power.

Solution: Common problem areas include the actuators, oil control solenoids and timing components such as the timing chain, tensioners and guides.

Control module issues

The engine control module (ECM) — in essence, the engine’s computer — monitors and regulates all aspects of engine performance. A problem with the computer or its circuit can cause the engine to struggle.

Solution: This job is typically best left to the pros. The technician will check that the computer’s circuit is working, and then check for any technical service bulletins. If there aren’t any, the ECM will need to be replaced. This can sometimes be pricey.

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