Why Your Car Jerks When Accelerating

Why Your Car Jerks When Accelerating
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Simply driving your car shouldn’t feel like you’re damaging it. Yet when something goes wrong, you’ll get that alarming, surging-and-hesitating jerking that feels like the engine is about to explode. It’s dangerous in traffic and can be scary for the driver. If you find yourself wondering “why is my car jerking when I give it gas?” know that it usually isn’t an issue that’s too expensive or difficult to fix. Here’s what you need to know.

How is car acceleration supposed to work?

Normally, when you step on the accelerator, your vehicle just seems to go. What you don’t notice is that the throttle opens, sensors deliver the right calculations to the engine control unit (ECU), the right amount of gas mixes with the right volume of air and the engine smoothly delivers power.

Everything from the transmission to the tires needs to work as designed to get you easy acceleration onto the highway or solid power when passing.

If your car jerks when pressing the gas pedal, here are your likely problems.

Why is my car jerking when I give it gas?

Jerking/surging problems usually involve the fuel or spark in the engine, followed by possible mechanical issues. Most of those issues trigger the somewhat helpful check engine light, and a genuinely helpful diagnostic code. All of that covers a lot of ground, so let’s break it down, starting with the most likely culprits.

Clogged fuel injectors

Fuel injectors spray droplets in gasoline commercials, but it’s really more like a super-fine mist. The tiny openings in the injector help atomize the gas which helps your fuel mileage, but they also can clog with use and age. With a clog in the injector, it sprays fuel unevenly or not at all, causing reduced power, misfires and jerking when giving it gas. While a bottle of fuel injector cleaner is a great maintenance item, it’s not going to save your clogged injectors here. Cleaning clogged injectors requires removing them from the engine for proper cleaning.

Blocked fuel filter or lines

Blocked fuel filters and fuel lines can cause similar symptoms as clogged injectors. That’s because the blockage in the filter or fuel line causes the same issue: sporadic delivery of fuel into the combustion chamber. The only solution here is replacement. Fuel filters are usually cheap and easy. Fuel lines are significantly more expensive and require more work, but are seldom the culprit.

Failing spark plugs

Damaged or severely worn spark plugs cause dramatic misfires at idle and alarmingly jerky acceleration. Fortunately, spark plugs often cost well under $10 and are easy to replace, even for a beginner.

Throttle position sensor

A throttle position sensor (TPS) monitors the position of the throttle and sends a signal to the ECU for best performance, gas mileage and emissions. When a TPS wears out and fails, the ECU is lacking critical information and you get poor performance and jerking acceleration. Since this is all but impossible to identify visually, using a code reader to run a diagnostics test is your best bet here.

Camshaft position sensor

Similar to the TPS, a camshaft position sensor reads the position and speed of the rotating camshaft, sending a signal to the ECU so it can adjust other engine parameters for best performance. It’s a lightning-fast flow of information, and when the sensor fails, the ECU can’t tell exactly what speed the engine is running. This creates a mess in engine management, and you get poor performance and jerky acceleration. You’ll get a check engine light as well, so have the code read (either yourself or by a professional) and get the sensor swapped out.

Mass air flow sensor

The mass air flow sensor measures the volume of air entering the engine, allowing the ECU to adjust other engine calculations for optimum performance. When it gets dirty or damaged, the sensor readings go off the charts and the ECU makes incorrect engine adjustments. This gives you that stumbling acceleration feeling. You’ll likely see a diagnostic code too. This one is pretty easy, and it might be worth it to buy an inexpensive bottle of sensor cleaner to possibly resolve the issue.

Filthy air filter

Ever noticed how the quick-change oil shops want to replace your air filter with every oil change? While they’re likely after an easy buck, there is some logic in changing out the air filter when changing the engine oil. As the filter works, it gets filled with debris and dirt, reducing the incoming airflow. The engine computer can adapt to some reduced flow, but a seriously clogged filter will eventually cause problems.

Worn piston rings

If you’ve ever been married, you know that rings are expensive. This is especially true for piston rings. These little metal rings keep your compression and oil pressure up by preventing blowby between the piston and cylinder walls. Symptoms of worn piston rings include terrible engine performance, oil burning and low compression, with lackluster acceleration and jerkiness. This one is not cheap, requiring a major engine teardown to repair.

Blocked catalytic converter

A catalytic converter needs to be able to flow exhaust gasses through it in order to function. If the converter is worn out or clogged, exhaust gasses pile up and reduce the flow of the exhaust, causing poor performance and jerky acceleration. You’ll also probably notice the other classic symptoms of a bad cat, including a terrible sulfur smell and a check engine light.

Damaged throttle cable

Throttle cables have mostly disappeared from brand-new vehicles. But if your ride is old enough to vote, this could be its problem. Before today’s drive-by-wire systems, the throttle cable physically connected the accelerator pedal to the butterfly valve on the throttle body. Step on the gas, it pulls the cable, which pulls the throttle open and your car accelerates. However, if there is too much slack in the cable, you’ll pull the slack before moving the throttle at all, giving you inconsistent acceleration.

Carburetor needs to be tuned

This one usually only applies to certain 1980s cars and older, as pretty much everything from around 1987 on has electronic fuel injection (EFI) instead of a carburetor. These computerless fuel and air-management systems use engine vacuum and sometimes even gravity to precisely measure fuel into the engine. While carburetors are simpler than modern EFIs, carbs need to be adjusted occasionally to keep in tune, much like a piano. Carb-tuning is both an art and a science. You might need to look around for an older mechanic if you believe this to be the issue.

Moisture in the distributor cap

Speaking of older cars, distributors have also disappeared from the highways, starting in the mid-1990s, and mostly replaced by the coil pack by the early 2000s. Since electrical components aren’t water-tight (one reason you can’t drive through a flooded street), water can get into them through the seams. If you notice sluggish starts and hesitant acceleration on high humidity days, this could be your problem. That moisture inside the cap makes it harder for the electrical system to spark, pushing a failing system over into the failure range.

Failing coil packs

Both distributor caps and coil packs accomplish the same task of sending voltage to the spark plug that will fire next. When a coil pack wears out after years of use, you often see the same symptoms as a failing spark plug. Expect rough idle and stumbling, jerky acceleration. Usually, you’ll get a very specific misfire code telling you which cylinder is having trouble. Then all you have to do is swap spark plugs. If the misfire moved to a different cylinder, the spark plug is toast. If it stayed, replace the coil pack.

Jumped timing chain/belt

This repair is usually beyond the skills of a beginner, and can easily run four figures at a mechanic’s shop. There are a lot of spinning bits inside an engine, all perfectly keeping in time relative to each other thanks to the timing chain or belt that connects them all. As the crankshaft rotates, it moves the timing chain, which in turn rotates the cams, and so on. If the chain gets too much slack from years of use, it can stretch enough to fall off a few of the cam or crank gear teeth. This means the chain has jumped timing, with one or more of the spinning components out of time relative to the others. This usually makes for poor performance, jerky acceleration and sometimes misfires.

Bad transmission control module

Just as the engine has an ECU controlling it, transmissions have a transmission control unit handling all transmission functions. Normally everything works great, and you get smooth shifting, powerful acceleration and decent fuel economy if you’re light on the throttle. Problems pop up as the vehicle ages, and electronics like the transmission control module can fail. Symptoms include poor gas mileage and a louder engine, and you’ll also find hesitant, jerky acceleration, as the transmission doesn’t consistently go into the next gear.

What to do if your car is still jerking

Checked out the above and you’re still having jerking symptoms during acceleration? It might be time to take it to a dealership service center or an independent mechanic. You could be looking at a previous owner that performed an improper ECU tune, bad gas from your last fill-up or a bad circuit that is difficult to pinpoint. After some troubleshooting, a pro should be able to quickly narrow down why the car jerks when pressing the gas pedal.

Conclusion

Any vehicle that jerks excessively during acceleration has a problem, and it needs to get checked out. Acceleration should be smooth and linear, not sporadic and alarming. Start with the cheap and easy suggestions above to narrow down the cause of your issues and you should be able to get this problem solved.

Frequently Asked Questions

Is it safe to drive a jerking car?

Driving a car with jerking acceleration is like driving a car with unpredictable braking. It’s not safe because you need consistent power delivery when on the road with other vehicles. Save yourself from a collision by getting this fixed ASAP.

How do I check if my fuel filter is clogged?

If your vehicle is hard to start, has stumbling idle and poor acceleration, or you hear an unusual noise from the fuel pump, your fuel filter could be clogged. If it’s the replaceable type, look in your owner’s manual for recommended service intervals, or replace every 30,000 miles.

Can transmission cause a car to jerk?

A transmission issue can cause a car to have jerky acceleration. Note that this will feel different than an engine issue that causes similar symptoms, as the transmission will be affecting acceleration by delayed shifts rather than misfires. Pay attention to your ride and you may be able to figure out if it’s the engine or transmission.


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