Ask the mechanic: Why does my car jerk when accelerating?

Simply driving your car shouldn’t feel like you are damaging it. Yet when something goes wrong, you will 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 acceleration?”. Here’s what you need to know.

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 fuel commercials, but it’s really more like a super-fine mist. The tiny openings in the injector help atomize the fuel 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.

Throttle position sensor

A throttle position sensor (TPS) monitors the position of the throttle and sends a signal to the ECU for best performance, fuel 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.

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.

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.

Filthy air filter

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

These little metal rings keep your compression and oil pressure up by preventing blow by between the piston and cylinder walls.

Symptoms of worn piston rings include terrible engine performance, oil burning and low compression, with lacklustre 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 gases through it in order to function. If the converter is worn out or clogged, exhaust gases 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 sulphur smell and a check engine light.

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.

Jumped timing chain/belt

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 mechanical workshop. 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 accelerator 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.

The writer is Team Leader Germax Auto Spares & Garage

Lukadde Road, Naalya.

0779250886/0752088734