6 Reasons Why Your Car Shakes When It’s Idle

Anyone who’s been around cars for long enough has probably experienced one or two inconveniences or malfunctions. Despite their durability and capacity to provide years of reliable service, the vehicles we drive are not indestructible and are subject to wear and tear over time. While keeping up with routine maintenance and caring for your car properly are great ways to ensure its long-lasting performance, age and mileage will still take their toll. Ultimately, there’s no guarantee that your vehicle won’t develop at least a minor issue at some point. 

Advertisement

If that does happen, it’s vital that you’re able to recognize the signs and address the problem before it becomes more severe. Most drivers are aware of the fact that cars can make some pretty strange noises when something’s wrong. However, noises aren’t the only sign of mechanical trouble that your vehicle can display.

Another issue that you may encounter if your car is experiencing a malfunction is shakiness and vibration. This can happen at any time while your engine is running and can be the result of various problems. However, if your car shakes when idle, there are usually a handful of conditions that may be to blame. Some of the most likely reasons for a car to shake when idling include things like worn-out motor mounts or a problem with the ignition system. 

Advertisement

As a former professional mechanic, I’ll shed some light on the issue. We’ll cover the most common causes of this problem, as well as some of the potential fixes and what your next steps should be if you experience the issue yourself. So, from a dirty engine air filter to vacuum leaks and a damaged serpentine belt, here are six reasons why your car shakes when it’s idle.

Automotive engines are exceptionally heavy. Even smaller four and six-cylinder engines can weigh between 200 and 450 pounds, while larger V8s and diesels can weigh up to 700 pounds. All of that weight can bounce around a lot during normal driving, and it requires a robust support system to hold it securely in place. Motor or engine mounts are built using thick rubber bushings and are responsible for supporting the engine’s weight, while also allowing it to move with the vehicle as you traverse bumps and uneven terrain. Most modern cars have between two and four motor mounts. However, if even one of them fails, it can cause some serious issues for your vehicle, including some pretty unsettling shaking and vibration.

Advertisement

Failed motor mounts can cause excessive vibration while your car is idling. However, they may also cause lurches or shakiness while driving, as well as clunking or rattling noises. One of the first things you should do if you suspect a motor mount problem is inspect them visually. Engine mounts are typically located near the sides and back of the engine, and you should look for things like corrosion to the metal brackets and cracks or dry rot on the rubber bushings. 

Next, grab a pry bar and place it between the mount’s bushing and bracket. Apply light force and observe the mount — if it moves excessively or you can see cracks in the rubber, it may be bad. Finally, you can do what’s called power breaking. For this method, you’ll need a friend to sit in the driver’s seat and hold the break while revving the engine slightly. If the engine lurches when your friend hits the gas, you may need to replace one or more engine mounts.

Advertisement

Your car relies on various systems to keep the engine running and the wheels spinning. One of the most vital of those systems is the ignition system, which, for gas-powered cars, includes things like the spark plugs, ignition coils, and ignition switch. While the system is robust and designed to provide thousands of miles’ worth of service, it’s not invulnerable to failures or wear and tear over time. In particular, if the spark plugs, spark plug wires, or ignition coils fail, your vehicle is likely to develop several unpleasant symptoms, including a rough idle.

Advertisement

That’s because the spark plugs are responsible for igniting the mixture of fuel and oxygen inside your engine’s combustion chambers. They rely on an electrical signal sent through the wires and ignition coils to do so, but if one of those three components is malfunctioning or damaged, the spark plugs may be unable to perform their job properly. This can result in incomplete combustion or engine misfires, a rough idle, and a host of other problems that can cause extensive damage over time. 

While most modern cars require an engine tune-up (which includes a spark plug replacement) roughly every 100,000 miles, older cars typically need their spark plugs replaced every 30,000 to 40,000 miles. However, spark plugs and the related parts can fail at any point in a vehicle’s life. It’s vital that you monitor and inspect these components as a part of your routine maintenance plan, but if you notice signs of worn-out spark plugs, like engine misfires and a rough or shaky idle, you should visit a professional mechanic as soon as possible to avoid the development of more severe damage.

Advertisement

The fuel system is another one of your car’s most critical components. As the name implies, it’s responsible for sending gas or diesel from the fuel tank to the engine for combustion. It comprises various parts, including the fuel tank, a filter, fuel injectors or a carburetor, a fuel pump, and more. If the fuel system experiences a malfunction, like a clog or leak, it may be unable to send the proper amount of fuel to the engine. Understandably, that can cause a lot of problems for your car, including a rough or shaky idle.

Advertisement

The issue is likely to occur if the engine receives too much or too little fuel. When an engine gets too much fuel, it’s referred to as a rich condition. It can cause the engine to begin performing erratically, a bit like how you may feel jumpy and jittery if you drink too much coffee in the morning. You may notice a rough idle or jerky and erratic RPM movements on your dashboard tachometer. On the other hand, when the engine receives too little fuel, it’s known as a lean condition. When this occurs, the engine will struggle to perform the combustion process properly, and it may stutter or feel jittery as it attempts to run without the right amount of fuel.

Fuel system problems can be caused by various issues. Some common malfunctions that can lead to a shaky or rough idle include things like clogged fuel injectors, a worn-out fuel pump, a damaged fuel filter, and more. Other symptoms of fuel system issues can include general performance problems, a gas odor, and a check engine light. If you notice any of these issues, the smartest move is to visit a mechanic as soon as possible.

Advertisement

In addition to depending on various systems to help keep the gears and wheels moving smoothly, your vehicle also relies on several engine accessories. These parts include things like the air conditioning compressor, power steering pump, and alternator. In most vehicles, these parts have a pulley that connects to a drive or serpentine belt. The serpentine belt wraps around the crankshaft pulley and transfers that rotational energy to the various accessories. However, if the belt becomes damaged or degraded over time, it can slip or cause damage to the accessory pulleys. When this occurs, you’re likely to hear some strange noises, and you may notice that your car shakes or rattles while idling.

Advertisement

Serpentine and drive belts are generally highly durable components. Depending on your car, you can usually expect to get between 60,000 and 100,000 miles of life out of the belt, but it can fail prematurely or due to old age if you never replace it. When that occurs, the belt can develop damage and wear, like cracks or glazing, and it may start slipping on the pulleys. If the belt deteriorates sufficiently, it’s likely to begin affecting the engine accessories. 

A bad belt can damage the accessory pulleys, causing them to spin erratically or unevenly. If you don’t catch the problem quickly enough, the pulleys themselves are likely to begin rattling and causing excessive vibrations or shakiness as the engine runs, and it may be easier to notice these symptoms when the car is idling. Outside of these issues, a bad belt will most likely cause a squealing noise. If you experience these issues for yourself or notice signs of belt damage, you most likely need to replace your serpentine belt.

Advertisement

If your car uses a gas-powered engine for its power, it also depends on a vacuum system to help create that power. The vacuum is created by different parts of the engine at work, like the pistons moving downward in the cylinders during the intake stroke, the throttle body opening as you step on the gas, and the intake valve opening to suck in air for combustion. Vacuum is essential for various vehicle functions and contributes to everything from your power brake booster to the emissions control system. 

Advertisement

The vacuum system comprises various components, including a series of hoses that send vacuum pressure to other systems and parts. While the engine vacuum system is a vital part of your vehicle, its reliance on rubber hoses puts it at risk of developing leaks. When that occurs, your car is likely to experience some unpleasant symptoms, including a rough or shaky-feeling idle.

A vacuum leak can cause a rough idle because it can throw off the combustion process. Similar to what may happen if you have bad spark plugs, a vacuum leak can disrupt the air to fuel ratio inside your engine’s combustion chambers, resulting in things like engine misfires, a rough idle, and general performance issues. You may also notice symptoms like a loud hissing sound if air is being sucked through cracks in vacuum lines, decreased acceleration, increased fuel consumption, stalling problems, and the presence of a check engine light. Diagnosing vacuum leaks usually requires a professional inspection and may involve the use of a smoke machine to check for leaks. Other methods for diagnosing a vacuum leak include pressure tests and spraying something flammable like carburetor cleaner onto the hoses to see if the engine sucks in the fluid, indicating a leak.

Advertisement

We’ve already alluded to the fact that internal combustion engines require a precise combination of fuel and oxygen to burn. The air intake system is responsible for pulling in outside oxygen for the combustion process. It relies on a series of components, including a filter, intake tubing, and the throttle body. Those parts are durable and long-lasting, but they still require maintenance, especially the engine air filter. Many automakers provide replacement intervals for the engine air filter, and it’s good practice to inspect the filter each time you change the oil. Generally, you’ll need to replace your air filter roughly every 15,000 to 30,000 miles. However, the intervals can change based on the filter’s condition — if it becomes extremely dirty or clogged, you’ll need to replace it to avoid some serious issues, including a very rough engine idle.

Advertisement

If the air filter does get clogged up by dust or debris, your engine may not be able to receive the oxygen it needs to run. That can cause it to run rich, which, as mentioned, means there’s too much fuel and not enough air in the combustion chambers. That can lead to issues like engine misfires and a rough idle, and you may notice the appearance of the check engine light on your dash. Fortunately, changing your engine air filter is quick and simple. We already stated that you should inspect your air filter regularly, but it’s an especially important — and easy — component to check if you experience a rough or shaky engine idle.

If you notice that your car is shaking more than normal while idling, there are a few steps you should take. First, you should inspect your engine air filter. As mentioned, it’s an incredibly easy process, and the replacement is one of the best DIY auto projects for beginners. If the air filter is in good condition, you should move on to checking the serpentine belt or drive belts. Look for cracks, fraying, and glazing, as well as any signs that the belt has slipped or is no longer centered on the pulleys. 

Advertisement

You can also observe the serpentine belt and pulleys while the engine is running to check for signs of damage like rattling or shaking parts. Next, you should inspect your motor mounts. We covered a few ways to do so above, and it’s generally something you can do, even with very little automotive experience.

If all of those components appear to be in solid condition, you can choose whether you want to move forward with your DIY inspection or visit a professional. Checking the fuel system and things like the spark plugs and ignition coils often requires a few specialty tools, like spark plug sockets and a multimeter, while inspecting the vacuum system may require a smoke machine or pressure tester. You can absolutely check these parts at home, even if you’re inexperienced. However, you’ll need to follow the right guidance, like a vehicle-specific repair manual. If you’re not an experienced home mechanic or confident DIYer, the smartest move is to visit a professional mechanic with the tools and know-how to accurately and quickly diagnose the problem.

Advertisement