Cars That Shakes When Idle But Smooth When Driving: Fixed!

You might be wondering why your car is shaking when idle but smooths out when driving. Maybe you thought it was just a normal part of car ownership, but that’s not necessarily the case. In fact, the cause of this problem can range from a minor issue to something more serious.  In this blog post, I’ll explain why your car shakes when idle but smooth when driving, and how to diagnose the cause of the problem.

Cars that shake when idle can be caused by a variety of different issues, including worn-out spark plugs, bad battery, bad motor or transmission mounts, stuck PCV valve, clogged fuel injectors, bad MAF sensor, dirty throttle body, bad idle air control valve, clogged fuel filter and clogged engine air filter.

You can also read my guide on car sputters after refilling the gas to learn more.

First Thing To Do When Car Starts Shaking At Idle But Smooths Out When Driving

The first step to do when your car starts shaking at idle but smooths out when driving is to see whether the check engine light illuminates on the dashboard. It’s a diagnostic tool that can alert you to any problems or faults in your engine or other components. If the check engine light is illuminated, it’s an indication that something is wrong, and needs to be checked out.

If car is shaking at idle and check engine light is on, If the check engine light is on, use an OBD-II scanner to read the codes and identify the issue. However, in some cases, you can’t see confirmed trouble codes. Such codes appear for some time due to intermittent faults in the engine and then disappear from the engine’s memory. Bluedriver OBD2 scanner has a section of pending trouble codes that can detect these trouble codes. For this, you can read my guide on check engine light with no codes.

Causes Of Car Shakes At Idle But Smooth When Driving

1. Bad Motor And Transmission Mounts

Motor and transmission mounts are metal and rubber brackets that attach the engine and transmission to the chassis of the vehicle. They are designed to reduce vibrations and noise coming from the engine and transmission which can be caused by the powertrain components moving around. 

When the motor mounts are worn out, the engine and transmission are not held in place as well as they should be. This can cause the engine and transmission to move around more than they should when the car is idling. When the car is driving, the movement is dampened by the motion of the car.

How to spot?

You can use your vehicle’s owner’s manual to find the locations of the motor and transmission mounts in your engine.

Here are some tips for inspecting the motor and transmission mounts:

  1. Check for loose bolts: Inspect the motor and transmission mounts for any loose bolts. If the bolts are loose, it could mean that the mount is worn out and needs to be replaced.
  2. Check the rubber: Inspect the rubber of the motor and transmission mounts for any cracks or tears. If the rubber is cracked or torn, it will need to be replaced.
  3. Check the mounting bracket: Inspect the mounting bracket of the motor and transmission mounts for any signs of wear or damage. If there is wear or damage, it could be a sign that the mount is worn out.

2. Fouled Spark Plugs

fouled spark plugs

Spark plugs work when a voltage is sent to them from the car’s ignition system. The voltage travels down the spark plug’s center electrode and arches across the gap to the outer electrode. This creates a spark that ignites the fuel and air mixture in the cylinder. As the mixture ignites, the piston is forced down and the crankshaft turns, creating power.

When spark plugs become worn out, they do not create a strong enough spark to ignite the fuel mixture. This causes an engine to misfire, which can cause the car to shake when idle. However, because the car is moving when it is driving, the engine is able to generate enough power to ignite the air-fuel mixture, thus creating a smooth ride. 

How to spot?

The first step is to locate the spark plugs in your engine. Once you have located the spark plugs, you will need to remove them. This can be done with a socket wrench or a spark plug socket. Once the spark plugs are removed, you’ll want to check them for signs of wear, such as corrosion, carbon buildup, cracks in the porcelain, or an improper gap between the electrodes. 

You can read this PDF to learn about signs about bad spark plugs.

3. Malfunctioning Idle Air Control Valve

One of the most common causes of an engine shaking while idle is a malfunctioning Idle Air Control (IAC) valve.

An idle air control valve (IACV) is an electronically operated valve that is used to regulate the amount of air that flows into the engine at idle. The IACV works with the vehicle’s ECU (engine control unit) to ensure smooth idling. IACV is located on the side of the throttle body.

At idle, the engine needs a certain amount of air to function properly. The IACV helps to regulate the amount of air that enters the engine. When the IACV is working correctly, the engine runs smoothly.

You can watch below Youtube video to learn the working of the Idle Air Control Valve:

If the IACV is malfunctioning, it will no longer be able to provide the correct amount of air to the engine, resulting in the engine running rough. This is what causes the car to shake when idling.

How to spot?

The idle air control valve can be susceptible to carbon buildup. 

To test a faulty IACV, begin by idling the engine. Place your finger over the hole in front of the throttle plate, blocking it. The idle should drop. As soon as you remove your finger, the idle should jump up before returning to its usual rate. This sudden change in RPM from low to high and back to its initial state is the PCM detecting the blocked bypass air port, retracting the IACV, and then allowing air into the bypass port. This air causes the RPMs to rise and the PCM detecting the high RPMs extends the IACV to return the idle to its normal rate.

Another way to test IACV is by measuring voltage and resistance across the terminals of its connector using the multimeter. You can watch the below Youtube video for a better understanding.

3. Stuck PCV Valve

A PCV (Positive Crankcase Ventilation) valve is a small, one-way valve located inside the crankcase of most engines. It is designed to allow the crankcase gases to be recirculated back into the combustion chamber, instead of venting them into the atmosphere. The gases are recirculated in order to reduce pollutants created by the exhaust. The PCV valve is usually found on the intake manifold, near the crankcase.

The PCV valve:

  • Closes slightly to reduce airflow at low RPM (idle), when the vacuum is high.
  • Opens more to increase airflow at high RPM, when the vacuum is low.

If the PCV valve becomes stuck in the open position, it will allow an excessive amount of air to enter the intake manifold. This will cause the air/fuel mixture to become too lean, which will result in a rough idle. 

How to spot?

PCV valve can become stuck opened or closed due to oil deposits. To test if the PCV valve is stuck closed, you can remove the oil filler cap and place the paper on its opening. If it holds the paper against its opening, it means that the PCV valve is malfunctioning.

4. Clogged Fuel Injectors

A fuel injector is an electronic device that is mounted to the fuel rail and injects fuel directly into the engine. It is controlled by the engine control unit and is responsible for ensuring the proper delivery of fuel to the engine.

A fuel injector contains three components: a pin, a coil, and a spring. The spring keeps the pin closed until a current is applied to the coil, which opens the pin and allows fuel to flow through it.

fuel injector construction

When fuel injectors become clogged, they cannot efficiently provide the correct amount of fuel to the engine. This can cause the engine to run rough, rev on its own and can even lead to stalling.

How to spot?

Spotting a bad fuel injector can be tricky, but there are some telltale signs that can alert you to a potential issue.

One test for a bad fuel injector is to observe if it is emitting a clicking noise or not. For this, you would need a long screwdriver. Now, pull out the fuel rail. But make sure that it is connected to the fuel supply hose. Turn the key on to prime the fuel pump and pressurize the fuel line. Now, hook the 9v or 12v battery one by one to each fuel injector and try to hear its ticking noise with a long screwdriver.

That ticking noise is basically the opening and closing of a pin in the fuel injector. Also, see if the fuel injector is injecting fuel in a certain direction without leaking or not. If it’s leaking, you would need a new fuel injector.

You can check out the following video from 2:40 to get an idea of how to test fuel injectors.

Apart from that experiment, check these things on the fuel injector:

  • Look for burrs on the injector inlet
  • Check nozzle holes for hole erosion or plugging
  • Inspect the end of the nozzle for burrs or rough machine marks
  • Look for cracks at the nozzle end

5. Malfunctioning MAF Sensor

MAF sensor

The mass airflow (MAF) sensor is a component of the car’s electronic engine control system that measures the amount of air entering the engine. This air is then used to calculate the fuel injection rate, which is necessary for optimal engine performance.

The MAF sensor is located in the intake system between the air filter and the intake manifold and is typically composed of a plastic housing and an interior sensor.

When the MAF sensor malfunctions, it can feed incorrect information to the engine control system, resulting in a poor air-fuel mixture. This can cause the engine to run too rich, resulting in reduced power and torque. The reduced power will cause the engine to shake.No

How to spot?

The MAF Sensor can become bad due to a few different reasons. The most common reason is due to the buildup of dirt, debris, or oil. This buildup can interfere with the MAF Sensor’s ability to accurately detect the amount of air entering the engine, resulting in an incorrect air-fuel ratio. This can happen due to a clogged air filter, or due to dirt and dust entering the intake manifold through a vacuum leak.

To clean the MAF sensor, follow these steps:

  • Disconnect the negative terminal of the battery.
  • Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
  • Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner. Avoid touching the wires of the MAF sensor. It will damage it.
  • Reinstall and reconnect the battery terminal.
  • Start the engine and let the ECU relearn idle for 15 minutes.

Note: Not all vehicles have MAF sensors. Instead, some vehicles have MAP (Manifold Air Pressure) sensor. The MAP sensor cannot be cleaned. It should be replaced.

To test the MAF sensor, you can watch the below Youtube video:

6. Clogged Engine Air Filter

An engine air filter is a part of the air intake system of your car’s engine. This filter traps dirt and debris, such as dust and pollen, that can enter the engine when the air is drawn in through the air intake. This helps to keep the engine clean and running smoothly. An engine air filter is located before the MAF sensor.

A dirty or clogged engine air filter can cause your car to shake when idle, but be smooth when driving. This is because a dirty or clogged filter can restrict the airflow to the engine. When the airflow is restricted, the engine has to work harder to draw in the necessary air, which can cause the engine to run rough and cause the car to shake.

It is important to note that this car shaking may not be noticeable when you are driving because the engine is receiving more air and is able to function more efficiently. However, when the car is idle, the engine is not receiving enough air and is unable to function properly. This can cause the engine to vibrate, resulting in your car shaking when idle. 

How to spot?

You can pop the hood, locate the engine air filter and visually inspect it. Clogged engine air filter will have smell of dust. It will also be dull in color due to stains of dust and smoke. The color of a clean engine filter is either pinkish or white.

The best way to prevent a bad engine air filter is to regularly check and replace it. The recommended interval for replacing your engine air filter is every 15,000 to 30,000 miles, depending on the type of vehicle and driving conditions.

7. Bad Engine Battery

If you’re experiencing your car shaking at idle but running smoothly when driving, it could be related to an issue with the battery. For more information on these potential causes, be sure to check out our other post “My car starts sometimes and sometimes it doesn’t” for more information on possible causes and solutions for this problem.

The battery is an essential part of the electrical system in a car. It supplies the power to start the engine, and it can also provide additional power to the electrical components (spark plugs, ignition coils while the engine is running. If the battery is weak or dying, it may not be able to supply the necessary power, resulting in a number of problems. 

How to spot?

A healthy battery has a voltage of:

  • 12.6V when fully charged and the engine is not running
  • 13.5 to 14.5V when the engine is running
  • 9.6 to 10V when the engine just cranks before starting (it only drops to 9.6V for a couple of seconds when the engine cranks). As the engine starts running, the alternator supplies the current to the battery.

So, you should use a multimeter and check voltage across battery terminals of your car.

8. Dirty Throttle Body

dirty throttle body

If a throttle body is dirty, it can cause a car to shake when idle but run smoothly when driving. 

A car’s throttle body is an essential component of its engine management system. It regulates the air/fuel mixture that gets drawn into the engine as the driver accelerates. The throttle body is connected to the gas pedal, so when the driver presses the gas pedal, it opens the throttle body, allowing more air to flow through. This increases the engine’s power and efficiency.

The mechanical throttle body is driven by a throttle cable that links the accelerator pedal to the throttle plate. If the throttle cable is too tight or sticking a little, it will not be able to have full control over the throttle plate. As a result, your car engine’s RPM will boggle without pushing a gas pedal.

The electronic throttle body is controlled by the ECU. The accelerator pedal sensor sends signals to the ECU so that it can compute how much the throttle valve should be opened.

How to spot?

A dirty throttle body can be caused by a number of things. The most common cause is a build-up of dirt and debris in the air intake system. This build-up can be caused by a number of things, such as a dirty air filter or a leaky air intake hose. The dirt and grime can then get mixed with the lubricating oil and form a sticky substance that affects the throttle body’s operation.

Now when there are sticky dirt and carbon deposits in the throttle body, they will prevent the butterfly valve of the throttle body to move freely, which will disturb airflow to the engine.

You should use this throttle body cleaner to clean the throttle body. Make sure that the throttle plate of a throttle body is not binding when you try to move it with your finger.

Note: You have to perform the ECU relearn procedure for an electronic throttle body after cleaning. I have explained the procedure of throttle body relearn in my guide on the P1516 code.

Important: A bad fuel pressure regulator can cause the throttle body to become dirty again with carbon deposits. So, you should also check the fuel pressure regulator of your engine. If you have (Gasoline Direct Injection) engine, you will have a fuel pressure sensor instead of a fuel pressure regulator. In that case, you don’t have to worry about it.

9. Vacuum Leaks

vacuum hoses in engine

A car’s engine air intake system works by creating a vacuum when a piston moves down in the engine cylinder. If you look at the air intake part of the engine, there would be small hoses originating from it. These hoses are connected to the PCV valve, fuel pressure regulator, charcoal canister, brakes, etc. 

When a leak occurs in vacuum hoses or air intake hoses, extra air is sucked into the engine, which is not measured by the MAF sensor. This air is termed ‘unmetered air’. 

When the engine is idling, it has maximum vacuum as the throttle body is not that much opened. So, when there are vacuum leaks, a large amount of unmetered air can enter the engine. The ECU can’t detect this air and adjust fuel injection accordingly. As a result, the air/fuel ratio of an engine is disturbed, due to which the car starts shaking at idle.

When you press the acceleration pedal and start driving, the vacuum decreases. As a result, a large amount of air cannot be sucked into the engine due to vacuum leaks as it happens at idle. So, the engine smooths out when driving.

How to spot?

Here is how to spot vacuum leaks in engine:

  1. Worn or cracked vacuum hoses: Vacuum hoses are the tubes in the engine air intake system that help to create the vacuum. If these hoses become worn or cracked, it can cause a vacuum leak.
  2. Faulty or damaged intake manifold gasket: The intake manifold gasket seals the engine air intake system and helps to create a vacuum. If this gasket is faulty or damaged, it can cause a vacuum leak.
  3. Defective intake valves: Intake valves are the valves that open and close in the engine cylinder to create the vacuum. If these valves are defective, they may not be able to create a vacuum, which can lead to a vacuum leak.
  4. Faulty PCV valve: The PCV (positive crankcase ventilation) valve is responsible for controlling the flow of air and fuel in the engine. If this valve is faulty, it can cause a vacuum leak.

You can use a carb cleaner to detect vacuum leaks in the engine. You can watch the below Youtube video for a better understanding.

10. Dirty Engine Oil

Engine oil is a lubricant that helps keep the moving parts of the engine, such as the pistons, cylinders, and valves, running smoothly. Over time, the oil can become dirty and thick due to a buildup of debris and sludge.

Sludge is a thick, gooey substance that accumulates in your engine oil as a result of engine wear and tear. It’s made up of a combination of dirt, debris, and contaminants and can reduce the flow and efficiency of your engine oil. This can cause the engine oil to not circulate properly and can lead to friction that is felt as a vibration in the car.

At idle, your engine is running at low RPMs, meaning the oil is not circulating as quickly as it would be at higher RPMs. This means any sludge that has built up in the engine oil will not be properly circulated, resulting in an uneven flow of oil and an overall decrease in engine efficiency. This, in turn, can cause your car to shake when idle. 

How to spot?

You can check the quality of the oil using a dipstick. It is recommended to change the oil every 5000 km to ensure smooth engine operation. Furthermore, you should use recommended oil viscosity grade. You can read my guide on difference between 5w20 and 5w30 engine oil to learn more.

Final Thoughts

Cars that shake when idle but smooth when driving can be a tricky problem to diagnose. It can be caused by a variety of issues, ranging from a worn-out engine mount to a misfiring cylinder. It is important to get the car checked out and fixed as soon as possible to avoid more serious problems.