10 Reasons Your Car Shakes When Idle

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Feeling your car shake when you idle is one of the first signs of an issue with your car. Unfortunately, not only is this unpleasant to experience it is also difficult to determine the reason your car is shaking while idling. 

Don’t worry. I’m here to help. 

This post will go over the 10 reasons your car shakes when idle and what you can do about it. 

Let’s get started. 

1. Broken Motor Mounts

If your motor mounts are broken or disconnected from the engine, your vehicle will shake while idling. When your motor mounts are broken, this gives the engine more space to move around in the engine bay, causing your engine to shake.

The vibration from the engine can be felt from the driver set when motor mounts are broken, disconnected, or damaged.

You may hear banging, rattling, and clunking noises due to the broken motor mounts.

If the hood is open with the engine idling or revving, you will notice the engine is moving side-to-side excessively or moving back and forward. This is a clear sign your motor mounts are broken or disconnected from the frame.

The engine will move around more aggressively during acceleration, hard braking, and turning on a vehicle with broken motor mounts.

Driving for an extended amount of time with broken motor mounts could cause damage to your fan belt and radiator hoses. The vibrations coming from the engine and the misalignment of the engine create damage.

Most standard vehicles typically have two motor mounts located on both sides of the engine to keep the engine secured in the correct location. 

Motor mounts most commonly will fail due to age and wear. 

The rubber on the motor mount over time can lose its elasticity causing your engine to lose its support. Cracks could form on your motor mount causing failure.

2. Misaligned and Worn Out Belts

Misaligned or worn-out Serpentine belts can cause vibrations while your vehicle is idling.

Typically if your serpentine belt is misaligned, the belt will make a screeching or a chirping sound due to the loose-fitting belt slipping on the underdrive pulleys.

A worn-out belt or a misaligned belt connected to your engine or other components could be the reason why your vehicle shakes while idling.

The serpentine belt powers your AC compressor, water pump, power steering, and your alternator.

If your belts are worn out or loose-fitting on the underdrive pulleys, you could experience shaking or vibrations inside your vehicle.

Your vehicle is working harder to stay running while idling.

Inspect your vehicle belts by looking for cracks, rips and also check for tight tension.

If you see damage or excessive wear on your belts, we recommend that you replace your belts. Belts are inexpensive and easily accessible.  

3. Bad Fuel Intake System

Over time your fuel take can get dirty and worn down. Your fuel intake system is more likely to fail when your car has lots of miles.

Fuel intake systems can begin to get dirty and clogged. This decreases the flow of air and fuel into the engine.

A clogged or restricted fuel intake will decrease performance and also make your vehicle shudder and vibrate while idling.

A restricted fuel intake can also make your vehicle drive rough at low and high speeds. Therefore, when diagnosing your vehicle for fuel delivery issues, it would be best to double-check your fuel pump and filter to make sure that is not your issue.

A restricted or bad fuel pump could give out some similar symptoms as a clogged fuel intake.

Your engine fuel/ air mixture ratio will be affected without proper fuel delivery, causing your vehicle to stumble and hesitate while idling or accelerating.

Using a fuel intake cleaner or replacing your old worn-out fuel intake will improve or eliminate your shaking while idling issue. If your fuel intake is old and clogged, we recommend replacing it to ensure your engine receives the proper amount of fuel/ air mixture.

4. Bad Spark Plugs

Fouled-out or worn-out spark plugs can also cause your engine to shake when idle. 

The most common reason is when your spark plug is fouled out. This means that your fuel to air mixture is running rich. This means that your mixture is running rich or with excess fuel. 

This typically causes your engine to misfire. When your engine misfires or after it misfires, this can trigger your engine to shake. 

An engine misfiring can cause your vehicle to shake during idle or hesitate during accelerating.  

Spark plugs, spark plug wires, coils, and distributors all play a big part in the ignition process to keep the air/fuel mixture in the correct operating range. 

We recommend replacing your spark plugs every 35,000 – 50,000 miles. If you suspect your spark plugs are not sparking, igniting, causing your vehicle to misfire before the 35,000-50,000 mark. Replace your spark to ensure your vehicle is running properly to avoid future vehicle issues. 


Diagnosing your spark plug misfire 

  • OBD reader will help diagnose your engine misfires
  • A spark plug tester will tell you if you need to replace your spark plug.  

Spark plug reader chart helps identify possible issues by looking at the outside of your spark plug

Replacing your spark plugs is an inexpensive job, and spark plugs are very accessible to attain. Most local automotive stores typically keep spark plugs in stock for your convenience.   

5. Clogged Air Filters

A restricted air filter can make your vehicle shake or vibrate while idling because the combustion engine isn’t receiving enough air to balance out the air/fuel mixture.

When a combustion-style engine is receiving limited amounts of air, your vehicle engine will run rich.
Running rich means your vehicle engine is receiving more fuel than air, causing your vehicle to run rich. When your vehicle engine is running rich, you could experience a rougher idle and decrease in fuel economy.

A restricted air filter will reduce the engine horsepower causing your vehicle to hesitate and misfire during acceleration. While your vehicle is idling, the restricted air filter will drop your idling RPM’s drop to 600-700 range causing your vehicle engine to shake and spotter.

The good news is replacing air filters is inexpensive and regularly accessible at your local automotive parts store. Replacing your air filter is an easy do-it-yourself job with just a couple of hand tools to get the job done.

An air filter replacement cost will range from $10-$30 dollars depending on the brand of the air filter. Replacing your current air filter allows you to purchase a reusable air filter or a traditional air filter that can only be used once.

6. Bad Vacuum Line

A loose Vacuum line or leak can cause your vehicle engine to run rough while idling or accelerating. Over time vacuum rubber lines develop leaks in the system due to old age and heat.

A vacuum line is connected to your intake manifold, the primary source of the engine vacuum. The throttle control determines how much air is allowed to enter your cylinder heads.

If your vacuum line on your vehicle engine starts leaking, your vehicle engine could run lean. Running lean means your vehicle engine is receiving more air than fuel, causing your engine to run lean. Driving your vehicle engine lean can cause serious engine damage.

Vacuum lines are needed to balance out the air/fuel mixture to prevent misfires, increase RPM while idling, running lean and rough idle.

Replacing a vacuum line is inexpensive, but the main issue with leaks is finding the leak and seeing where it’s coming from. If you cannot find the vacuum leak, contact your local maintenance shop and schedule an appointment for repair.

7. Clogged fuel Filter

A clogged fuel filter pump starves your engine of fuel, lowering fuel pressure. The low fuel pressure makes your vehicle engine run rough while idling or accelerating. A clogged fuel filter takes the air/fuel ratio range of balance, causing your engine to misfire.

If you notice your fuel pressure drops while driving, pull over and turn your vehicle off immediately to avoid permanent damage. When your fuel pressure drops too low PSI levels that means your vehicle is running lean, or taking in more air than fuel. Running lean could possibly cause complete engine failure.

If you suspect your fuel filter is clogged replace your fuel pump before driving your vehicle again to prevent running your vehicle engine lean on fuel.
Your fuel filter is typically located in the Fuel tank. The fuel filter can be mounted outside the fuel tank near the back of the vehicle, depending on your make and model vehicle.

Replacing a fuel filter can be more difficult for the average vehicle owner. The actual fuel filter is inexpensive and attainable at most local automotive parts stores.

We recommend towing your vehicle to a shop where a trained professional mechanic can replace your fuel filter and diagnose your fuel delivery issues. Having low fuel pressure shouldn’t be taken lightly.

8. Bad Fuel Injectors

Bad fuel injectors will fail to provide your engine with enough fuel to run properly. This can cause your engine to shake and sputter as your engine fights to get more fuel. 

Fuel injectors spray gasoline into your engine based on throttle position. The more throttle pressure applied to the pedal, the more fuel will be sprained into the engine.

One of the main reasons fuel injectors begin to fail is because they get clogged. Fuel injectors use pintless to spray fuel with wear and tear, dirt, and debris. This will cause your fuel injectors to fail or stop working properly. 

Some common signs of a bad fuel injector include: 

  • White or gray smoke coming from the tail pipes
  • Engine Knock 
  • Low fuel pressure
  • Bucking with a throttle load
  • Decreased performance  
  • Rough idle
  • Failed emissions test
  • Engine limited to low RPM levels 
  • Detonation causing your engine to catastrophic fail 

Diagnosing your bad fuel injector can be difficult. However, it only requires that one fuel injector not be working properly to cause performance issues with your engine. 

The majority of vehicles have one fuel injector for each cylinder. It only takes one fuel injector to be clogged for your vehicle to run rough while idle or accelerating. 

Using fuel injector cleaner could help clean out the dirt and debris from your small injector nozzles. You just add an injector cleaner to a full tank of gasoline and simply drive your vehicle. As you drive along, your fuel injector is getting cleaned out since the injector clean is mixed in with your gasoline.  

9. Holes In Your Exhaust Pipe

Holes in your exhaust pipe sound unpleasant and also increase the vibrations inside the vehicle. Over time your exhaust pipe can get rusty creating holes within the exhaust system. It’s possible your exhaust pipe suffered an impact from underneath creating a hole in the exhaust. 

Holes in the exhaust pipe can allow exhaust fumes to seep into the interior space of your vehicle. The exhaust fumes exposure can give your carbon monoxide poisoning from the gas fumes cause you to feel sick. 

Symptoms you make the experience

  • Vomiting
  • Nausea
  • Coughing
  • Flu-like symptoms

If you notice your vehicle is increasingly louder than normal check your exhaust system for holes. To fix the exhaust leak is inexpensive, but if your full exhaust system needs to be replaced. Expect higher prices due to the full exhaust system must be replaced. 

10. Bad Mass Air Flow Sensor

 A bad mass airflow sensor can cause your vehicle to run rough while idling or accelerating. The mass airflow sensor is responsible for calculating the best air/fuel ratio for the engine to run efficiently. 

If your mass airflow sensor is bad your air/fuel ratio will not be balanced. An unbalanced air/fuel ratio causes your vehicle to run lean or rich and misfire under a load. The driveable of your vehicle is difficult and rough while your air/fuel mixture is unbalanced. 


Signs Of A Bad Mass Air Sensor 

  • The engine will run rich or Lean 
  • Hard to start 
  • The engine will stall out 
  • Jerking and hesitating
  • Decrease performance 
  • Misfires 

Replacing your mass air sensors would be recommended if you are experiencing some of the bad mass air sensor signs. Visit your local repair shop to get your vehicle properly diagnosed and repaired. 

Mass air sensor replacement charges are reasonable and will not require a lot of labor time to replace. The sensor is located right before your cabin filter on the left or right side of your engine bay depending on the make and model of your vehicle. 

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