Car Shakes at Idle But Smooths Out While Driving

The miniature, controlled explosions of an internal combustion engine are always going to produce a little extra vibration. Though there’s a point where you’re only right to wonder if the car is shaking a little too much when you aren’t heading down the road.

It’s not just garden-variety paranoia to wonder why your car shakes at idle but smooths out when driving?

Sometimes this is par for the course with a larger engine like a V8, and sometimes it can be linked to a problem like bad motor mounts or fouled spark plugs.

7 Common Reasons Why a Car Shakes at Idle But Smooths Out While Driving

A lot of times, the reason a car shakes at idle but smooths out while driving is linked to a fuel system issue. Though other things like bad motor mounts, foul spark plugs, and worn-out belts can also be a foot.

  • Fouled Spark Plugs
  • A Bad Idle Control Valve or IAC Sensor
  • Bad Motor Mounts
  • A Fuel System Problem
  • Clogged Fuel Injectors
  • Loose Hoses
  • Worn-Out Belts

1. Fouled Spark Plugs Causing A Hard Idle

Fouled Spark Plugs Causing A Hard Idle

Spark plugs play a critical role in the ignition process needed to maintain internal combustion in the engine’s cylinders. When spark plugs are fouled by carbon deposits, oil residue, or age, they don’t fire correctly. This can leave unburned fuel in the cylinders leading to misfires, stress on the engine, or causing the engine to run hard.

Symptoms of Fouled Spark Plugs

  • Hard Idle
  • Poor Gas Mileage
  • Occasional Misfires
  • Poor Acceleration
  • Hard Starting

How to Diagnose Fouled Spark Plugs

The easiest way to diagnose fouled spark plugs is to pull the plugs and examine the tips. If they look dirty, blistered, or burned in some way, they’re probably at least part of the reason why your car shakes at idle but smooths out while driving.

You can also diagnose a spark plug problem by hooking the car up to an OBD II code reader. The codes for spark plugs typically start with P03, and then a number indicating the cylinder. For example, P0301 is the code for a bad spark plug on the first cylinder.

How to Fix Fouled Spark Plugs

The best fix for fouled spark plugs causing your car to shake at idle is to have any/all the bad plugs replaced. This is something you should do every 75,000 to 100,000 miles as part of an overall tune-up. Though if your car has been running hot or had a fuel system problem recently, it can accelerate the age of the spark plugs fouling them prematurely.

2. A Bad Idle Air Control Valve or IAC Sensor

Much of the air intake system on modern-day vehicles rely on the throttle body, which can sometimes be fouled by carbon deposits and a process known as coking. When your car is idling the Idle Air Control Valve and its related sensor work to maintain the “Air” portion of the fuel/air ratio.

If the idle air control valve is bad, or the sensor regulating it has a fault, the air ratio can be off causing the car to shake at idle but smooths out while driving as the ECU demands more air for the combustion chamber.

Symptoms of a Bad Idle Air Control Valve or IAC Sensor

  • A Dying Battery or AlternatoAn Unusually High or Low Idle
  • Car Shakes at Idle
  • Stalling While Idling
  • Check Engine Light
  • Code Reader Throws Code P0505

The Code P0505 typically means that the IAC sensor has a fault in it. Sometimes it’s just the sensor, though the majority of the time, the IAC valve itself also has a problem.

How to Fix a Bad Idle Air Control Valve or IAC Sensor

If a bad idle air control valve is causing your car to shake at idle, you can usually clean it yourself using the following steps.

  • Step One: Remove the air filter box.
  • Step Two: Completely remove the air duct, which might include carefully disconnecting some hoses.
  • Step Three: Pull the air duct from the throttle body.
  • Step Four: Locate the opening to your idle air control valve, and inspect I for any deposits.
  • Step Five: If it only looks slightly gummed, you can clean it by following the direction on the throttle body cleaner.

If the IAC sensor is bad and throwing code P0505, you can usually reset the IAC valve sensor via the following steps.

  • Step One: Turn the car on or turn the key into the ON position.
  • Step Two: Make sure the car is in park and set the electronic parking brake.
  • Step Three: Slowly press down on the gas pedal.
  • Step Four: Release the pedal and leave the car in the ON position for at least 5 to 10 seconds.
  • Step Five: Turn the car off for at least 10 seconds.
  • Step Six: Turn the car on to see if it’s idling correctly.

This reset of the IAC is usually just a short-term fix that will help smooth out the idle until you can get a permanent repair done.

3. The Car Has Bad Motor Mounts

The Car Has Bad Motor Mounts

The motor mounts serve a critical role in connecting the car’s engine as well as the transmission to the frame. They also help to cushion some of the vibrations that you feel in the cab. As the vehicle ages, the rubber in the motor mounts can break down, causing an increasing amount of vibration.

A bad motor mount is not something you should drive with. Especially at high speeds. It can also cause serious engine damage.

Symptoms of a Bad Motor Mount

  • Hard Idling
  • Hard Starting
  • Excess Road Noise
  • Impact Noises When Driving Over Potholes & Obstacles

How to Fix Bad Motor Mounts

Bad motor mounts typically require professional repair. If you attempt to fix one yourself, and the mount later fails causing damage to another person, vehicle, or property, you could be held liable.

A lot of times when one motor mount goes bad causing a car to shake at idle, the other mounts are soon to go as well. So, it might make sense to have a mechanic address them all at one time.

Having a mechanic repair/replace a single bad motor mount usually costs between $200 to $500 per mount.

4. A Fuel System Problem

A Fuel System Problem

When a fuel filter goes too long without being replaced, it can affect the natural flow of fuel to the engine, not to mention causing severe strain on the fuel pump.

When an insufficient volume of fuel is delivered to the fuel injectors and the combustion chamber it can cause the car to shake at idle.

Then when you step hard on the gas and demand increases from the ECU, it smooths out. Though a badly clogged fuel filter and/or a failing fuel pump are problems that will only get worse in a short order of time.

Symptoms of a Fuel System Problem

  • Rough idle
  • Car shakes at idle
  • Stalling
  • Foul Smelling Exhaust
  • Misfires
  • A Humming Noise Coming from the Fuel Pump

How to Fix a Fuel System Problem

Fixing a fuel system problem that causes a car to shake at idle but smooth out while driving, typically starts with replacing the fuel filter. This is something you can do on your own for a cost of less than $100. You can also have it done when you get an oil change.

The concern is that a badly clogged fuel filter and time have compromised the fuel pump. If you replace the fuel filter, and the car still shakes at idle, with a buzzing sound coming from somewhere around the fuel tank, chances are good you need to have a mechanic replace the fuel pump as well.

The cost to have a mechanic replace a bad fuel pump ranges between $250 to $1,200.

5. Clogged Fuel Injectors

Clogged Fuel Injectors

Clogged fuel injectors are another reason why a car might shake at idle but smooth out while driving. Especially if you’ve been lax about replacing the fuel filter, and the fuel pump is still good, but the shaking at idle persists.

Clogged fuel injectors might also throw codes P0171 and P0174. These indicate that the engine controller is seeing a lean or rich condition and needs to be addressed. Sometimes the fuel injectors are partially clogged allowing some, but not the correct amount of fuel in to maintain a properly balanced fuel/air ratio.

Symptoms of Clogged Fuel Injectors

  • A Rough Idle
  • A Dancing RPM Needle. …
  • Frequent Stalling
  • Poor Gas Mileage. …
  • Engine Misfires

How to Fix Clogged Fuel Injectors

If you’re lucky and the fuel injectors are only partially clogged, you might be able to simply add some fuel injector cleaner to the gas tank the next time you fuel up. It can take at least half a tank or more before you notice the shaking at idle improving.

If they are badly clogged, the fastest and easiest fix is replacing the clogged fuel injectors completely. This rides the line on what the average home mechanic can handle on their own.

The cost to have a mechanic replace your clogged fuel injectors ranges from $350 to $800 for all the injectors. Though they might charge more for a car with a performance-tuned engine.

6. Loose Hoses

The hoses in the engine bay play a variety of roles. Though when a car shakes at idle but smooths out while driving it could mean an air leak or an exhaust leak caused by a loose hose. This might be a hose clamp that’s succumbed to time, or it could be a hole or crack in a hose.

Finding a loose hose can feel a little bit like chasing a gremlin around your engine bay. I’ve thrown up my hands more than a few times when I thought I found it, only to be wrong.

It starts with a visual inspection for loose hose clamps, cracks, ruptures, or signs of rotten rubber. You can also start the car and listen for a hissing sound as you follow the vacuum diagram for your make and model of car.

Symptoms of a Loose Hose

  • A Hard Idle
  • Hissing Sound in the Engine Bay
  • Engine Feels Down on Power
  • Trouble Starting
  • Check Engine Light

How to Fix a Loose Hose

If it’s just a matter of a hose clamp coming loose or breaking, you are usually better off replacing it than you are trying to tighten it again. I’ve had more than one tightening attempt just to repeat the problem 10 miles down the road.

If it’s a hole, crack, rupture, or rotten section of rubber then you’ll have to replace the hose itself completely. Duct tape is a bad fix for something like this and something you should only consider if you simply need to limp home to affect a more permanent fix.

You might be able to replace a bad hose yourself for $80 to $150 depending on which hose has failed. Otherwise, a mechanic will charge you between $120 to $450 to replace a bad hose.

7. Worn-Out Belts

Your car’s serpentine belt interlinks a variety of pulleys that coordinate a wide range of engine components. Not the least of which is the cooling fan. All serpentine belts have a limited lifespan, and as they start to slip or affect the performance of the engine, it can lead to a car that shakes at idle but smooths out while driving when the revs come up.

Symptoms of a Worn-Out Belt

  • Visible Cracking & Fraying on the Belt
  • Slapping Sounds
  • Rough Idle
  • Engine Running Hot
  • Pulley Engine Components Have Problems

Ironically one of the places where a worn-out belt starts to affect first is often in the power steering system.

How to Fix a Worn-Out Belt

You can usually fix a worn-out serpentine belt on your own with a replacement belt and a tensioner tool via the following steps.

  • Step One: Check the belt diagram & take copious pictures of how the serpentine belt snakes through the engine.
  • Step Two: Release the tensioner
  • Step Three: Inspect the belt and all the pulleys to see if an alignment issue damage the belt.
  • Step Four: Carefully thread the new belt over all the pulleys, following the diagram, and using the photos as your guide.
  • Step Five: Tighten the tensioner.
  • Step Six: Start the car. Let it run for a minute or two and see if it still shakes at idle, before taking it for a test drive.

Frequently Asked Questions

Is it normal for the engine to shake a little?

A little amount of shaking at idle is normal for most cars. Especially V8 engines. Though these natural vibrations are supposed to be minimal in a properly functioning vehicle and shouldn’t be described casually as “Shaking.”

Is it safe to drive when your car is shaking?

If your car shakes at idle but smooths out while driving, you should be safe to drive less than 10 miles home and start the troubleshooting process.

If your car starts shaking while you’re in gear driving down the road, then you absolutely need to pull over and get it towed, as you likely have a more serious problem brewing in the engine bay.

Conclusion

There are a lot of things that can cause a car to shake at idle, but smooth out while driving. Your first suspicion should start with fouled plugs and some sort of problem with the fuel system. Hooking the car up to a code reader will go a long way toward helping to dial in just where and what the problem might be.

With most fuel system problems, you need to replace the fuel filter, and then assess if the fuel pump or fuel injectors need to be replaced.

If an air or exhaust system leak is causing the car to shake at idle, but smooth out while driving you should inspect the throttle body, including the idle air control valve. While you’re at it, take a closer look at all the hoses and belts.

Sometimes you can fix a problem like replacing fouled spark plugs or replacing a worn-out serpentine belt on your own. More serious problems like a faulty fuel pump or bad motor mounts should be left to a professional mechanic.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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