When your car is on, warmed up, and in park, it should purr like a kitten. This means it should settle into a steady idle speed instead of randomly revving up and down. Moreover, the car shouldn’t shake at all.
If it surges, vibrates, and feels generally composed, you have a problem.
When is a Car Idling?
Your engine is idling when your car is on but not moving. It could be in the park with accessories on or in gear without moving. Even though the two idle positions can need different engine RPMs, the engine should always be smooth and not surge trying to find a balance.
A rough idle is best described as a shaking and bouncing sensation. If you’ve ever been in a new or different car, you will immediately tell when another car is idling rough.
Other prominent signs of a rough idle include:
- Odd sounds from the engine
- Inconsistent RPM
- RPM way below or above the recommended idle RPMs
Top Reasons Why Your Engine is Idling Rough
A couple of things could make your engine idle rough. The most notable are those that affect the air-fuel mixture or the fuel ignition system. These include:
A Clogged Air Filter
Your air filter controls what type and amount of air getting into your engine. Under normal circumstances, your air filter should block dust, debris, and other dirt.
Over time, the accumulated dirt starts blocking the filter’s porosity. That is why manufacturers recommend changing the filter every time you service the car or at least once a year.
When the air filter is clogged, it will starve the engine. Luckily, inspecting an air filter is simple. Take it out and look at how dirty it is. If it is almost time to service your car, have it replaced.
ProTip: You can replace your filter more often if you live in very dusty places.
Vacuum Leaks
A vacuum leak happens when some air bypasses the Mass Airflow sensor when going into your engine. It could also occur when some of the hoses to other parts of the car that need a vacuum to run, for instance, your brake booster, have a leak.
Fixing the problem is as easy as installing a replacement hose to the leaky one or replacing a gasket. We did a post about vacuum leaks and fixing them. You can check it out to learn more.
Faulty Spark Plugs or Ignition Coils
The spark plugs and your ignition coils are the heart of the firing system in a petrol car. If one of them fails, your car won’t fire right.
The spark plugs produce the spark needed to ignite the fuel. Ignition coils deliver power to the spark plugs.
If this happens, our engine will spontaneously misfire. In severe cases, the engine will shake violently, and you will get a check engine light with a misfire error code.
We did a post on when to change spark plugs. You can check it out to learn more about fouled spark plugs and signs that it is time to change it.
A Failed or Failing Fuel Pump
The fuel pump is responsible for pushing your fuel along the plumbing and onto the engine. While fuel pumps rarely go wrong, your engine will not run if they do.
Some fuel pumps will go silent on you. This happens if they are burned out. IF this happens, you could also have a blown fuel pump fuse.
In other cases, the fuel pump’s performance will dip over time. In this case, your engine won’t get enough fuel pressure meaning the injectors can’t deliver the required amount to the cylinders during each ignition cycle.
Note that some cars have more than one fuel pump. Each pump sits at different sides of the fuel pump. This is to ensure that your vehicle never misses fuel, especially when cornering hard.
Pro Note: Clogged or faulty fuel plumbing could also make your fuel pressure dip below nominal.
Fouled or Faulty Fuel Injectors
Sometimes the fuel pump might be OK, but your fuel injectors are clogged or not firing right.
Fuel injectors introduce oil into the combustion chambers. While different engines use different injector positions, the theory is always the same. Spray the required amount of fuel into the engine at the right time.
A clogged or damaged fuel injector can’t do this. Either it won’t deliver enough fuel or won’t fire at the right time. The result is misfires or a weak running engine that vibrates a lot.
The Wrong Idle Speed
Every engine should idle at the proper RPM to give the car sufficient power to run all its electricals without stalling. If anything makes it idle slower, you will feel vibrations from the engine.
Idling speed can be messed up by something as simple as a dirty throttle body or a poorly calibrated idle position. Luckily, most cars with an electronic throttle body have a throttle relearn procedure you can follow.
An Idle Air Control (IAC) also plays a role in regulating airflow. It can clog due to carbon buildup.
If this is the problem, you can fix it by cleaning the two components using a throttle body cleaner.
A Faulty Mass Airfloor Sensor
Fuel-injected engines use a Mass Airflow Sensor to determine how much air is going into your engine. It uses this information to calculate the right fuel to inject into the machines.
If your MAF is faulty or dirty, the engine will idle rough because it will probably run too lean or idle too slow.
MAFs are very sensitive and prone to damage if you mishandle them. They can be costly too. Before cleaning yours, ensure to do a lot of research, buy the right products, and avoid damaging what you already have.
A Blocked EGR Valve
Some engine designs use an exhaust gas recirculation (EGR) valve to regulate emissions. The valve recirculates exhaust gases when the engine is running at high RPM to burn again.
Since the valve is constantly exposed to carbon, it clogs easily. It could be stuck open when this happens, letting the engine recirculate exhaust and run rough when at idle.
A Faulty Coolant Temperature Switch
A coolant temperature sensor gathers information on the engine coolant (hence engine’ temperatures. This information goes to the ECU that uses it to determine what fuel mix and timing to use on the engine.
If this sensor sends the wrong information, your engine will run lean all the time. The result is a weak engine that idles rough.
A Bad Oxygen Sensor
Another common culprit when an engine is idling rough is a faulty oxygen sensor. Your ECU uses information from the upstream sensor to determine how efficient combustion is.
It determines the efficiency and adjusts the mixture to lean or rich depending on how much oxygen is in the engine’s exhaust.
A faulty sensor will affect the air-fuel mixture making your engine idle rough.
Can Too Much Oil Cause Rough Idle?
Yes, overfilling your engine with oil can make it misfire, idle rough, and shake. Filling up with more oil increases pressure and the possibility of the oil going where it shouldn’t.
The most common problem that leads to rough idling is when the extra oil leads to more pressure that bursts old head gaskets.
This will leak oil into sparkplug holes. Spark plugs and ignition coils don’t like oil. It will damage them and make the engine start misfiring or idling rough.
Can a Bad Battery Cause a Rough Idle?
This is very rare. But it can happen in some cars. Usually, cars are designed to idle at an RPM where the alternator is producing enough power to keep the engine running without needing a battery.
That’s why you can jumpstart a car and take off the jumper cables even before the battery charges up, and the vehicle will keep running. If it doesn’t the culprit should be your alternator and not the battery.
If your car works differently and relies on battery power to keep going at low RPM, it could idle rough. This could occur in vehicles with smart alternators that avoid loading the engine at low RPM unless necessary.