Idling your car can tell you a lot about your engine’s health. At idle, the engine produces the least power at low RPM, just above its stall speed. If anything goes wrong, your engine will shake, vibrate, surge, and even stall.
Signs of trouble start with shakes and vibrations when idling. If the shaking and vibration go away when the engine RPM picks up, the problem is in its infancy, and increased engine power output overshadows it.
Why Does the Engine Shake More When Idling
A problematic engine will shake more at idle mostly because the RPMs are still low enough to register as an uncancelled out upwards or downwards force on the crankshaft.
Even a perfect engine is less smooth at idle compared to how smooth it will be once you get to cruising RPM with the engine loaded.
Moreover, when idling, you don’t have the road feedback to distract you from the engine’s vibrations. This makes it easier to pick up the ruckus as opposed to when driving.
- At low idle RPM, the shakes are at a lower frequency and with a bigger amplitude making them easier to pick
- At higher RPM, the vibrations are at a higher frequency and less amplitude, making it harder to feel them (you hear the vibrations more at those frequencies)
- The noise of a revving engine will distract you
- If the vibration is due to imperfections in fuel delivery or combustion efficiency, revving up the engine could compensate for this, making the engine feel smoother.
- Once you start driving, more vibrations come from the wheels rolling over the tarmac and the car slicing through the wind. This can mute out engine vibrations.
While your car could still be drivable to highway speeds, you still have to fix it, as the shake is the first sign of looming failure. Here are the top reasons why you could be experiencing these shakes.
Bad or Fouled Spark Plugs
Gasoline engines use spark plugs to ignite the air-fuel mixture and create power. If one or more spark plugs don’t fire, you will experience the dreaded misfire shake.
The vibration intensity varies depending on how many spark plugs didn’t fire or how often they misfired.
Even one fouled spark plug is enough to make your engine shake uncontrollably. Luckily, you will get a misfire code that triggers the check engine light. An OBDII scanner can pull out the error codes and tell you which cylinders – hence spark plugs- are faulty.
We did a post on faulty spark plugs. You can check it out here to learn more about fouled plugs and how often you should change yours.
Bad Motor Mounts
Regardless of how smooth your internal combustion engine is, it will always make a ruckus. We don’t get to feel much of these vibrations because all engines mount to the engine frame through a set of ‘shock absorbers’ called motor mounts (engine mounts).
Engine mounts are hefty metal and rubber constructs that bolt the engine to the car chassis.
- One metal clamp/harness attaches bolts to the engine while another bolt to the car’s chassis.
- The two clamps are linked by a thick rubber bushing, polyurethane, active electronic, or a hydraulic motor mount, depending on what your vehicle’s model was designed to use.
The shock absorbing section of the mount is the hydraulics, the solid rubber, polyurethane, or active electronic mechanism. It smooths out all the vibrations and jerks from the motor, isolating them from the car’s body.
If they fail, all those normal vibrations from a running engine will carry through to the cabin shaking up the entire car.
You will note the vibrations more when parked since the engine runs slower, and there are no tire and road vibrations to drone it out.
Damaged Ignition coils
Gasoline cars also use ignition coils or systems with spark plugs to ignite the fuel-air mixture. The ignition coil creates the high-voltage spark plugs needed to create a spark.
A faulty ignition coil that doesn’t boost your basic 12V power to the voltage spark plugs need to run will leave your engine misfiring.
While such misfires remain even when the engine is running at speed, there could be the possibility that the coil is failing intermittently and fires enough to keep your engine and the entire vehicle from vibrating and shaking violently.
A Faulty Idle Speed Controller or Throttle Body
Idling an engine is walking a tightrope between stalling and staying on. The revs have to be as low as possible to minimize fuel use but still high enough to overcome friction power loss and produce enough power to run accessories like the alternator and AC compressor.
In most cases, this is somewhere between 600 and 1000 RPM. The engine relies on the power train control module to control the throttle body and maintain this RPM or an Idle Air Control valve to keep the engine running in the correct range.
The system has to adapt to varying load demands depending on whether you are drawing from the electrical system or the AC compressor is running.
It has to modify:
- Fuel injection
- Throttle position
- Idle air control
Any fault in the idle speed controller or throttle body will make such adjustments impossible. If the engine idles too slowly or doesn’t bump up power output to compensate for extra loads kicking in, it will start stalling intermittently, leading to a notable stutter and shaking.
In this case, the shaking will vanish once you blip the accelerator and bring the revs up.
Faulty Fuel Injectors
Fuel injectors deliver fuel into the combustion chambers. For perfect ignition, they must spray fuel into each cylinder correctly.
Blocked or damaged fuel injectors will send less fuel into the engine, forcing it to run lean and weaker. This could translate into more vibration when at idle, but the problem will remedy itself once you start accelerating, forcing the injectors to pump more fuel.
A Problem in the Fuel Delivery System
Fuel injectors are not the only thing that can starve your engine of fuel. You will also get a trickle or no fuel flow if there is a problem somewhere in the fuel delivery system. This could be:
- Partly blocked fuel lines
- Faulty fuel pump
- Malfunctioning high-pressure fuel pump
If you are not getting sufficient fuel, your engine will be weaker. It will be weak enough to shake and stall at idle but will run without shaking when you accelerate.
The engine will not be as powerful as usual, but it won’t shake as it does when idling.
Very Dirty Air Filter
Both fuel and air are very crucial for perfect combustion. As we have seen, starving the engine of fuel will make it idle rough, and vibrate.
The same will happen if you starve it of air. A clogged air filter is a perfect example of how you could starve the engine of air, making it produce less power than it needs to idle smoothly.
However, a filter clogged to this extent is very rare. You could only have to face this if you drive in dust, sand, mud, or pollen-rich places a lot but don’t take the time to dust or replace your air filters.
A Vacuum Leak
Another problem that could make your engine shake at low idling RPM is a vacuum leak. Vacuum leaks introduce excess unmetered air into the system. This makes the air-fuel mix sporadic, and the engine produces different amounts of power in every ignition cycle.
The erratic power variations will cause rough kicks to the crankshaft hence leading to a rough idle. This will manifest in a shake you can feel on the staring wheel and a tachometer that jumps around a lot.
Accelerating to increase RPM could compensate for the air leak making the engine smooth out once you start driving.