8 Reasons Your Car Vibrates When Stopped or Idling
1) Dirty or Clogged Fuel Injectors
Over time, parts of the fuel system, namely the injectors, become clogged with carbon deposits, varnish, and other debris. The result is an uneven amount of fuel delivered with each engine cycle.
Clogged injectors should be cleaned or, in severe cases, replaced. Using a good fuel system or fuel injector cleaner will often do the trick. Cleaning the throttle body, which controls the amount of air that mixes with the fuel, is also a good idea. There are throttle body cleaners for exactly this purpose.
2) Worn or dirty spark plugs
Another common cause of a car shaking when stopped is a faulty spark plug. Dirt or carbon buildup prevents the spark plug from firing properly and igniting the fuel. This causes the engine to misfire and makes the vehicle feel like it’s vibrating when idling.
Replacing the spark plugs will often fix the problem, unless they’ve been replaced recently. There are many types of spark plugs, so you’ll need to buy the right ones. Always check your user manual for accurate information. The best spark plug for your car may be completely different from the one your neighbor is raving about.
3) Dislodged vacuum hose
Often overlooked, but more common than you might think, is when the vacuum hose in your engine becomes dislodged while driving or accidentally while working on your car. When the hose comes loose, it can cause your engine to shake, vibrate, and even stall.
A similar shaking can also occur to a lesser extent if the hose is cracked or torn. Check all hose connections to make sure they are tightly connected. If a hose feels loose, it needs to be replaced or it will come loose again.
Small air leaks are the hardest to find. If this is the case, spray soapy water along the hoses while the engine is running. If there is a crack anywhere, the soapy water will temporarily plug the leak and you will notice a change in idle speed.
4) Cracked or torn timing belt
When a timing belt (or any other fan belt) stretches, breaks, or is partially broken, it can cause the car to shudder when idling. In the case of a timing belt, this is a warning sign of impending timing belt failure that you should heed. If the timing belt breaks completely, it can cause serious damage to the engine.
It is important to follow the manufacturer’s guidelines for timing belt replacement intervals. Replacing a timing belt is never cheap, as you will almost always also need to replace the water pump, but not replacing it can cost thousands of dollars.
5) Dirty Air Filter
In addition to an interruption in the fuel supply (see dirty/clogged injectors above), an interruption in the engine’s air supply can also cause the engine to vibrate and not run as smoothly as it should. If the air filter is dirty or clogged, not enough air will get into the combustion chamber and not enough fuel will be injected.
This combination will cause the RPMs to drop below the normal idle range (usually around 700 RPM), causing the car to shake a bit when stopped. In some cases, it may even slow down so much that the car stalls and you have to restart it.
Luckily, changing the air filter is an easy do-it-yourself job and a new filter will only cost around $10-20. Alternatively, you can buy a reusable air filter (such as K&N) that you can clean instead of replacing. It will cost more up front, but it will be cheaper in the long run.
6) Clogged Fuel Filter
When the fuel filter is clogged, it will not get enough fuel from the fuel pump to the injectors. This will cause the engine to not get enough fuel and will cause the same kind of shuddering as clogged injectors.
If your fuel filter is bad or clogged, it will need to be replaced. Most people are better off leaving this job to a professional, since the fuel filter is often located in or near the fuel tank, but changing the fuel filter on most vehicles isn’t too difficult if you have some experience.
7) Bad Motor Mounts
Since one of the jobs of a motor mount is to dampen engine vibration so individuals in the car won’t feel it, a broken motor mount will make itself known very quickly. While a faulty engine mount will be more noticeable when the engine is working hard (like when accelerating), shaking and rattling at idle is usually noticeable as well.
If you suspect a bad motor mount, you’ll want to take your car to a mechanic as soon as possible to confirm the issue and have the mount replaced. If left unchecked, it may cause other engine mounts to fail and even cause damage to other parts of the engine.
For instance, the blades in the radiator fan can break from hitting the surrounding shroud because the engine is sitting lower than it should be due to a broken motor mount.
8) Faulty Mass Air Flow Sensor
Probably the least common reason for your car shaking while stopped, a bad mass air flow sensor (MAF) can be the culprit. Since its purpose is to properly calculate the air-fuel ratio for the engine, if it goes bad and sends incorrect readings to the car’s computer, the wrong amount of fuel can get injected into the engine at the wrong times.
As already explained above, when not enough fuel gets into the engine than what’s needed, excess vibrating or shaking can be felt. A bad MAF will almost always cause the check engine light to go on so a scan of any stored diagnostic trouble codes can confirm the issue.