Does your car bounce or shake when you put it in idle? Does its engine rev inconsistently or idle below 600 RPM? Do you hear shaking or skipping sounds? It sounds like your vehicle has a case of rough idling. This problem can lead to engine damage that’s costly to fix. Or maybe not. But it needs to be take care of.
There are several possible reasons behind the rough idle in your vehicle. Knowing the common causes of rough idling makes it easier to resolve the issue and restore your vehicle.
Here are the most common reasons for rough idling in your car:
Spark plugs ignite the fuel in the cylinder by generating an electric spark between the center and grounding electrodes. A bad car spark plug degrades the consistency and efficiency of fuel combustion. It can cause the air-fuel mixture to burn incompletely, causing rough idling. Sometimes, you can clean a dirty spark plug to restore its performance. However, a damaged spark plug will require replacement.
Find out if the engine is misfiring by holding a cloth by the exhaust pipe at the rear of the car. If it’s not skipping, you’ll feel a steady purr of exhaust. When it misfires you’ll feel a “puff” that feels different from the “purr.” See this video for a random intermittent misfire and what caused it:
This kind of misfire often won’t set a misfire code. A plug that is partially fouled or gapped a bit too close will also cause a random idle misfire like the one you see in the video.
The fuel injector has a pintle that lifts off its seat to allow fuel to spray out in an atomized form. On non-GDI engines, the fuel is delivered in the intake manifold right behind the intake valve and the fuel mist is carried with the air into the cylinder when the intake valve opens.
A dirty fuel injector can get stuck, interrupting the engine’s fuel supply and causing issues like rough idling, though this is uncommon. If a vehicle has regularly received fuel from a rusty fuel supply tank on a farm or from a rusty can, those rust particles can be small enough to pass right through the fuel filter and clog the injectors. Where there’s little or no fuel flow, there’ll be a misfire.
You can check the fuel injector for issues by touching a long screwdriver against each injector (if you can get to them) and hold the handle end of the screwdriver to your ear. Better yet, get a cheap mechanic’s stethoscope and do this. If the injector ticks erratically or goes “cluck cluck” when the other injectors go “click click” or if it doesn’t make a sound, you might need to clean or replace it.
But here’s a caveat: if the PCM detects a misfire for some other reason, it’ll shut down the injector (on most vehicles since the mid ‘90s) on that cylinder to prevent damage to the catalyst, so a non-functioning injector that starts out clicking and then stops clicking after the engine has been running for a minute or so may not be the cause of the skip; something to keep in mind.
Older car models rely on a carburetor to mix air and fuel in the correct ratio for efficient combustion.
As the vehicle and its carburetor put on the years, they become more prone to developing issues. A bad carburetor can produce a lean air-fuel mixture, leading to rough idling.
With the air cleaner removed and the engine idling rough, lay a rag over the top of the carburetor. If it runs better, look for a vacuum leak, and realize that the idle mixture may be adjusted by a mechanic.
Older, high-mileage (100,000 miles or more) cars are more prone to have low engine compression in a cylinder due to worn piston rings or issues with the valves.
If the engine has a misfiring cylinder, compare the way it sounds spinning to the way a healthy engine with the same number of cylinders sounds. If you can hear the engine pick up speed at one spot while spinning (it’s easy to hear), then one cylinder has low compression. This will be something deep in the engine that you may need to hire done, or you may need to replace the engine or have it done.
Rough idling can also happen because of intake vacuum leaks. Hoses, gaskets, etc. can leak and cause a rough idle.
The air filter stops dirt and other contaminants that can damage the engine. If it gets extremely dirty to the point of being clogged, it may cause a rough idle. Inspect the air filter regularly.
PCV valves are specific to the engine and system and you need to make sure you use the right one. They don’t usually cause a rough idle, even if they’re totally clogged, but they will cause a lot of oil steam to enter the breather when they clog.
As mentioned, some possible reasons why your car may idle rough include issues with the spark plug, fuel injector, or carburetor. As such, replacing them as soon as they show signs of wear is crucial.
CarParts.com has an array of high-quality replacement parts sourced from only the most trusted manufacturers in the industry. This means you can expect your replacement auto parts to last as long as stock parts, giving you your money’s worth.
We also have warehouses strategically located across the US to speed up the delivery process and increase shipping efficiency. Expect your order to arrive in as fast as two business days.
Shop now, and get the best deals today!
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.