Car Idling Problems? 9 Most-Common Problems & Solutions

Your car engine should idle smoothly without surging up and down or stalling. Most causes of idling problems are easy to diagnose, and even easier to fix.

We’ll cover the most common problems plaguing your car engine with idling issues and the solutions to fix them.

#1 – Your PCV Valve Has Gone Bad

As your engine runs, it builds up crankcase pressure from the combustion process. This pressure must be vented, and these vapors collect oil residue as they travel from the engine to the intake manifold.

There is a one-way valve called the Positive Crankcase Ventilation (PCV) valve that fills with contaminants and can block the pressure being released. When this happens, the air-fuel ratio tends to change and causes a rough idle.

How to Fix It

Diagnosing a bad PCV valve is a simple process of removing it from the intake manifold and shaking it. The valve should rattle in your hand indicating that the internal valve is still functional.

If the valve doesn’t make a sound, the internal baffling is either stuck open or closed due to contamination. Either situation is bad, and the valve should be cleaned or replaced.

Most mechanics will recommend replacing the valve because they are relatively cheap and cleaning the valve will only fix the problem temporarily.

Contamination will build up again inside the PCV valve and it will cause a rough idle.

#2 – Your Engine Has A Vacuum Leak

Your engine uses rubber hoses to balance vacuum (or negative pressure) to ensure a proper air-fuel mixture. These rubber hoses will eventually dry out and become brittle.

When that happens, they will crack or break, and the vacuum pressure will be lost. It can cause a rough idle, an engine misfire, and a surging idle.

Beyond the rubber hoses, the intake manifold can have a leak at a connection point using a gasket.

How to Fix It

Typically fixing a vacuum is a simple process of replacing the part that is leaking. That may be a gasket at a connection point or a rubber hose that has dried out and cracked.

The hard part of the process will be finding the exact spot of the leak. Two common methods of finding the leak include a smoke machine or an aerosol can of cleaning fluid.

The smoke machine method doesn’t require the engine to be running. You will disconnect a vacuum line from the intake manifold and connect the end to the smoke machine.

The machine will force smoke through the vacuum lines, and you just watch where the smoke comes out. It should find the point of the leak and smoke will come through the hole or crack.

Once you find the leak, you can replace the part with the issue.

The second method requires the engine to be idling, which can be a little riskier to undertake. You can use carburetor cleaner to find the leak. Spraying the aerosol cleaner along the length of each vacuum line will eventually find the leaking spot.

When the cleaner is sucked into the leak, it will find its way to the engine and the idle will change. The idle should briefly increase in RPM and run better.

Once you find the spot that causes the idle to change, you just need to replace the part with the leak.

This method is more dangerous as the engine is running, and the cleaner is flammable. You need to keep it away from hot parts of the engine and exhaust to prevent a fire.

#3 – Your Air Filter is Dirty And Needs To Be Cleaned Or Replaced

Your engine air filter collects contaminants like bugs and dirt to prevent them from entering your engine.

When the filter becomes dirty, the airflow is hindered making your engine run rich. This means there is not enough oxygen for an efficient combustion process.

How to Fix It

Most air filters have a paper element that is not able to be cleaned. When they are dirty and clogged with contaminants, you should replace them.

Your filter should be checked at each oil change to verify their cleanliness and replaced when you can’t see light through the element.

There are filters that have a specialized element that can be cleaned with soap and water. Once they are allowed to dry, the element can be sprayed with a topical oil that will collect contaminants until it is cleaned again.

These are more expensive than the replacement paper filters, but they can last up to a million miles with proper care.

#4 – Your Fuel Injectors Are Dirty

Fuel injectors have very small holes in the tip that spray fuel into the intake manifold or directly into the combustion chamber.

These small orifices can be blocked with contaminants that reduce the spray amount or completely block the spray. The reduction of fuel causes an imbalance between the cylinders, which subsequently causes a rough idle or shaking.

How to Fix It

Most fuel injectors can be cleaned with a pour-in fuel injector cleaner that goes into the fuel tank when you add fuel to your car.

The treatment will clean the injectors as you drive the car, and you can continually add more cleaner at each tank until the problem clears up.

If the injectors are too clogged, they can be removed and cleaned by a professional service. You will need to have them removed from the car, sent to the company for cleaning, then install them back in the car when they are returned.

As a last solution, you can also buy new injectors to replace the clogged ones. That would be the most expensive solution.

#5 – Your Carburetor Has Problems

Fuel injection has been around since the 1980s in many forms, but cars made earlier may still have a carburetor to inject fuel into the engine.

The carburetor has small needles that fit into seats to meter how much fuel is required for combustion and jets that spray the liquid fuel into the engine.

Contaminants can prevent the needles from sitting correctly or clog the jets similarly to the fuel injectors. When that happens, it can cause too much fuel or too little fuel to be sprayed into the intake manifold for combustion.

A second issue with carburetors is the choke staying on after the car warms up. The choke initially adds more fuel when the engine is cold to decrease the warm-up time.

When the engine is warm, the choke should turn off. If it doesn’t, it will continue to add more fuel than required. That will cause your car to produce black smoke from the exhaust and it can smell like gasoline.

How to Fix It

Most contaminant issues can be fixed by using the fuel system cleaner that pours into the gas tank. If the carburetor is significantly clogged, it can be removed from the engine and rebuilt with new parts.

If parts are not available, you may be able to buy a newly remanufactured unit or an aftermarket carburetor.

#6 – Your Spark Plugs or Wires Need Replaced

Your spark plugs and wires transmit the ignition power from the ignition coil to the combustion chamber.

The spark plugs generate an electrical spark that should ignite the fuel and air mixture. If the wires are defective, they won’t transmit the power to the spark plugs.

If the spark plugs are defective, they won’t generate the spark needed to ignite the fuel and air mixture required for your engine to run.

Read our article, What Is A Spark Plug? to find out more.

How to Fix It

With plugs and wires, you’re dealing with two different problems.

It’s best to diagnose the problem by looking at each component separately. The first issue to diagnose is if the wires do indeed transmit power.

You can use a spark plug tester connected to the engine that will show the spark. If the wires transmit power, you can further test if they are good by measuring the resistance from end to end.

The wires, in general, should not have more resistance than 5,000 to 7,000 ohms per linear foot. That is dependent on their type and the manufacturer’s specifications.

If you measure a wire with more resistance than required, it may be causing an issue with the cylinder it’s connected to. The wire should be replaced, and you should replace the complete set of wires.

The spark plugs should transmit the energy from the wires and cause a spark inside the combustion chamber. A damaged electrode (the part that generates the spark) could prevent the spark from occurring.

If you find any damage to the spark plug or contamination on the electrode, the plug should be replaced. It’s recommended to replace all the plugs to ensure a balanced ignition system.

#7 – Your Ignition Coil Is Defective

The ignition coil generates the energy that is sent through the spark plug wires to the spark plugs.

If the coil is defective, it may only generate the energy part of the time. That can cause a rough idle and a loss of power.

How to Fix It

Once you determine an ignition coil is defective, it should be replaced with a new one.

Most coils are sealed to prevent water intrusion and they can’t be rebuilt with new parts. Buy a new one and get your car back running correctly.

#8 – Your Oxygen Sensors Are Giving Bad Information

Your exhaust system has oxygen sensors that monitor the emissions from your engine.

The information is sent to the Engine Control Unit for adjustment of the fuel amount injected into the engine. If the sensors are sending bad information, you may have a rough idle, a misfire, a loss of power, and poor fuel economy.

How to Fix It

Oxygen sensors are critical to your engine’s performance and are designed to be replaced at approximately 75,000 miles.

A defective sensor sending bad information should be removed and replaced with a new one. It’s recommended to replace all oxygen sensors at the same time.

A defective sensor may trigger a Check Engine Light (CEL) to illuminate and a trouble code to be logged indicating replacement is required.

#9 – Your EGR Valve Is Stuck Open

Your exhaust system has an Exhaust Gas Recirculation valve that opens when the engine is warm. It allows exhaust to recirculate back into the intake manifold to reduce the emissions from your car.

If it is stuck open, it will allow the exhaust to recirculate when the engine is cold. It will cause a rough idle or stalling until the engine has warmed up to operating temperature.

How to Fix It

The EGR valve can be removed and cleaned with an aerosol cleaner. If that doesn’t fix the issue, the valve can be replaced with a new one.

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