Engine Rough Idle Problems

Updated on March 25, 2017
With so many systems, sensors, wires and connections, it's hard to diagnose a rough idle.
With so many systems, sensors, wires and connections, it's hard to diagnose a rough idle. | Source

Engine rough idle problems arise because car engines are demanding. Components in the fuel, ignition, emission and other systems should work correctly. It only takes a minor problem like a fouled spark plug to cause your engine to vibrate.

Idle problems may originate in a worn out component, a failed part, or a blown gasket. The number of potential sources of a rough idle can make your diagnostic a little difficult. Still, you have at your disposal several tools to help you troubleshoot and fix the problem sooner.

Even more, on modern vehicles the car computer monitors many sensors and actuators and usually can detect small deviating operating parameters and store related trouble codes as necessary. The code may indicate the component, circuit or system involved in the fault. So this gives you an additional advantage.

In this guide, you'll find many of the components that, when going bad, will usually cause engine rough idle problems. The components are ordered from the most frequent to the least, either because they usually receive little attention or because they are bound to fail within a number of miles of operation.

To make the most out of this list, pay attention to the 'Other symptoms' at the bottom of each component description. And compare these symptoms and conditions to the ones you've noticed in your car. This will help you zero in on your diagnostic faster.

For example, when your engine idles rough, you may notice the problem only shows up before the engine warms up; then idle smooths out. This may indicate a bad ECT sensor as described in Other symptoms in that section.

So pay attention to all the symptoms and conditions when given in each component listed to make your diagnostic easier.

Do You Have a Driveability Problem?
1. Spark Plugs
2. Air Filter
3. EGR Valve
4. MAF Sensor
5. ECT Sensor
6. PCV Valve
7. Oxygen Sensor
8. IAC Valve
9. TPS Sensor
Five Tips for Finding Hard to Find Rough Idle Sources
10. Throttle Valve
11. Fuel Pump
12. EEC
13. Vacuum Leak
14. Head Gasket Leak

Do You Have a Driveability Problem?

Before starting hunting for the source of a rough idle, you can do this simple method to confirm that you have a driveability problem. It'll work in most cases.

1. Set your transmission to Park (automatic) or Neutral (manual), and apply the parking brake. Start the engine and let it idle.

2. Now, listen to the sound coming out of the tail pipe. If you hear a clear popping sound, most likely you have a performance problem; if not, check the engine mounts. A worn out, loose or damaged engine mount can cause the engine to vibrate.

A fouled spark plug can cause a rough idle.
A fouled spark plug can cause a rough idle. | Source

1. Spark Plug

A faulty spark plug may cause several engine performance problems, including an engine rough idle. The same conditions under which the spark plug operates, along with poor engine maintenance, may foul the tip with ash, oil, or carbon deposits.

On top of this, miles of service cause the plug electrodes to wear out, widening the gap. This causes an increase in the voltage needed for the spark to jump between the electrodes; but this condition may burn the plug tip and other ignition components, resulting in a rough idle and possibly other engine performance problems.

Remove the spark plug to inspect it. Check the condition of the plug tip. Look for signs of contamination like oil (which may indicate worn rings, a scored cylinder or leaking valves), ash (poor fuel or oil leak), carbon (weak ignition, rich mixture), damage (perhaps from preignition or use of a wrong plug for your model).

Also, check the gap between the electrodes using a wire feeler gauge and regap the plugs. You can find the correct gap for your plugs in your car owner's manual or vehicle repair manual. If you don't have this manual, you can buy an inexpensive aftermarket manual online or with your local auto parts store. Your vehicle repair manual also has the service schedule for your spark plugs.

Spark plug wires represent another potential problem in the system. Check spark plug wires resistance using a digital multimeter, and replace them according to the maintenance schedule as described in your vehicle repair manual. Some manufacturers suggest replacing the spark plugs every 2 or 5 years or more, depending on the type of plugs your engine is using. Also, check other ignition components like rotor, distributor cap, and ignition coil(s) for wear or damage.

Other symptoms: you may notice misfires, an increase in fuel consumption, loss of power and hard starting.

Replace the air filter as suggested by your manufacutrer to avoid driveability problems.
Replace the air filter as suggested by your manufacutrer to avoid driveability problems. | Source

2. Air Filter

The air filter traps dirt, dust particles and other matter out of the air going into the intake manifold.

Most filters use paper as the filter element and after a few months of work, the elements clogs, reducing air flow rate. If you haven't changed the filter in more than two years, you'll notice a rough idle.

Most car manufacturers suggest replacing the air filter every year. Check your vehicle repair manual or car owner's manual.

When installing a new filter, thoroughly clean the filter housing. Carefully close the air box and properly re-attach any ducts you had to disconnect to prevent unmetered and unfiltered air into the intake manifold; otherwise you'll cause more driveability problems.

Other symptoms: A clogged air filter may also cause an increase in fuel consumption, stalling, in extreme cases, and other driveability problems.

Carbon buildup can render an EGR valve inoperative.
Carbon buildup can render an EGR valve inoperative. | Source

3. EGR Valve

The Exhaust Gas Recirculation (EGR) valve allows a certain amount of exhaust gases into the intake manifold to help lower engine temperature and harmful emissions.

When the valve fails, it may get stuck close or open, or even leak. It is when the valve remains open when engine idle suffers. Sometimes, the engine may stall at idle because of the lean of the air-fuel mixture.

It is possible for you to test the EGR valve at home, using a hand held vacuum pump.

Other symptoms: An EGR stuck open may include a rough idle, rough performance at low engine speeds, stalling at idle, increase in fuel consumption, fuel odors, and possibly a check engine light on.

4. MAF Sensor

The mass airflow (MAF) sensor can also cause a rough idle. This sensor measures the amount of air flowing into the engine using a sensing element.

A dirty sensing element is the most common fault, but the sensor itself can develop other faults.

On most cases, a malfunctioning MAF sensor will cause the computer to store a trouble code. So scan your computer for trouble codes.

Also, you may test the sensor at home using a digital multimeter. Consult the vehicle repair manual for your particular make and model.

Other symptoms: power loss, surging at idle, hesitation, stalling during acceleration and other driveability problems.

5. ECT Sensor

A bad engine coolant temperature (ECT) sensor can also cause engine performance problems.

ECT operation has an effect on the fuel system. Your car computer, uses this sensor's signal to lean or enrich the fuel mixture, depending on coolant temperature.

An electrical open in the sensor circuit may cause the computer to lean the fuel mixture trying to compensate for conflicting signals from other sensors.

On some modern vehicles, a malfunctioning ECT sensor can upset ignition timing.

You can test your engine ECT using this guide.

Other symptoms: With a bad ECT, you may notice a continuous lean or rich mixture, an increase in emissions and fuel consumption, and engine stalling when cold; operation may smooth out once engine temperature increases.


If a rough idle happens along with a flashing check engine light, you need to look into the problem as soon as possible.

The flashing light indicates that a detected malfunction can cause severe damage to the catalytic converter. An expensive repair. This usually happens because the converter is receiving unburned fuel, and the fuel will gradually destroy it.

Scan your computer for trouble codes yourself (some auto parts stores will do this at no cost), or have your car s

Check the pcv valve regularlty to avoid performance problmes and engine damage.
Check the pcv valve regularlty to avoid performance problmes and engine damage. | Source

6. PCV Valve

The PCV valve allows your engine to move blowby gases out of the crankcase by reintroducing them into the intake manifold to get reburn.

Over time, the valve may fill with dirt and sludge, blocking blowby gases, or simply the valve may stop working.

Car manufacturers suggest servicing the valve at regular intervals to prevent sludge from blocking the valve and gradually destroying the engine.

However, it is when the valve leaks when you may notice a constant lean air-fuel mixture with a consequent rough idle.

Other symptoms: misfires, hard starts, and increase in oil consumption.

8. IAC Valve

Besides regulating idle speed, the car computer uses the idle air control (IAC) valve to supply air to an engine with fuel injection system.

It's not uncommon for buildup to block IAC valve passages over time, upsetting engine idle. Also, the valve itself may fail and cause similar symptoms.

On most vehicle models with an IAC sensor, the valve is accessible and you can remove it to inspect air passages underneath.

Consult your vehicle repair manual to test the valve using a digital multimeter. And watch the video below so that you have an idea how to clean valve ports and passages.

Other symptoms: You may notice a constant idle speed, either high or low.

Wear and carbon buildup will prevent an oxygen sensor from operating properly.
Wear and carbon buildup will prevent an oxygen sensor from operating properly. | Source

7. Oxygen Sensor

The oxygen sensor measures the amount of oxygen in the exhaust gases.

The sensor sends this measurement to the car computer as a voltage signal. Then, the computer uses this information to increase or decrease the amount of fuel delivered to the engine.

Over time the oxygen sensor (O2s) wears out or may become soiled with carbon buildup, causing the engine to idle rough.

Usually, a failing O2 sensor will cause the computer to store a trouble code, but you still need to confirm that your oxygen sensor needs replacement.

You may be able to test the oxygen sensor at home using a digital multimeter.

Other symptoms: A bad oxygen sensor will reflect primarily in the fuel injection system or the computerized carburetor system of your car, resulting in a rich or lean fuel mixture, increase in fuel consumption, or misfires.

9. TPS Sensor

The throttle position sensor (TPS) reports the opening angle of the throttle plate to the computer. For this, the TPS uses a voltage signal it sends to the computer.

This simple circuit can also cause problems. If the TPS variable resistor wears out, or the circuit shorts out, it can cause the computer to enrich the fuel mixture; however, if the sensor's circuit opens, it'll cause the computer to lean the mixture with a consequent rough idle.

Other symptoms: A bad TPS may also cause stumbling, hesitation, surging, stalling and a hard starting.

Don't ignore check engine light warnings.
Don't ignore check engine light warnings. | Source

Five Tips for Finding Hard to Find Rough Idle Sources

These tips give you some extra help when the source of the rough idles seems hard to diagnose or after finding the source of the problem.

1. For a better diagnostic, scan your computer for possible trouble codes, even if you don't see the light coming on. You may have some pending codes that can steer you in the right direction.

2. If you, or your shop, recently did some work on your engine and soon after you noticed a rough idle, make sure you didn't leave something unplugged, a loose vacuum hose, or didn't properly reassemble the air cleaning assembly. Even a small air leak can cause engine performance problems.

3. Idle speed is the speed at which the crankshaft rotates when the throttle plate or valve is closed or at rest. On modern vehicles, the computer controls idle speed, although a bad or failing sensor or mechanical malfunction can upset it.

On older car models, though, adjusting idle speed is part of the tune-up procedure. Refer to your car repair manual for the procedure and engine rpm specifications at idle.

4. Ignition timing (spark timing) means how early or late spark fires in relation to the position of the pistons in the cylinders. An engine slightly out of time can idle roughly since the spark is not in sync with the cylinders.

Too much timing advance or retardation causes serious driveability problems. Many problems may upset timing. However, the most common is an overstretched timing belt or chain, or a bad tensioner after miles of operation.

Check your vehicle repair manual for the service schedule, and how to check the timing belt or chain. And be warned that on interference engines, a broken belt or chain can cause severe engine damage as well.

5. You may have replaced a bad component that was actually causing your engine to idle rough or erratically. But after installing the part, things didn't improve. However, some engines, specially modern ones, seem fussy about the type of parts you install.

A poor quality component may not work as expected, and you may thing that you haven't found the source of the problem. So, save yourself some money and trouble. When replacing ignition components or some other electrical or electronic parts, try to use OE or a buy from a known quality brand.

Check and clean the throttle plate at regular intervals.
Check and clean the throttle plate at regular intervals. | Source

10. Throttle Valve

The throttle valve in the throttle body or carburetor connects to the accelerator pedal mechanically or, in some modern vehicles, electronically.

This valve helps control engine power, speed and airflow.

Overtime, carbon buildup may grow around the engine valve and the horn--the walls inside the throttle were the valve seats--restricting or obstructing the operation of the valve, and cause an engine rough idle.

You can check the throttle valve by removing the air cleaner intake on engines with carburetors or by disconnecting the air duct off the throttle body on modern vehicles.

If the throttle walls or throttle plate is covered with buildup, spray carburetor cleaner on a shop rag and thoroughly clean the components.

Other symptoms: Poor engine performance when cold or as it reaches operating temperature, hard starting, poor acceleration, rich fuel mixture at idle and stalling.

11. Fuel Pump

Although you won't see this often, an engine rough idle can be a sign of problems with the fuel pump or fuel pressure regulator.

A worn out fuel pump can fail to supply fuel at the adequate rate or with the correct volume to keep up with engine demand; also, the fuel pressure regulator can fail to hold pressure, causing too little fuel to reach the engine.

Other components in the fuel system can also cause your engine to idle rough. For example, a dirty (clogged) fuel filter, or an obstructed or bad fuel injector.

On some models, the car computer can detect problems in the system and store trouble codes in memory that can guide you to the source of the problem. So scan your car computer, specially if the check engine light (CEL) has come on.

You can use the repair manual for your car to make a quick check of the fuel pump and pressure regulator using a fuel pressure gauge.

Other symptoms: Noisy fuel pump, hard starting, power loss at highway speeds, engine stalling after a few minutes of operation.

12. EEC

The evaporative emissions control (EEC, sometimes EVAP or ECS) is another system in your vehicle that can cause rough idle problems.

The EEC system routes fuel vapors inside the fuel tank into the engine for burning to prevent harmful emissions.

Components in the system may consist of a purge solenoid, filler cap, relief valve, canister, purge valve, filter and hoses, depending on your particular vehicle make and model.

System faults may cause the computer to store a trouble code. So scan your car computer, if necessary, and consult your repair manual to troubleshoot the system.

Other symptoms: A fault in one or more components in the system may cause the engine to idle poorly, stall, leak fuel and increase pressure inside the fuel tank.

Check routing and condition of vacuum hoses.
Check routing and condition of vacuum hoses. | Source

13. Vacuum Leak

Vacuum leaks can cause your engine to idle rough.

The leak may happen for a variety of reasons: You were working on your engine and inadvertently pulled loose a vacuum hose, or you forget to reconnect a hose; also, a hose may become brittle after miles of operation and crack, or a gasket may blow.

So what can you do to locate the source of the leak amid many hoses, specially on Asian models? Here's a simple method:

First: check that all the vacuum hoses are properly connected at each end.

Second: trace each hose with your fingers to feel for possible damage.

Third: Set the parking brakes, start the engine and let it idle.

Then, using a length of rubber hose, you can put one end of the hose against your ear and use the other end to probe vacuum hoses around the engine. Don't forget the hose between the charcoal canister purge valve and the intake manifold. Consult your vehicle repair manual to locate it, if necessary. Also, check the gaskets around the intake manifold and throttle body. You'll hear a noticeable hissing sound once you get close to the leak source. Make sure to keep away from moving parts.

An alternative method is to use a solution of soapy water: Spray along the length of each vacuum hose, at connectors and fittings, and around the gasket of the intake manifold and throttle body. If you notice bubbles after spraying at a particular spot, suspect a leak.

Other symptoms: Besides a rough idle, a vacuum leak may produce backfiring, hard starting, stumbling, high idle, hesitation, misfiring, poor power brake action, and poor acceleration.

A blown head gasket can cause a rough idle as well.
A blown head gasket can cause a rough idle as well. | Source

14. Head Gasket Leak

Although not as common, you may be dealing with a blown head gasket, a leaking or burned valve, or even a warped intake manifold.

Usually, this type of problem shows up on a model with high mileage, or a grossly neglected engine. In either case, an idle problem from a blown head gasket usually shows up as a high rough idle.

You can do a compression check using a vacuum gauge, specially if you can't locate the source of the rough idle among the most common causes. The test can not only diagnose a blown head gasket, but uncover the condition of piston rings and valve seats.

Other symptoms: Missing, oil leakage, blue smoke, and overheating.

Finding the cause of engine rough idle problems can be tricky at times, but paying attention to other symptoms besides the rough idle (shaking, noises, power loss) can help you close in on the source of the problem. And keep in mind that the most rough idles issues come from lack of proper maintenance. So check those components or systems you have neglected lately. And scan your computer for possible trouble codes, even if the check engine light is not on. But, whatever the cause, don't shrug off the problem, even if it seems to be a simple one, because it can result in a costly repair if you don't take care of it now.

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