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Troubleshooting Engine Rough Idle Problems

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

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.

How to Determine What's Causing Rough Idling in Your Car

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.

Index

Do You Have a Driveability Problem?

1. Spark Plugs

2. Air Filter

3. EGR Valve

4. MAF Sensor

5. ECT Sensor

WARNING Flashing CEL

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.

1. Spark Plugs

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.

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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.

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 drivability problems.

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

Carbon buildup can render an EGR valve inoperative.

Carbon buildup can render an EGR valve inoperative.

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 drivability 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.

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

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

6. PCV Valve

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