Rough Idling Engine – The exact cause, can be difficult to diagnose.

Rough Idling Engine - The exact cause, can be difficult to diagnose.
Rough Idling Engine - The exact cause, can be difficult to diagnose.

So, a rough idling engine, is a common problem of which, the exact cause, can be difficult to diagnose.

Also, the reasons for a rough idling engine can vary. With some being cheap easy fixes and some requiring, more complex repair procedures.

So, when your car is stationary your engine should idle smoothly; at around 1000 (RPM) and give off a, constant even sound.

Damaged or faulty components in the, fuel, ignition, emission and other systems, will cause, a rough idling engine.

So, if your car’s idle is rough; possibly rising and falling or struggling to maintain a steady rate of rotation, you have a, rough idling engine.

Since a smooth idle depends on, just the right mixture of fuel and air; there can be many possible places, where that combination is getting unbalanced. It only takes a minor problem like, a fouled spark plug, to cause your engine to idle rough.

Fouled Spark Plug Causing Rough Idling Engine
Fouled Spark Plug

Idle problems, may originate from a worn out component, a failed part, or a blown gasket. That’s why, the number of potential sources of a rough idle, can make your diagnostic a little difficult.

Always Start With Checking Your Computer For, Diagnostic Trouble Codes

When diagnosing a rough idling engine, there are certain things that must be done, prior to replacing parts.

Checking Your Computer For Diagnostic Trouble Codes
Checking Your Computer For Diagnostic Trouble Codes

So, if you have a 1981 or newer vehicle, it will most likely have, an engine management computer. The computer monitors the functions, of a number of engine compartment components. So, when a problem develops in a system, that is monitored by the computer; codes which relate to the particular system, will be set and retained, in the computer’s memory. Checking for computer codes first is important, because it may lead you to the, rough idling engine cause.

Next, Do A Visual Inspection

If you 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. Consequently, even a small air leak can cause, engine performance problems.

Mechanical Reasons For A, Rough Idling Engine

So, high mileage vehicles, are the most likely to start having, mechanical reasons for a rough idle. That’s why, the best place to start is with a, Compression Test and a Cylinder Leak Down Test.

Compression Test

Your car’s engine compression, can tell you a lot about the, overall health of the engine. So, if your car is blowing blue smoke out of the tailpipe; or losing oil, you could have, leaking piston rings.

Compression Tester
Compression Tester

When a engine’s compression, is low in any particular cylinder, it will cause, a rough idling engine; which could trigger a check engine light. As a result, low or uneven compression, will cause a rough idling engine.

Cylinder Leak Down Test

A cylinder leak down test, is similar to a compression test. But, instead of measuring pressure, it measures pressure loss.

Cylinder Leak Down Tester
Cylinder Leak Down Tester

Beyond getting an overall picture of engines condition, the engine leak down test; is an excellent way, to pinpoint where problems are, before tearing down the engine.

Listening for where the air is escaping, by ear to isolate the problem:

Intake Valve

  • Air whistling out of the intake, carburetor or throttle body indicates a leak, at the intake valve.

Exhaust Valve

  • Air heard hissing out of the tailpipe, turbocharger or exhaust manifold means, an exhaust valve is leaking.

Piston Rings

  • Whistling or hissing out of the (PCV) valve, oil filler cap hole or dipstick tube means; the air is pushing past the rings. As a result, suspect ring or cylinder wall wear.

Head Gasket

  • Air bubbles in the engine coolant, seen at the radiator filler cap could mean; air escaping into the coolant, past the head gasket.

Cracked Cylinder Head

  • Bubbles in the coolant or coolant being pushed up, out of the radiator neck, can also indicate; cracks in the cylinder head or cylinder walls.

Common Reasons Why You May Have, A Rough Idling Engine:

Vacuum Leaks

  • Most vehicles have a maze of hoses, which create a vacuum for both, fuel and air. In older engines with a carburetor, this vacuum, pulls the fuel into the engine. Most newer vehicles, however, have a throttle to regulate engine speed and airflow through the system.
Cracked Hose Causing Vacuum Leak
Cracked Hose Causing Vacuum Leak
  • This also creates a vacuum, within the intake manifold. These hoses can wear out over time and a vacuum leak may develop. As a result, too much air gets mixed with the fuel, causing a rough idling engine.

Spark Plugs

  • A faulty spark plug may cause, several engine performance problems, including a rough idling engine. Furthermore, poor engine maintenance, may foul the tip with ash, oil, or carbon deposits.

Dirty Fuel Injector

  • When they can’t deliver their normal dose of fuel, performance, fuel economy and emissions, are all going to suffer. Dirty injectors, cannot delivery the correct spray pattern, that is so essential for clean, efficient combustion. Sometime dirty fuel injectors, can be the root cause of a, rough idling engine.

Clogged Fuel Filter

  • A clogged fuel filter, can cause similar problems. The job of the fuel filter is to, screen out contaminants in the fuel. A rough idling engine, is one symptom of, a clogged fuel filter.

Defective Mass Air Flow (MAF) Sensor

  • A defective mass airflow (MAF) sensor, can be responsible for, a rough idling engine. A mass airflow (MAF) sensor, detects the amount of air, coming into the fuel injection system. Finally, the computer uses that data to deliver, the proper amount of fuel, to the air in the vehicle.
Mass Air Flow (MAF) Sensor
Mass Air Flow (MAF) Sensor
  • Over time, these sensors can malfunction or become dirty. As a result, one of the first symptoms of a malfunctioning mass airflow (MAF) sensor is, a rough idling engine.

Dirty Oxygen (O2) Sensor

  • Oxygen (O2) sensor measures, how rich or lean the gases are, as they exit the combustion chamber. So, depending on the results, the amount of fuel entering the engine is adjusted, by the vehicle computer. The ultimate goal is to maintain, an ideal mixture that, produces the lowest emissions. Finally, a dirty oxygen (O2) sensor, will usually trigger the; check engine light and can lead to a, rough idling engine.

Sticking (EGR) Valve

  • An (EGR) valve is designed to open at, high engine (RPM)s. The (EGR) valve, feeds a portion of the exhaust, back into the engine. As a result, reducing combustion temperatures and oxides of nitrogen emissions; the main contributor to smog.
Carbon Buildup On (EGR) Valve Causing Rough Idling Engine
Carbon Buildup On (EGR) Valve
  • The (EGR) valve, is a mechanical device that can develop carbon buildup, that prevents it from closing fully. When that happens, the (EGR) valve allows exhaust flow at idle, which causes a, rough idling engine.

Clogged Air Filter

  • So, your engine needs air from the outside, to mix with the fuel, in the internal combustion chamber. There is an air filter, which prevents debris from getting, into the combustion chamber. But, if this air filter is clogged or worn out, then there will be, less clean air passing through it. This will cause a, rough idling engine.

Sticking Valves

  • Carbon build up on valves, can cause them to stick. As a result, causing loss of compression.


So, idling issues can be frustrating. But, with a little troubleshooting, you’ll have a real chance, at figuring it out. Also, remember to always check your engine idle, with the air conditioning and defroster turned off. As with many car issues, the root cause of a, rough engine idle; can either be a quick, easy, inexpensive fix or it can be, something more serious and costly.

Thank You !