Rick's Free Auto Repair Advice

Top 10 rough idle causes

Rough idle causes

What are most common rough idle causes?

How to diagnose and fix rough idle

Many things can cause a rough idle, from something as small as a disconnected vacuum hose to major engine wear. But I’m listing the most common rough idle causes here in the hope that you can track down the cause of your rough idle.

Vacuum leak is tough idle cause #1

Because the engine runs as such a low RPM at idle, anything that upsets the air/fuel balance will cause a rough idle. If you’ve had your vehicle into a shop for service lately and you now notice a rough idle, chances are high that the shop checked the condition of your air cleaner and PCV valve and forgot to replace the vacuum lines. After all, it only takes a minor distraction to get those 12-year old “technicians” at the quick change lube places to forget what they were doing. So start by making sure the air filter cover is firmly re-installed and that all the vacuum lines are in place.

If the problem didn’t start after a recent visit to the shop, you can still start your diagnosis with a check of every vacuum line under the hood. Look for cracked rubber and plastic hoses or any hose that’s not connected.

Next, check the pleated air duct that runs from the air filter box to the throttle body. Those rubber ducts develop cracks over time and those crack allow unmetered air into the engine after the mass airflow sensor. If you find cracks in the air duct, replace it. Do NOT try and patch a broken air duct. Your repair will not last!

If you still can’t find a vacuum leak, use a scan tool with live data to read the short and long term fuel trims. If the computer is adding fuel to compensate for a vacuum leak or unmetered air, your fuel trim readings will be high, usually above 10% (the range is -25 to +25). If fuel trims are high, suspect an air leak.

Dirty electronic throttle body or dirty idle air control valve is rough idle cause #2

Late model fuel injected engines have electronic throttle bodies that no longer use an idle air control valve. Before you turn the key, the throttle plate is completely closed. When you turn the key, the computer measures engine coolant temperature, air temperature and barometric pressure to determine the proper air/fuel mixture. It then opens the throttle plate to just the right setting. However, carbon buildup in the throttle body can decrease the airflow at that setting. The computer doesn’t know the carbon buildup is there..all it knows is that the engine isn’t running right based on it’s calculated opening. Checking the condition of an electronic throttle body is easy. Just push the throttle plate open and look for black carbon buildup. To learn how to clean an electronic throttle body, read this post.

If you have an older vehicle, you may have a dirty idle air control valve. Some idle air bypass or idle air control valves can be cleaned. Others can’t. to learn more about idle air control valves, read this post.

Dirty mass airflow sensor is rough idle cause #3

The mass airflow sensor (MAF) is located right after the air filter and is responsible

Mass Airflow Sensor

Mass Airflow Sensor

for telling the computer exactly how much air is entering the engine. It does this by measuring how much power it takes to keep a wire or metal plate hot while air passes over it. Over time, paper fibers from the air filter and crankcase vapors can build up on the hot wire or plate and bake on. That backed on crud acts as an insulator, causing the computer to get incorrect readings. You can clean the MAF sensor yourself with an aerosol can of MAF sensor cleaner. Do NOT use brake cleaner and BE careful—the wires in the sensors are VERY fragile. For more information on how to clean your MAF sensor, read this post.

Dirty fuel injectors are rough idle cause #4

Just as I said earlier, at low idle speeds, any disruption in the air/fuel mixture will cause a rough idle. Over time, carbon buildup on the fuel injectors can cause them to deliver less fuel than the computer expects. You don’t need expensive fuel injector cleaning to bring them back to life. Simply add a bottle of fuel injector cleaner or fuel system cleaner to your tank and let the juice do it’s work. For more information on fuel injector cleaners, read these posts.

See this post for information on cleaning your fuel injectors

See this post for information on whether you should seafoam your engine?

Carbon buildup on gasoline direct injector engines is rough idle cause #5

In regular fuel injected engines, the fuel injector spray fuel in the intake manifold

dirty valves

Buildup on the back face of an intake valve

and a portion of that fuel spray hits the backside of the intake valves. Since fuel contains cleaners, the liquid fuel spray helps keep the back of the valves clean and reduces carbon buildup.

But gasoline direct injection engines work differently. In those engine, the fuel injector delivers gas right into the cylinder, so the valves never get fuel spray. After shutdown, crankcase vapors rise to the top of the engine and settle on the intake valves where they condense and harden. The result is carbon buildup that can cause a rough idle. Fuel injector cleaner will NOT remove this buildup because the cleaner never sees the backside of the valves. You must inject a cleaner through the intake using a procedure known as air induction cleaning. For more information on air induction cleaning read these posts.

What causes carbon buildup in gasoline direct injection engines

How to perform air induction cleaning

Worn spark plugs and wires are rough idle cause #6

This one is really simple: if your spark plug are worn or your spark plug wires

Worn spark plug

Worn spark plug

are old and leaking, your ignitions system will misfire and cause a rough idle. Misfires most often occur when the engine is under load. But they can still occur at idle speed because the idle RPMs are so low.

Replacing your spark plugs on time and using the right plugs and installation techniques are critical to the smooth operation of your engine. Check your plugs and wires so see what condition they’re in.

For more information on choosing the right spark plug, see this post

For more information on installing spark plugs, see these tips.

Stuck open EGR valve is rough idle cause #6

An exhaust gas recirculation valve is designed to open at high engine RPMs to feed a portion of the exhaust back into the engine to reduce combustion temperatures and reduce oxides of nitrogen emissions, the main contributor to smog. But the EGR is a mechanical device

plugged EGR valve

Plugged EGR valve and
P0400 Exhaust Gas Recirculation Flow Malfunction trouble code

and just like valves and the throttle body, it 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 idle condition. To check the valve, remove it and use a throttle body cleaner to remove the carbon.

Low fuel pressure is rough idle condition #7

Fuel injectors don’t really inject fuel. They’re just electronic valves that open and close rapidly. It’s the fuel pressure that forces the gasoline through the small ports in the injectors. If the fuel pump is failing and not delivering the proper amount of pressure or volume, it simply won’t deliver the amount of gas the computer expects to see. You can’t check fuel pressure by simply depressing the valve on the fuel rail. That’s meaningless. You must attach a fuel pressure gauge and actually measure pressure. Then, use the fuel pressure gauge to measure fuel volume. The exact amount is listed in the shop manual but is usually around 1-pint every 30-seconds. If your pump doesn’t deliver the right pressure or volume, replace it.

Incorrect engine coolant temperature sensor reading is rough idle cause #8

As I said earlier, the computer needs to know the engine and air temperature

where is the engine coolant temperature sensor

Engine coolant temperature sensor is usually located near the thermostat housing

to calculate the right air/fuel mixture. If it’s getting the wrong readings, it will deliver the wrong amount of fuel for a given amount of air. Engine coolant and air temperature sensors usually don’t fail completely. Instead, they give false readings. If your engine is hard to start on cold mornings, requiring you to depress the gas pedal, that’s a symptom of a failing engine coolant temperature sensor. Test an engine coolant and air temperature sensor by comparing scan tool live data to actual engine and air temperature measured with a thermometer. Or, get the specs from a shop manual and measure the sensor’s resistance values.

Clogged PCV valve is rough idle cause #9

The PCV valve is responsible for metering a set amount of airflow from the crankcase to the intake where it is burned. The PCV valve contains a safety plunger to prevent a backfire from entering the crankcase. So the plunger is an obstruction to airflow. Over time, oil

PCV valves

Find the correct PCV valve for your engine and replace it.

vapor and carbon can build up on the plunger, reducing airflow. That reduction in airflow can cause a rough idle.

In addition, a cracked PCV hose can also cause a rough idle. But that would also be considered a vacuum leak, and that was rough idle cause #1.

Head gasket leak is rough idle cause #10

Coolant in your oil, oil in your coolant, missing coolant, coolant overflow out of reservoir, overheating are all signs of a head gasket leak and they can easily cause a rough idle.

head gasket leak

Typical head gasket

What sensors don’t cause rough idle?

Oxygen sensors don’t cause rough idle

Lots of people think oxygen sensors can cause a rough idle. They can, but it’s rare unless you have a check engine light and trouble code. Oxygen sensors report what they see in the exhaust stream. If they see low voltage or high voltage, that’s because they see a lean or rich condition. In those cases, the oxygen sensor didn’t CAUSE the problem; it’s only REPORTING it. DIYers always replace the oxygen sensor and wonder why it still gives the same reading. That’s because it was telling the truth. You just didn’t believe it.

Throttle position sensors don’t cause a rough idle

Throttle position sensors (TPS) do fail, but they don’t fail in idle mode. TPS sensors tend to wear out in certain spots, like around 30-mph when you’re tapping on the pedal to get more speed. That worn out spot causes the engine to stumble/hesitate. Since the TPS doesn’t wear at idle speed, don’t suspect it as a cause. DIYers commonly replace the TPS and they waste their money doing it.

Air filters don’t cause a rough idle

The MAF sensor tells the computer how much air is getting into your engine. If the air filter is really dirty, the MAF sensor reading will tell the computer the engine is getting low airflow and the computer will respond by cutting back on fuel. What you’ll feel is a lack of power, NOT a rough idle. Ok, I have to qualify that; if your air filter is TOTALLY plugged, you’ll get a rough idle. But generally speaking, a dirty air filter will not cause a rough idle.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat

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