Causes of a Bad Idle & How You can Diagnose It

Although cars have varied greatly over the years in size, construction, and complexity, there are some attributes and issues that are common to pretty much any car ever made. One problem that is pretty universal is a rough or bad idle. When certain parts go out on a vehicle they can cause an annoying rough idle.

At its best a rough idle can seam harmless and may be ignored as simply a quirk of the vehicle. At its worst a rough idle in a vehicle may cause it to immediately cut off and can be a sign of a bigger issue. In this article I will discuss the various causes of a bad idle, what you can do to diagnose it, and how to fix it.

What is a Rough Idle?

When somebody says that there vehicle is “idling rough” this usually means that the revolutions per minute (rpm) gauge on your instrument cluster is rising and falling or maybe the car idles really low at first and then smooths out after a few seconds. Our vehicles are sensitive, they need the right combination of spark, fuel and air to idle smoothly. Anyone of these components has a problem and the vehicle will idle rough or not at all.

What Causes a Rough Idle?

how to diagnose rough idle instrument cluster

Unfortunately our cars are quite demanding. There are a number of components that have to work correctly for your vehicle to idle smoothly. A slip-up from a spark plug, a fouled fuel filter or even a tiny crack in a vacuum line can cause your car to run like crap.

There are some idling problems that are vehicle specific issues, but for the most part a bad idle can be caused by any of these parts in any car.

Just to give you a quick list of possible problems:

Vacuum Lines

Mass Air Flow Sensor

Intake Gasket

Throttle Body

Idle Control Valve

Ignition Coil

Spark Plug

Fuel Pump

Fuel Pressure Regulator

Vanos Seals (More of a BMW issue)

Valve Cover Gasket

Head Gasket

Oxygen Sensors

Whew! That’s a lot of possibilities isn’t it? There’s more than ten things on this list, and trust me there are other potential candidates as well. The problem in diagnosing a rough idle is that there are just so many possibilities. So how on earth does a DIYer like you and me narrow the list down?


I typically keep a log in my car with dates on all the repairs I have made on it. This helps me stay organized and reminds me when the last time I performed important services. It helps me gauge for myself the quality of certain parts that I buy online or from autoparts stores. The log also comes in handy when I am diagnosing a problem such as a rough idle. If I’ve replaced something well within the lifetime of the part I know it’s probably not that part. This doesn’t mean that I scratch it off entirely as a possibility, but I do put it on the backburner.

The absolute first step in fixing a poor idle is looking at any check lights you may have, unfortunately the ECU may not throw any check lights. “But Steve, my cars idling badly shouldn’t I have a check engine light?” No, not necessarily, I mean that would be nice if there was a check engine light for every issue that came up on your vehicle, but there is not. Vacuum leaks, bad ICVs, and frequently MAF’s will not throw a check code when they are causing a problem. So remember, you are not out of the woods just because your check engine light is not illuminating, but an illuminating check engine light can be a good indicator of what is wrong.

Have you ever heard the noise a really bad catalytic converter makes? Sounds like someone threw a bag of rocks into the exhaust whenever the driver hits the gas. My baby brother has been driving his E36 for months like that, it sounds awful. The car has lost power, and fuel consumption, but he is just too cheap to fix it. Guess what though? He passed an inspection with his converter like that (amazingly!) because his check light never came on, at least up until this past week.

So if you do get a check light, and it says something specific like an oxygen sensor, vacuum leak, MAF Sensor, Misfire, etc. Than that’s great, your diagnosis is pretty much over. If it doesn’t give you anything specific or it doesn’t give you anything at all, keep reading.


Not all rough idles sound the same, and not all of them act the same. What do I mean by this? Pull your mass air flow sensor connector off, the car will keep running, although it will usually run at a lower rpm and throw a check code. When an idle control valve gets bad enough your car won’t even hold an idle. How do you know it’s the ICV when it won’t hold an idle? Put your foot on the throttle, if the car holds an idle than it’s a bad ICV.

Vacuum Line Test

Causes of a bad idle

Most vehicles have a maze of vacuum lines running all kinds of ways above and below the intake manifold. Vacuum lines become brittle overtime and will develop hairline cracks. You would be surprised at how small of a crack on a seemingly unimportant vacuum line can cause a very big problem.

This is often the very first thing I will do with a rough idle if nothing is jumping out to me as the source. Before performing this test look around at your vacuum lines and connections. Identify if any hoses are brittle, every now and then I’ll get lucky and find a hose that’s not even attached. If nothing is jumping out, cut on the engine and listen for a hissing noise. If you’re not certain what a vacuum leak hissing noise is just pull one, you’ll here it.

There are a variety of ways to perform some sort of vacuum line test. I’ve seen a mechanic smoke a cigar and use that as his fog tester. Mechanics typically have an actual vacuum tester that pumps smoke into the system to find the vacuum leak.

DIY guys typically use brake cleaner, starter fluid, propane or soapy water. I sometimes use soapy water in a spray bottle and spray it on connections and gaskets, basically it will bubble up if there is a vacuum leak. I’ve seen plenty of tire places use this method as well to plug tires.

Starter fluid and brake cleaner should only be used when the engine is COLD! You wouldn’t want the vehicle to catch fire and create a whole new set of problems. If you decide to try brake cleaner/starter fluid, you’re looking for an idle surge. If you get an idle surge you’ve identified your vacuum leak.

Spark Plugs

Spark plugs are an extremely common source of a rough idle and on most vehicles they are extremely easy to replace. As a vehicle racks up mileage the spark plug electrodes wear out which causes the gap to widen. Other symptoms of faulty spark plugs are hard-starting, and an increase in fuel consumption.

Remove a single spark plug and inspect it. A really bad spark plug will cause the vehicle to misfire and therefore will throw a check engine light, however this does not always happen. If the spark plug is fouled with oil or carbon deposits you should replace them. Luckily, they only cost a couple bucks each most of the time.

Mass Air Flow Sensor (MAF)

The MAF is a really common cause of idling problems on higher mileage vehicles. Luckily, the good news is that most of the time the MAF does not need replacement! Usually you can get away with cleaning the sensor with some mass air flow sensor cleaner. Be careful when removing the MAF however, it is a very delicate sensor and should not be touched.

Fuel Injectors

Have you ever replaced a fuel filter before? If you have, I am sure you noticed how disgusting the fuel was when you poured it out from a certain side. All that gunk buildup is supposed to get trapped in the filter, but unfortunately the fuel filter is not 100% accurate 100% of the time. Fuel injectors have tiny, pinprick holes on the end of the nozzle. The fuel shoots through these holes. Since the holes are so small it takes very little debris to block a nozzle enough to cause an idling problem.

Throw in some fuel injector cleaner, I really like Lucas but Techron is really good to. Fuel injector cleaners can be picked up at your local auto parts store for a couple bucks, just follow the instructions on how much to dump in your gas tank.

Idle Control Valve

There are two ways an idle control valve will go out. Either the electrical component will go bad or the valve itself will get stuck due to carb build up. I’ve been able to diagnose a bad ICV by pressing my foot on the gas pedal when I start the car up. If the vehicle holds an idle when you hit the gas, but dies when you let go, you’ve probably got a bad ICV.


Causes of a bad idle fuel pressure

You can purchase a fuel pressure tester or you can rent one at a local auto parts store. Check what your fuel line fuel pressure should be at idle, and if it’s significantly lower or bounces around you’ve got a fuel delivery problem.

If I identify that a fuel delivery issue is causing a rough idle I immediately add seafoam or Lucas fuel cleaner to the gas tank and then change the fuel filter if it hasn’t been changed in a long time. The fuel cleaner may unclog the injectors enough to fix the idle if dirty fuel injectors are the source of the bad idle. If a fuel cleaner additive fixes the idle problem and you’ve been using cheap 87 grade gas, start buying 91 or even 93. Your fuel injection system will fall in love.

If this doesn’t solve the problem you should test the fuel pump.

Head Gasket

A blown head gasket will certainly cause an idling issue, but it will cause a number of other problems as well. If you think you have a head gasket leak, check your coolant and oil for fluid mixing. There are a number of other simple tests to perform in order to identify a head gasket leak as well. Nobody wants a blown head gasket, but it’s better to know than not to.

Oxygen Sensors

how to diagnose rough idle oxygen sensor

Oxygen Sensors can also cause a rough idle, but typically by the time they are bad enough to cause idling issues they will also throw check codes. If your vehicle has more than 100k on it and the oxygen sensors are original, assume that they are on the way out. Bad oxygen sensors will also cause a loss in fuel economy, and if they are upstream sensors they can cause the catalytic converter to go bad!


A rough idle can be one of the more time consuming diagnoses to perform simply because so many things can cause one. Luckily most tests are relatively quick and DIY friendly, and hopefully you can identify the culprit within an hour or so of testing. The last thing you want to do is to start shooting in the dark and change parts at a whim. This can get very expensive very quickly. It’s best to put in the time to identify the culprit as quickly as possible. That way you have your vehicle back on the road cheaply and quickly.

Check out more DIY articles!

Stephen Metellus

I am a BMW enthusiast and owner of! I have been repairing, flipping, and parting out BMWs for nearly ten years. I love these vehicles and I hope you will find my articles and YouTube channel helpful for whatever BMW project you have in store!

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