9 Causes of a Rough Idle (or RPM Running Too High While Idling)

When your car is experiencing a rough idle, your engine may chug, vibrate, or shake while it’s running. This will cause your overall driving experience to feel rough and you’ll likely be bouncing a little bit in your seat from it.

When the engine idles, there will usually be a misfire that you can feel just from holding onto the steering wheel. Furthermore, the RPM of your engine will be higher than normal when the vehicle is idling.

The RPM will usually go back to normal as you accelerate the vehicle, but will increase again after you stop. This is definitely not a normal situation.

Reasons for a Rough Idle or High RPM

If you are experiencing a rough idle or your RPM is running high when idling, then there are a variety of reasons for why this could be happening. It is not always easy to diagnose the cause because there are so many factors involved here.

Below are the top causes that a mechanic would look for.

1) Bad Spark Plugs

dirty spark plug

If you have bad spark plugs because they’re worn out or damaged, then it could be causing your engine to be idling rough. Remember that spark plugs are what send a current of electricity into the internal combustion chamber to ignite the mixture of fuel and air.

If the spark plugs are bad, then the electrical current won’t be sufficient enough for the ignition. This will cause the fuel and air mixture to burn inconsistently, which will result in rough idling engine.

2) Bad Ignition Coils/Wires

bad ignition coils

If your ignition coils or spark plug wires have gone bad, you will experience similar symptoms to having bad spark plugs.

Ignition coils, coil packs, and spark plug wires transmit a very high voltage to the spark plug which enables electricity to jump the gap. If the insulation on these components has worn away, they may be unable to produce a consistent high voltage output.

Any time the spark is unable to jump the spark plug gap, that cylinder has a misfire and no power is produced on that particular combustion stroke.

3) Vacuum Leak

vacuum hoses

There are a lot of hoses that help deliver both fuel and air to the internal combustion chamber. One very common cause of idling rough is a problem with a vacuum hose in the vehicle. The hose could be damaged, loose, or worn out, creating a leak.

This could result in an abundance of air mixing with the gasoline, which would result in a misfire. Then, the result of that would be a car that is idling rough or having a higher than normal RPM.

4) Carburetor Issues

Carburetors were previously used in vehicles before fuel injectors were invented. Perhaps you have a vehicle which still has a carburetor in it.

If so, the carburetor itself could be having issues resulting in rough idling of the engine. A telltale sign of this would be if your exhaust was blowing out black smoke.

5) Dirty Fuel Injector

fuel injectors

Fuel injectors are the modernized way in which vehicles deliver fuel into the internal combustion chamber. The only thing is that these fuel injectors can get dirty or clogged after a while.

If you are not using gas additives or fuel injector cleaners on a regular basis, then there’s a good chance you’ll get a dirty fuel injector at some point. This will result in your car idling poorly. You could also see your gas mileage take a nosedive as well.

Related: Best Fuel Injector Cleaners

6) Clogged Air Filter

clogged air filter

The engine needs air from the outside to mix with the fuel in the internal combustion chamber. An important component under the hood of every car is an air filter which prevents debris from getting sucked into the chamber.

But if this air filter is dirty and becomes clogged, then there won’t be enough clean air passing through it. This will cause the engine to idle rough.

7) Bad Idle Air Control Valve

bad idle air control valve

The idle air control valve (IAC or IACV) adjusts the amount of air that is allowed past the throttle body while the throttle plate is closed. This regulates the engine idle speed and allows idle to remain constant even if you switch on the air conditioning or turn on your headlights, both of which put more load on the engine.

If the idle air control valve did not compensate for these, the idle speed would drop when certain accessories are used. In severe cases, the engine may even stall. 

The idle air control valve is most important on a cold startup. Problems with the idle air control will be most obvious during this time.

8) Incorrect Cam Timing

bad timing belt

Have you recently replaced the timing belt on your engine? If the timing belt is loose or off by a tooth or more, the engine will likely end up with a rough and lopey idle, if it runs at all.

When the valves are not timed correctly, the intake and exhaust valves are open when they shouldn’t be, which reduces compression and causes the combustion gasses to travel where they shouldn’t on that particular stroke.

9) Incorrect Ignition Timing

timing light

While cam timing is mechanically controlled by a timing belt or chain, ignition timing is controlled by the ECU and the distributor. Some vehicles have a crank or cam angle sensor instead of a distributor that can be manually adjusted to advance ignition timing in much the same way.

If ignition timing is incorrect, the car could run rough and often try to stall. You should be able to check your own ignition timing using a timing light. The procedure is different for every vehicle, so check a repair manual if you’re not sure how to do this yourself.

Things to Remember

Your engine should normally get to around 700 RPM after you start the vehicle, depending on the model. If your idle is way off and you have one or more of the problems listed above, then you will want to diagnose the issue right away.

The cost of the repair or replacement job will depend on what the cause was. If you take your vehicle to a repair shop soon, then your engine should be okay.

But if you’ve continued to let it idle poorly for too long, then there could be irreversible damage to the engine. This in turn could cost you thousands of dollars in repair or replacement costs. So, don’t wait for this to happen.

32 thoughts on “9 Causes of a Rough Idle (or RPM Running Too High While Idling)”

  1. I have an exhaust leak I’ve got a broken bolt in the exhaust manifold so I can’t get a bolt in it I now have a jeep wrangled that idles too high just started today I’ve not drove it very far how much will this cost if I take it to a jeep dealership to fix it

    Reply
    • My Toyota corolla 2011 has a high rpm when the engine is switched on. At driving, the speed is fast, break becomes hard and takes greater pressure on the break pedal to stop or slow downs the car, what is the problem.

      Reply
      • It sounds like something is holding the throttle plate open, even when you are not stepping on the gas pedal.

        You should get this diagnosed as soon as possible to prevent unintended acceleration. If that’s what’s going on, it has the potential to be very dangerous.

      • Replace your brake booster. Problem solved. Or check the vacuum line from the brake booster to motor for leaks/cracks etc…

    • I have a chev aveo 1.5 2009 model hashback my idling is too high and sometimes is fine it is mulfuctining what mightvthe cause because i did replace the trottle sensor and idling sensor but still doing the same thing

      Reply
  2. I ascertained my probelm, it was not clogged injectors, not the fuel, not the fule filter, nor spark plus, rotor arm, ignition cap, distributor, iginition module. c.o is spot on. dwel angel is spot on. The issue is the lack of lead (my car claims it runs on unleaded by the cast mark and contacting Ford UK/ E.U). Took the head off, placed new valve seats and problem sorted out. I have now a engine which runs on uleaded that I know for sure will run on unleaded fule. You cant trust Ford or any manufactuerer just look at the V.W scam over emissions anti lock brakes freezing the list goes on.

    Reply
  3. Hi.
    I have a 2014 Rav4 and recently changed the Transmission. the RPM is Okey when i “start” the car, however when i push the gas to start driving, the RPM goes up to 4k and 5k and the car does not run or runs supper slow (an RPM of 5k with a speed of 5MPH). this also happens sometimes when i stop behind the red light. however, the car is doing fine on a higher speeds (runs smooth when above 40MPH). all this story started when i changed my transmission. Please advise

    Reply
    • It sounds like the engine isn’t able to transfer enough torque to the transmission, due to a slipping clutch or torque converter. Check fluid levels and if they’re topped off, consider bringing the vehicle to a local mechanic for further inspection.

      Reply
      • My car starts up good. But then starts to bounce and lights dim and junks in drive rpm go up and down but don’t die I changed the Sparks the coil and control module but still misfires on cylinder four I have 07 Chevy HHR 2.2 ECOTEC

  4. When I start my car the rpm goes so high and it does so even when I push the gear on park position. The rpm reduces when I engage a gear. What could be the problem?

    Reply
    • It could be a problem with the idle air control (IAC) valve, a vacuum leak, or a number of other things. You can check for vacuum leaks with a smoke test.

      Reply
  5. Good day guys I am having a Ford Focus model 2004 it has a higher rev I have tried changing lots of stuff but then nothing have changed yet I don’t know what to do anymore

    Reply
  6. My car is 2004 toyota mark II. When I start the engine rpm was was too high even the gear was parking. When I change the gear to drive mode, then engine rpm was reduce. Please be advice me.

    Reply
    • How high is “too high”? It could be a vacuum leak, but engines also tend to rev higher on cold start. It helps warm up the catalytic converter.

      Reply
  7. I have a 2002 Toyota Celica the rpms is fine intill I get up to 80mph then it wants to stay over 4000. What could this possibility be?

    Reply
    • It might just be the way that car is geared. Shorter gears mean higher engine speeds for a given ground speed. My Miata definitely sits around 4k RPM on the freeway and this is normal.

      Reply
  8. I have a 2005 ford focus 1.6i, engine starts and dies straight away. If I keep my foot on the gas pedal then engine will keep running but rough as if off a cylinder. Changed plugs, ignition coils, air filter,throttle body and intake manifold cleaned out, made no difference, engine check light is on but not getting any codes on my scanner. compression test is normal.

    Reply
    • Is the check engine light flashing? This usually indicates a misfire.

      Has the timing belt been changed recently? One other cause of the symptoms you describe is incorrect timing. If the belt skipped a tooth, it would certainly cause a rough idle. If the ignition timing needs adjusting, you may also experience similar symptoms.

      I would do some more testing before throwing any more parts at it. That game gets expensive pretty quickly.

      Reply
  9. I am having the same problem with my 2003 Malibu for the last month. My car idles high and will speed up to 40 miles an hour by itself. The RPM runs up between 2 and 3. I took it to the dealer and since the check engine light does not come on they are unable to figure anything out. They did say the got an alert that some gas stations are selling winter gas instead of summer gas. Never heard of such a thing. Today we drove it and it drove at a speed of 40 miles per hour itself and when I pulled up the driveway and put it in park I couldn’t hardly put it back in drive. ‘what is going on?

    Reply
    • It sounds like something is holding the throttle plate partially open, which could be very dangerous if it gets worse. A stuck throttle could lead to unintended acceleration. I would avoid driving the car until the issue is fixed.

      If your throttle does get stuck, be aware that you should still be able to shift into neutral. Just bump the shift lever into neutral and pull over immediately when it is safe to do so. The engine will make a lot of noise, but it is always better to lose an engine than your life.

      Consider finding a shop that can give you something better than the “winter vs summer gas” excuse. While gasoline additives do vary by season, they shouldn’t cause your car to behave that way.

      Reply
  10. I have a 2009 Benz Sprinter that’s a passenger van but, I have been delivering bulk packages to the offices and my rpm are staying around 20 to 30. Any suggestions?

    Reply
  11. I have a 2012 Dodge Journey Crew, Going high way speed Rpm was close to redlining just to pick up to 70mph from 60 mph with light gas .!

    Reply
    • High engine speeds won’t harm a healthy engine. On some vehicles (especially drive by wire vehicles), “light gas” could be close to wide open throttle. It depends on how the ECU is tuned.

      Whether or not your symptoms are indicative of a problem I can’t say for sure without more information. Does it feel like the transmission is slipping, leading to high RPM? Is it downshifting to get to redline?

      Reply
  12. Hi,

    I have a 2012 Elantra it just started revving high this week at normal speeds. Around 3 grand and louder than usual. Wonder what that could be?

    Reply
  13. Hi, I have been to plenty of mechanics even to honda dealership because my rpm sticks at 1300 even taking the foot off gas pedal and rpm will go up at 1300 and when braking the car will stop but the rpm wil jump up again causes vehicle to shake. No one is able to figure out the exact problems there no vaccum leaks. today mechanic told me that my alternator volts are should be 13.54 min but its 12.98v at 3000 rpm. IDK if alternator should cause this problem

    Reply
    • I don’t think the alternator is the cause, but that may be a piece of the puzzle. Keep digging and see what else you can learn from testing various components.

      Reply
  14. If your car is over 15 years old, suspect the main ECU capacitors.

    Caps only last 15-20 years. Causes misfires, smoke, bad timing, fuel overuse, and so on. Even transmission shift issues.

    Reply

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