While I'm sitting idle at traffic lights I put the car into neutral. The engine stays idling at about 1000 RPM. However every now and then it drops down to about 500 RPM and then quickly back up to about 1200 then goes slowly down to 1000 and stays there for a bit until I go again or it drops again.

It only happens for a second or two and about 3 times a minute.

Is this normal or does it indicate some sort of problem with my car?

  • 1
    Are you using the defroster or air conditioner when this happens? Is it soon after starting the car, or does it happen all the time? Is your Check Engine light on? Listen carefully - do you hear a click or a fan noise when it happens (or anything else - try at home with the hood opened).
    – JPhi1618
    Jan 21, 2016 at 13:21

6 Answers 6


I am presuming this is a gasoline engine. If so, the symptoms sound very much like the throttle internals are dirty. If gunge builds up on the butterfly valve, it can restrict the air required for a smooth idle when the butterfly is closed, so the engine ECU needs to open the Idle Air Valve more that it thinks it should have to and you get into a cycle of surging revs.

If you take the large air intake pipe off the throttle body, you should be able to clean the internals with some carb cleaner and a cloth. Make sure you don't add any dirt that can get sucked into the engine and damage it.

  • I believe you more than me. +1 and deleting my answer. Jan 21, 2016 at 8:50
  • How difficult would doing so be for an amateur (although I am a robotic engineer, so am somewhat handy, just never done anything on cars)
    – Aequitas
    Jan 21, 2016 at 21:39
  • 1
    It is a very easy job, see this video for example - m.youtube.com/watch?v=SCboJ8B6wRM
    – HandyHowie
    Jan 21, 2016 at 21:49

i've seen this caused by slight loss of pressure in cylinder(s)... its sometimes an early warning of more severe problems that are developing e.g. if there is some light damage to a piston ring or the head gasket that is going to degrade further if left unremedied.

a compression test can rule this sort of thing out.


I went through 4-5 different possible solutions (which include replacing the O2 sensor, the CAM accuator and all six plugs). There were low cost attempts and probably needed to be done anyway. The final solution? The MAP sensor. Bought it online for $5.80 and have not had a problem since...


It's called as Engine Hunting... It's the problem with the FIP(fuel injection pump) engine have. It's because of rack movement restrictions I.e, rack will not come to its original position it's slightly touching to the FIP cover so by adjusting the cover by bringing face to face. U better take it to the Service Shop

  • Are you referring to a diesel engine? Also, this question is over a year old. So, this may already be resolved.
    – CharlieRB
    Jan 25, 2017 at 21:20

I agree with the answers above about the air intake but in these circumstances, most of the time it's to do with the 02 sensors on the exhaust system. Depending on the car, their are X3 of these sensors and you can either replace them all or find the broken one. Just one question is your engine management light on or not pal, if so, it will be the 02 oxygen sensors or sensor that just needs replacing. 😀


Most likely emissions. Some (most) stuff burns off better at higher RPM (for example CO). Controller will drop down RPM when it can. Or you controller is just having problems. If you put the car in neutral you will put less load on the engine.

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