Shaky idling?

[chitown]

Last summer my secondary air pump went and just had that replaced along with oil change not too long ago, but in the last few weeks when I'm driving and shift back to neutral when coming to a stop (or if the car is just sitting), the RPMs move up and down between 0 (and slightly below) and 1000 constantly like the engine is kind of struggling. I thought this would be the air pump but wasn't. I've had the car die on me when coming to a stop a couple times as well, but turning the car on again made it run again.

Any ideas as to what this is? Even when starting from a cold start it seems like its not starting as smoothly as it should.

And just yesterday I got an error and flashing light telling me to check the brake light circuit. Too much going wrong...

[propr'one]

my e36 has a harsh idle, moves between 450-550. hasen't been a problem yet though

[Justin e36]

Originally Posted by chitown

Last summer my secondary air pump went and just had that replaced along with oil change not too long ago, but in the last few weeks when I'm driving and shift back to neutral when coming to a stop (or if the car is just sitting), the RPMs move up and down between 0 (and slightly below) and 1000 constantly like the engine is kind of struggling. I thought this would be the air pump but wasn't. I've had the car die on me when coming to a stop a couple times as well, but turning the car on again made it run again.

Any ideas as to what this is? Even when starting from a cold start it seems like its not starting as smoothly as it should.

And just yesterday I got an error and flashing light telling me to check the brake light circuit. Too much going wrong...

Your idle is controlled by two things. Your o2 sensors, and your ICV. Since your tach keeps jumping up and down, I suspect it is a sticking ICV. Sometimes they are a bit tacky, and it gets worse as it gets colder outside. You can clean it yourself. If your o2 sensors are dying you'll just get an idle that either stays too high, or too low and soon after your check engine light may come on.

If your idle is just high, and you get a check engine light---then it could be a vacuum leak, or your MAF is dying.

If your engine is ALSO running rough (jumpy), and likes to stall unexpectedly.. then I would suspect an ignition coil is gone. Let your car idle and listen carefully to your ignition packs.. if you're lucky you'll hear a "spark" popping sound once in a while.. not very loud. If you take off the cover, you can feel each pack.. they should be only warm to the touch.. if one is scorching hot, it's bad. You can also use a volt meter to test them individually. If your ignition coils are good, then next step would be to check the spark plugs for any damage. It's possible that a ceramic is cracked.

J

[windsor318is]

justin bad plug wires could do it as well?

[chitown]

Hey Justin,

The idle isn't high, it's pretty normal but just dips down to where you can feel the engine kind of stutter and then go back to normal, and over and over.

The check engine light has been on since summer, it came on when the secondary air pump died. But after getting that replaced, it's still there.

I'm not sure if it's just the cold and windy weather but sometimes it does seem like the car is a bit rough, like it has lost a little bit of its "pickup" but that could just be mental

Thanks J

[Justin e36]

Originally Posted by chitown

Hey Justin,

The idle isn't high, it's pretty normal but just dips down to where you can feel the engine kind of stutter and then go back to normal, and over and over.

The check engine light has been on since summer, it came on when the secondary air pump died. But after getting that replaced, it's still there.

I'm not sure if it's just the cold and windy weather but sometimes it does seem like the car is a bit rough, like it has lost a little bit of its "pickup" but that could just be mental

Thanks J

windsor - yep a bad plug / connector can do it too...

chitown - get a code reader to check why your check engine light is on, or if you have an OBD II e36 you can do the self-diagnostics to check this. It sounds like your oxygen sensor is dead... but here's a way to check if it's a vacuum leak:

1 - put your car in park (or neutral as it may be).
2 - allow your engine to warm up to normal operating temperature.
3 - rev your engine to 5000rpm, hold it there as steady as you can for a full 40+ seconds. (the longer the better)
4 - let go of the gas slowly until it is back to idle speed.

now, is the idle still unstable? if your idle is now fine, then you likely have a vacuum leak, or your MAF plug (as windsor mentioned) can be the culprit, there is also a remote possibility that your air filter is clogged and your car is having difficulty getting the proper air/ fuel mixture, and only after revving up can it suck in enough air to even out.

if the idle is stable at first after doing this, but then it begins to get bad again after driving around for a few minutes---then your MAF is probably dead.

and last, if doing this test makes no difference at all and your engine is still idiling weird, then your oxygen sensor (o2 sensor) is likely dead.

chances are that it is not your ICV, as if your ICV sticks your engine will usually rev up to 3000 rpm, then nearly stall, and go back and forth like this. But my understanding is that your car is not revving this high when at idle though.