Idle Issues

[Emily]

My 'check engine' light has been on and off for about a year now...And I have noticed that my idle is inconsistent. Some times it idles high other times it is so low it feels like it is going to die. It seems to change the idle based on the temperature. I know that I have a small tranny issue but I didn't think that would affect the idle.

I had a free (meaning possibly unreliable) diagnostic on it and it registered a problem with the O2 sensor.
This is a dumb question but how many of these things do I have? I was thinking it was two, but I don't know for sure. Is there a way of determining which senor needs to be changed so I don't buy more than I need?

I am the second owner of this car and have all the repair papers. I believe that one of the sensors has already been replaced (under warranty or recall). If this is the case, could it be the same one?

If it is the O2, any tips on good places to order one from? *CHEAP*

[Legacygt_97]

The oxygen sensors, I believe most Subarus only have two. (someone correct me if I'm wrong) One upstream, before the converter; and one downstream, after the converter. The former plays a role in determining air/fuel ratio which could affect idle quality. Personally, I would be leaning more towards a vacuum leak, which could also cause the PCM to see the AF ratio as lean. If possible, knowing what DTC's are stored causing the MIL would be awesome.

[Huffer]

Irregular idle usually isn't an O2 issue - an O2 sensor failing will result in poor performance, MPG and a smelly (way rich) exhaust.

Irregular idle is usually a vacuum leak, Idle Air Control valve malfunction or even an airflow measurement inaccuracy.

I would reset your car (unplug battery for a few mins, foot on the brake to discharge the system, reconnect battery) and invest in a decent diagnostic scanner (see the Toolbox section for my review on the ScanGaugeII or any other similar scanners).

O2 sensors can be purchased online for under $50-70, the costlier ones are direct plug n play, but before purchasing one I would see what those Check Engine Light codes are first.

[Reason]

The oxygen sensors, I believe most Subarus only have two. (someone correct me if I'm wrong) One upstream, before the converter; and one downstream, after the converter. The former plays a role in determining air/fuel ratio which could affect idle quality. Personally, I would be leaning more towards a vacuum leak, which could also cause the PCM to see the AF ratio as lean. If possible, knowing what DTC's are stored causing the MIL would be awesome.

So IIRC you think the O2 is causing the CEL to go on becasue the PCM is casuing the AFR to do whacky things to the DTC and the MIL?? It all makes since now :lol:

[Emily]

Yeah...I'm a very limited in my automotive knowledge. We are going to have to break it down a little further.

So basically, I don't have to change my sensor...and I am going to have to spend more money?

[Emily]

I will try the reset and see if it makes a difference.

What would be involved in correcting a vacuum leak?

[d1giPhux]

Dont spend anymore money.. dont buy a scanner.. as you can go to a local 'autozone' or 'advanced auto' or 'pepboys'.. or whatever to use theirs for free. If your car is over the year 95, you will ask to use an 'OBD 2 Scanner'.. and they will let you use it for free. I had a very rough idle on my legacy at one point, and im pretty sure it was due to the MAF (Mass Airflow Sensor) on the intake of your car. Im assuming yours has a MAP sensor.. and this may have something to do with it.. im not sure..? I think it serves the same purpose.

When you pull up to a stop light and and pushing down on the brakes to stop.. does your idle drop out suddenly.. and almost cause your car to stall?

Any more signs like this?

It could also possibly be your Idle air control valve like states.. but give us a few more details as to when the RPM's on your car drop out / spike up.. etc.

Also, if you do the reset.. pull the negative (black) side off your battery for like 15 minutes.. then press on the brakes a few times, then re-connect and let your car idle / drive it around for a few mins.

[Huffer]

Buying a scanner is a good investment. Sometimes an autoparts store will not allow clearing of codes to see if they come back.
Resetting the car all the time isn't good automotive practice. Clearing codes is ok.

[Emily]

Thanks for the info.

Really the idle seems to change with temperature more than anything else. It really isn't consistent.

It seems to idle low with colder temperatures and higher with warmer temperatures. But this isn't always the case. There are times the idle is normal during temperature changes. I notice it more for the first few minutes of driving. But occasional it will start after the car is warmed up. It is hard to determine what the exact pattern is because it doesn't seem to stick with one.

I did get the code from Advance or AutoZone (they are kind of interchangeable to me) and that is why I mentioned it could be unreliable. I don't know how exact they are. They checked it and said it was an O2 sensor. It has been happening for about a year now. If it was something like a vacuum leak or a idle air control valve would it have gotten worse or probably stayed the same?

It is really only bothering me because it affects my acceleration. When the idle is low it will barely move. But when it is idling high it takes off without me! (I don't mind that so much...) :twisted:

[Wiscon_Mark]

They really can't screw up a code at autozone, they're using an OBDII code reader just like you would.

Did they tell you the exact code? It's an alphnumeric sequence.

[PRNORS]

I would be changing your temp senor

temp sensor will affect idle also try cleaning your AFM with electrical contact cleaner and check all vacuum lines for leaks

pull off the idle air control valve and give that a clean too

Reggy

[Emily]

No, they didn't tell me the code. The guy just plugged it up and then told me it was the O2 sensor.

I don't really remember...It is been a year...It didn't seem all that dangerous. Now it is just annoying.

[Wiscon_Mark]

Sounds like you should go back in and get it read again. There may be multiple codes now.

[Huffer]

I still say chunk the change, buy a good code reader and reset the car. First CEL, pull the code. Fix that.
Clear the code.
Keep driving.
Simple, really!