'15 IS250 with ~66k miles. I used 93 octane. I just replaced my alternator and battery. To be able to gain access to my alternator, I had to move and unplug the ECU box/computer. Before replacing alternator everything was fine. Now my car is acting funny. It will idle at a red light at 500-600rpm than go to 1k rpm than back down and repeats. It will even do thing when I am reversing or in park. I tired resetting my throttle position, but it still kept on doing it. Could it be the issue of carbon build up on my throttle body or a case of bad gas? Any help would be great. I will post a video of it tomorrow

 

Disconnect the battery again...clean the throttle body...and reconnect the battery.
Take the car for a 20 minute drive and you should be fine.

 

^ Agree, this sounds like it could be throttle body. My first thought was spark plugs, but I think that'd be something to check after the throttle body if you haven't ever cleaned it.

 

This video is in drive. This video is in park. I looked inside my throttle body and did little bit of cleaning and drove it around still seems to have alittle rough idle. I do notice from time to time that the battery light would randomly appear but goes away super fast like 0.1 seconds.

 

I remember I had my Corolla's battery changed at the dealership around 100K miles. Ironic, before battery change service, they offered to do a throttle body cleaning for around $200-300 (to which I declined).
They changed the battery and my car was driving like complete dog **** (idling really bad, worse than this video) as soon as I hopped in. I went back into the service center and told them this. I waited another 20-30 minutes and they said 'it's fine now' and I asked them what happened and they said "we cleaned the throttle body for you"... and then it was idling fine (which till this day I don't believe because they wanted $300 for this)

My point is, I think there's some kind of link between changing battery / alternator and the idling being rough, not sure what it is. But whatever they did at the dealership fixed it and they claimed it was them cleaning the throttle body... (even though I never had ANY idling issues prior to bringing it to them).

 

How exactly did you clean the throttle body? The flap should have been opened manually and cleaned all around the bore and the back of the flap, and all around the edge of the flap.

You are correct that there is a link between the battery replacement and the rough idle.
Over the course of the vehicle life as the carbon builds up in the throttle body the ECU will compensate and adjust the flap position to maintain a proper idle.
When the battery is disconnected, the ECU's learned value of the throttle body flap position over that life span is lost.
The ECU reverts to the programmed value of a new throttle body as the starting point for the new lifetime learning.
But, if the throttle body has carbon buildup, that means the ECU is not going to open the flap enough to allow the correct amount of air in, so the vehicle is going to idle rough...remember it thinks it's anew throttle body.
Simply cleaning the throttle body thoroughly restores the proper idle...for there the learning curve is a lot faster for the ECU.

 

[QUOTE=Sasnuke;11089438]How exactly did you clean the throttle body? The flap should have been opened manually and cleaned all around the bore and the back of the flap, and all around the edge of the flap.

I disconnected the battery, removed my intake, manually opened the flap and cleaned the bored, the back of the flap as well as the edges. I will drive for 100 miles to see if the computer will remember my old idle, if not i will take it to a local shop for a better diagnosis

 

I disconnected the battery, removed my intake, manually opened the flap and cleaned the bored, the back of the flap as well as the edges. I will drive for 100 miles to see if the computer will remember my old idle, if not i will take it to a local shop for a better diagnosis

 

Sounds like a thorough cleaning...your rag should have been fairly black after that.
Some will even use an old toothbrush to really get in there and then the intake cleaner to wash it all away.
Once you can get it to as shiny and new as possible, then that should be good enough.
Taking it for a nice boot after will help burn off all the chemical residue.
The computer should be able to learn and adjust the throttle flap in short time now for a smooth/stable idle.

 

**UPDATE** Battery light would pop up from time to time because of the faulty rebuilt alternator from AutoZone that caused my vehicle to have a weird idle in drive/park. I wasn't to positive on it until I had my voltage tested and it would read 12.8v. I also noticed my lights would go from bright to dim inside my car. Now the replacement alternator, it is working much better. No more weird/rough idle. No more battery light.

 

If you changed the battery without the car on or plugging an external battery.
The computer (ECU) will be reset and if your throttle valve is not clean (not fully closed),
your idle rpm would go crazy.

Either change the battery with power connected:

Or make sure the throttle valve is cleaned after ECU is reset.

 

My car does this exactly in Park and Drive and I always thought it was a vacuum leak. Going to try cleaning the TB with the battery unplugged and if that does not solve it, I will test my alternator lol.