Idle surge while car is moving.

Hi.

I recently had my throttle body replaced. The car starts and warms up fine, but has a "bobbing" idle between 1100 and 1500 RPM after the car moves. Odd, I know. Everything is fine until rolling to a stop with the load off of the engine (either clutch in or in neutral). The engine slowly drops to 1100 RPM and then surges to 1500 and back to 1100. This continues until the car comes to a complete stop. At zero MPH the car idles rock solid at 850RPM. The problem doesn't start until the engine has warmed up.

Could a vacuum leak cause this? It seems like a data issue between the ECU and throttle body to me since it only happens when the car is moving; a vacuum leak wouldn't know the car was moving at 1 MPH while in neutral. I had a shop install the new throttle body. The first one they installed was an aftermarket part that had the same issue. They then installed an OEM throttle body, performed a relearn procedure twice, called an outside vendor to check the programming, and was unable to fix the issue.

Any ideas? I can't find anything on the internet with these symptoms. The car is stock with 152,00 miles on it.

Thank you for any insight you can give.

I just realized that I didn't mention my car is a 2009 Speed3...

 

Battery is dying

 

Thanks! I replaced the battery a few months ago. I'll see if I have a loose cable.

 

I reset the cables and tested the battery.... it is at 12.7V with the engine off, and puts out over 600 amps. It looks okay.

 

I had the exact same problem battery seemed fine but had a bad cell. New battery fixed the exact problem you are describing

 

Thank you so much! The battery disconnection/terminal clean fixed the problem. Your comment saved me hours hunting for nonexistent loose connections.