Low Idle when warm - P0506 -- strange solution

[hayedid]

Hi. I have a 2007 Subaru Outback 2.5i.

 

It's been fixed (more on that later)... but here were the symptoms... and the strange solution:

 

Idle was low (400 RPM) only when the car was warm. Often threw Code P0506 (Idle lower than expected). On a cool morning, the car idled fine. The car drove fine, but when it was warm and the idle was low, it would often die at a stop light. There is no throttle control valve. Basically there's the gas pedal, the computer, and a sealed throttle body control motor (stepper motor). To fix it, I tried things such as replacing the air-filter, checking for vacuum leaks, cleaning the mass air flow sensor and temperature sensor, checking the throttle body for anything stuck in it and for jammed movement. No issues. I finally gave up on my diagnostic skills and took it to the Subaru.

 

They fixed it with no replacement parts. They said they had to 're-teach it to idle'. I asked how they did this and they said something about running at idle for 15 minutes, running at idle with AC for 5 minutes, then running on the interstate at 65 for 5 minutes. They also said, they had no idea why it happened or why it was necessary to do this. They said it should have figured it out on it's own.

 

Now, according to Subaru, this 'teaching' is done to the main computer - ECM. However, this problem was occurring for 4 weeks and 1 week ago, they re-flashed the ECM due to some emissions control recall (unrelated). I was really hoping that this flash would have fixed the issue but it didn't. The Subaru dealer said they thought it should have.

 

My job is embedded programming, so I'm a bit baffled by the whole thing. Why wouldn't the default settings on a freshly flashed ECM run correctly? Why does the car have to be 'taught' anything? Why wouldn't reflashing the ECM fix the issue (the only reason I can think of is if it reads some saved parameters, flashes the chip, and then re-writes the saved parameters)?

 

Lastly, in case it happens again, does anyone have any detailed steps on what the real procedure was to re-teach the car to idle? -- I wasn't able to get any details out of the dealership. I have a shop manual, but it was sending me off on a wild goose chase for things that didn't even make sense.

 

Hopefully, this will help someone else in case they have a similar situation.

 

Thank you very much.

Dustin

Edited August 9, 2012 by hayedid
clarification

[jarl]

Most probably the new firmware doesn't overwrite a memory area, or some parameters are stored outside whatever IC they flash.

 

It would be extremely fun to do some reverse engineering