P0507 code

[SouthernHoo]

2000 S40 1.9T 115k miles

Last week my S40 started having "fluctuations" in the RPMs as I drove around town. It would rev up or down about 200 rpm and then back to normal a couple of times during my 20 minute stop and go commute. This week the frequency increased to about once per minute. Then it began to dramatically increase at startup (up to 2500 - 3000 on the tach) and gave the P0507 code. I looked up the code (P0507 Idle Control System RPM Higher Than Expected) and searched this forum and others and last night I removed and cleaned the IAC. No improvement.

This morning I replaced the IAC (and gasket) with an OEM part. Still no improvement. In fact, it revs worse at startup and idle than it did before. It has not given any other codes and has only given the p0507 once.

Any ideas? Thanks in advance.

[SouthernHoo]

Okay...Tonight I removed the throttle body and cleaned it with throttle body cleaner. It was pretty gunked up. I also cleaned the Mass Air Flow sensor with MAF cleaner. That fixed the massive surge at startup/idle and it now idles normally at 700 - 800 rpms, but it still surges/pulses down then up 300 - 400 rpm while driving. What should I check/clean/replace next?

[zulu-v40]

I'm currently having the same problem on a 2003 v40 - same code, P0507. The tachometer dips about 200RPM while decelerating, then surges 200RPM as I'm braking. If I let off the accelerator while driving, the tach drops dramatically. I've ordered the new IAC. I have yet to clean the throttle body, but I did clean the MAF sensor.

[trickstar20]

Bump.

I have a 2004 s40 with p0507
Wondering if anyone has any advice?
I have already:
cleaned throttle body
Cleaned IAC valve
replaced plugs and coils
replaced battery
checked for vacuum leaks with a can of carb cleaner

it used to give me P0506 instead, it went away and now i have P0507...
not sure what the next step should be. Yes it is only code P0507 and it causes the car to die after a cold start and even at stop lights, signs, etc.