P0505 Code

I'm having an issue with my Prius and was hoping to get some advice from the members here. I have read through the threads, but haven't seen anything exactly like this.

I have a 2004 Prius with 196k on it. Over the past 3 months, I have had the P0505 CEL come on about 3 times. After looking over everything and not noticing it running differently, I have cleared the code and kept going. The CEL stays off for a 3-4 weeks, then comes back on.

Yesterday, the light came on and was accompanied by a rougher idle. I checked the code and the only one shown is P0505 reads as Idle Air Control System. I removed the airbox and cleaned the MAF sensor with MAF spray and cleaned the throttle body valve with throttle body cleaner spray. They were both dirty, but didn't seem too bad.

After I replaced everything, I started the car back up and now the engine idle pulses between 1600 to 1800 rpm over and over while it is in the open loop state. If the car is cold, it idles correctly for about 10-15 sec during intial warmup, then starts pulsing the rpm. When it goes to closed loop during idle or when it is in drive but not moving, but the brake is on, it idles high at about 1600 rpm then drops back to about 1000rpm. When driving, if the gas pedal is released, then the idles drops from 1800 down to 1000 rpm over the span of 10 sec, and then stays low.

Things I have done:
-Replaced the MAF.
-Uninstalled and reinstalled the airbox to make sure the surrounding hoses were good.
-Removed the PCV hose from throttle body(engine stalled out), then plugged the throttle body hole and the engine was still pulsing rpm.
-Checked the movement of the throttle body valve visually.(~20% at startup, ~16% at idle).
-Throttle position as read from the OBD II is not moving as the engine rpm is pulsing.
-IAT, Coolant temp, and O2 sensor appear to have normal readings.

Things I will do next:
-Evap system to see if there is a leak.
-Check vacuum hoses again.
-Replace PCV.

Anyone run into this problem before or have any suggestions on what to check?

 

Thanks for the suggestion S Keith, I just went out, unplugged the battery and it appears to be idling and running great after taking it for a short drive. Sounds like that is what it needed after replacing the MAF and cleaning the throttle body. I will report back if it goes back to a rough idle.