I have a P0505 Iacv code and have cleared the code once and now it is back. I reved the car up to 3000 rpms until the fans came on, unplugged the IACV valve at idle and the car seems to drop in rpms but doesn't die. The car seems to rev from about 500 rpms to 1500 rpms while idling. It only does this when the car is warmed up and it also does this in 1st and 2nd gear up to about 45 mph. It seems to run fine after you get into third , fourth and fifth gear. No other codes are present so should I replace it? Any help would be greatly appreciated.

 

What year Civic and engine do you have?

 

Sorry[]Its a 1999 Honda Civic 2 DR EX with 5 speed trans. and from the best of my knowledge the motor is a 1.6 (D16Y8)

 

The troubleshooting diagrams should help you figure out the problem. Given that the idle speed decreased after disconnection of the IACV, the problem may be either dirty ports in the throttle body or a vacuum leak.