1999 civic lx a/t. D16y7. Only 78,000 original miles.
The problem: CEL keeps returning with 505 code. Can’t get the idle below 1000 rpm. No surging.
What I have done:
Replaced IACV with OEM Honda, new engine temp sensor. Cleaned throttle body. Replaced all vacuum hoses. Tested for vacuum leaks, none. Adjusted idle air screw. All the way seated it still idles at 1000. Performed countless idle relearn procedures. I can drive the car a 100 miles or more on the highway, it’s fine. Get in town with stop lights and it throws the code. Still idles fine at 1000. When accelerating it weirds out at 1600 rpm’s, then accelerates nicely after 1700rpms. The one thing that I have discovered is when idling with the air cleaner off, if I hold my finger over the IACV port, the idle will drop down to about 500 rpm. It’s the same whether the IACV terminal is connected or not.
Does that mean that my new valve is bad. Not closing?
I’m about to pull my hair out. Any ideas?
UPDATE: Nov 9, 2025. Problem solved.
Searched through my shop and found my original IAC valve and cleaned it twice with WD-40, carb cleaner, blow out with air. Car now runs like new. Idles at 700rpm. No thanks to this website. It’s pretty much useless. No reply’s to my post after two weeks.
Looking at other posts from the last year confirms that. I do hope that my post helps someone down the road.