hello, a couple months ago i bought a used 2000 honda civic ex.
would like to keep it running well for my son,

it developed a surging idle problem. i have taken it to 2 mechanics around here and hey have not been able to solve the issue.
I looked on line for solutions and of course the most talked about thing is IACv...........replaced valve and nothing. still idle surging and code P0505.
after changing my intake, checking TPS, ECT sensor, Map sensor and everything else i could think off I finally noticed what I now know is the Air Assist valve was not closing when up to temp.
I was so happy when i found information on it and of course my luck would be it is only on 99-00 cars and most people don't know about it,
ordered it, replaced it and crank that baby one more time ..........IDLE SURGE. WTF?????
did an idle relearn multiple times and about 20 different ways........... same crap.
what gets me is its almost instant. as soon as i turn the car on it goes into an iddle loop.........IAC opens surges up untill like 2k rpm then closes goes down to 1.5k rpm and wash rinse repeat...........it drives me crazy now.

if i unplug the iacv it purs like a kitten but as soon as i plug it in it goes buzurk.

does anybody have any suggestions? I'm out of ideas and can't find anything else to try.
Thanks so much in advance for your help.

 

Check that cooling system is full. Check in the radiator when cold, not just the white plastic tank.

If it is a manual car, the adjustment procedure is to warm up the engine, with IACV unplugged adjust the air bypass screw on the top of the throttle body for 450 rpm, i.e. barely running. If you cant get it to slow it down that much, find the air leak. Automatic cars with 3-wire IACV are not this simple but the overall concept is that the IACV can only let more air into the engine and speed it up. If it is getting too much air via some other path the ECU will be unable to slow it down.

 

I've checked and re-topped off the coolant a couple times, i did read a comment on it causing surging idle, not sure how.

when the IACV is unplugged i don't think i can get it down to 450rpm maybe 600ish is the lowest. i'll try again.
Also I've checked for leaks using the Carb cleaner/Throttle body spray method with no luck...........i've sprayed that stuff eveywhere and don't notice a vacuum leak anywhere.

Is blocking the Throttle body also a good way to check for leaks?
if it stays running it means it is getting air somewhere else if it dies where good?

 

Just had the same issue with mine only the idle would surge after it warmed up. Took the throttle body off then took the IAC off soaked it in (believe it or not) lemon juice of all things because it is a mild acid until the flap inside the IAC rotated with almost no resistance. Then cleaned all the passages with a good THROTTLE BODY CLEANER not carb cleaner put it all back together with new gaskets and now car idles great.

 

Is it manual or automatic transmission? The manual EX has a 2 wire IACV and the automatic EX three wire. The three wire can't be tested by unplugging. The two wire has a spring mechanism that will close it fully with no electricity applied.

There are holes in the side of the throttle body before the throttle plate. These go to the IACV, the cold idle valve, and the air bypass screw. If you block all three the engine should stall.

 

it is a manual EX, intake only has one hole, its one of those wonky air assist systems that controls the cold idle by letting in air by the injectors.

i had a friend smoke test for vaccum leaks and there are none.
i got an official honda iacv and still idle surging from 1500-2500.

im out of ideas.
would a bad oxygen sensor be able to cause this, does the ecu even take that into consideration when trying to keep idle?

 

OK you have a 2 wire IACV you should unplug it with the engine warmed up and idling, and confirm that it drops to much less than normal idle (450 rpm). If it does not, too much air is getting in somehow and the ECU will not be able to control the idle.