UPDATE: The hose that goes from the lower intake vacuum port to the PCV valve had crack in it from the UNDERSIDE. This is why it was hard to diagnose without pulling more items. After pulling the throttle body assembly and the EVAP hose (the upper one), I tried to remove the PCV hose. As soon as I yanked on it, it broke. Upon further inspection, it was very hard and brittle. The vacuum leak actually sucked part of the hose inward over time.


Let's see who is the best detective in this generation.

The suspect : 1999 Honda Civic LX - D16Y7 - Manual transmission

The crime : High Idle

The notes :

Purchased this car sight unseen. I have not had a D16 myself. I have had D17 and various accords F22,F23 etc.
The idle was high when I took it home. It would around 1500 at cold and when it warmed up would bounce going to 2000 and dropping.
Drives fine otherwise.

I did pull out the IAC valve and clean it. It was mildly dirty and a little bit sticky but not too bad. After cleaning you could move it smoothly.
As a test, when plugged in, it will go full closed. Started car, still high idle.
Unplugged the IAC, super high idle. When plugged in, less, but still high.

There is a CLEAR hissing noise from somewhere. Even with the stock airbox on and everything, you can hear SSSSSSS loudly from the intake area.

My suspects are the hose that goes to the PCV or the EVAP port. I couldn't really get to those. It should be easy to spot, I sprayed some carb cleaner around those areas which usually changes the idle but it did not.
Maybe the IAC is actually bad?
Otherwise, maybe a TB or intake manifold gasket?




If you can correctly find the problem, I will give you a cookie.