This problem just started today, never happened before.

I have a 99 civic ex with the d16y8, and when I let off the brake pedal, the idle speed goes up from about 1000 to about 1500, and if I pump the pedal it climbs all the way up to 2k.

Some background:
This is my third post here, as I've been trying to resolve a rich condition/high idle. I thought I had it figured out when I disabled the FIAC (makes the car idle fast when cold, similar to the thermo valve in older civics). Started it up after that and did the re-learn procedure and it ran great for about 30 mins. After I took it for a short drive, the light came back on and now I'm getting a code 10, as opposed to the 45 I had before. Incidentally, both indicate a rich or lean condition. It smells like raw gas all the time andI'm about 100% sure there's no random misfires, so that leaves it with a rich condition.
Unfourtunately, I don't have a scope, only a multimeter, so pulling O2 voltage is completely useless.
Next, I cleaned out the IAC, which was pretty dirty, but didn't seem excessive. Anyway, that didn't solve anything. I still get the code 10. Though I'm curious what happened to the 45...
Just as an aside, I also checked the IAT and ECT sensors and they're both testing perfect according to that graph of resistance vs temp that's floating around the internet.
Vacuum leaks; I've tried pinching off every line in turn to see if anything changes, as this should indicate a leak. At one point the line leading to the purge solenoid caused it to change, but it's not doing that anymore, so I may have just pinched that one off while the solenoid was open.

I think that covers everything I've checked so far, and I'd love to hear some more input.

 

Mine drops idle when pedal is pressed & idles out at times. I think its an electrical issue, possible ground in my case.

 

vac leak... u got power breaks?if so cheack for fluid on outside of boooster (eattin away paint) if so u got a LEAK IN THERE. which can also be a source of the vac leak

 

Have you cleared the ECU to see whether code 10 returns? Could be a bad check valve in the vacuum hose leading to the brake booster.

 

Maybe it's just me but any car with power brakes draws vacuum when the pedal it pushed and naturally makes the idle rise