I have a 1999 Prelude (H22A4 I believe) with a Jackson Racing Supercharger (and a few other high performance mods.) I started having bad idle problems with it (the car) a few months ago. Prior to that, even with the aftermarket engine parts, it ran beautifully.

The idle jumps up and down, and when driving I get power surges. The ECU gave me a code for the IAC valve, which I have replaced. Same problem! Removing and cleaning the throttle body did not help. No codes now after replacing the IAC by the way.

Trying to figure out what could still be causing this problem, I disconnected the vacuum line from the supercharger to the MAP sensor. The idle immediately stabilized, though it is idling at around 850 RPM (too fast!)

Today I replaced the MAP sensor, but I still get the same problem. The idle jumps with the vacuum hose connected to the sensor, but when disconnected it runs smoothly, though at a couple hundred RPM too high.

I've tried connecting the MAP sensor straight in to the throttle body (i.e. the stock setup) but still get the idle jump.

My fuel/air ratio guage shows the engine running rich (and I get some black smoke out the exhaust as well). My guess there is that with the vacuum line disconnected the MAP sensor thinks the engine has just started and the ECU is running the engine rich to try to warm it up, even though it is at operating temperature.

I'd appreciate any help solving this problem. I've manually adjusted the idle as low as possible (still stuck at 850 RPM unfortunately), and that is liveable for the moment, but I'd really like to get things back to where they are supposed to be. Thanks for your time.