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.

 

Shamelessly bumping this. Anyone have any ideas at all? Please?

 

i have the same problem too , somone said the pvc valve , i tired cleaning it but didn't "replace" it, but it still does it

 

I tell you what my buddies civic was doing the exact same thing. Was your speedo possibly jumping and lights dimming. I mean the speedo would go from 20 to 80 in like .3 seconds. The idle was jacked up. It was his wiring harness where it goes thru the fire wall was short circuiting and causing bad grounds also!

 

Well, I figure by this point is has to be electrical. I've replaced the IACV and MAP sensor, bypassed the fast idle thermo valve, and replaced the PCV valve.

Other possibilities I've considered are vacuum leaks and a leaking intake manifold gasket. But like I said, it's running rich with the MAP sensor disconnected. A manifold gasket leak should make it run lean shouldn't it? And if it's a vacuum leak why would the idle problem disappear when I remove the vacuum line from the sensor?

Now... I DO have an A/C problem. Can't turn the air conditioner off (even with the switch off it is blowing ice cold air), and the A/C is icing over (low freon undoubtedly). But I don't see how the A/C problem could be connected to the idle....

 

It could be bad ground wires.

 

i agree, try your ground

 

So where would I look to find the ground or test it?

 

check your EGR Valve, Fast idle valve, and (don't laugh) your idle set screw.

brad

 

I've bypassed the fast idle valve, and the idle set screw is screwed all the way in. Not sure what or where the EGR valve is, sorry. None of those should explain the change in behavior of the idle with/without vacuum running to the MAP sensor though.