Odd idle, humming sound and slipping OH MY..

[Ty]

Stay tuned, kind of long...

Up front, I don't know much about my 92 S13, even what the stock engine is, just that it says twin cam on it :P. I came from the mitsu world after a few dozen 4g 6 bolt builds, but I've fallen in love with this car.

--to the chase--

Idle -When clutching or putting the transmission in neutral, the RPMs drop to about 200-400 with a rough shake. This is the case all the way through breaking to a full stop. Soon as I stop completely, the idle pops up to ~750-850. I've been reading around the forums and have seen some problems like this, only it smoothed out eventually. This one doesn't. Every time without fail it drops like that then jumps up. Racing it to 2500 and holding, the engine seems to sputter a little bit.

What I've tried so far - Replaced Mass Airflow Sensor. -- unfortunately that's it, as it seems to start raining every time I go looking for the Idle Air Adjustment unit. Is the IAA attached directly to the throttle body? I couldn't see it, under it? On other threads people have seen success with replacing the EGR system and temperature sensor.

Humming - Regular driving, humming gets prettly loud faster I go. Sounds like coming from rear. Dear god I hope it's just a bearing. Stops immediately when I clutch so I'm not thinking transmission right yet.

Slipping - Had to drive down to the Valley Area today (i'm in SoCal) and traffic picked up to about 80MPH. As I'm cruisin along my *** end kind of slips a little bit. Dry weather, highway road isn't the best, but it's a quick slip needing slight correction on steering. - Wondering about the rear end, do the 96+ models' rear fit/bolt-up to the '92 S13? If they do, is there any benefit, or is there a year I should stay away from if I'm going to end up replacing everything in the back?

ANY/ALL input is welcome.

[corn322]

the stock engine is the KA24DE. 160 lb-ft, 155 hp. Not much topend, most of your torque is between 3k to 5500 rpm, then it falls flat on its face around 6k.

the idle adjustment screw is on the back of the intake plenum, right next to the firewall. but I don't think that's the problem, because your rpm's come back up to normal.

[Ty]

Found it. Thanks for that. It stopped raining for 5 minutes, long enough to check it out. I'm gonna pop it off and clean it out soon as this rain passes.

[Ubernoober]

With the car at a complete stop and the transmission in neutral push and release the brake. Does the idle change? If so you have a leaky brake booster. Technically, a good booster will be able to apply boost to the pedal for at least 10 seconds after the engine is shut off. If you press and hold the brake then while holding shut the engine off, the pedal should not begin to "rise" for at least 10 seconds. Otherwise... bad booster.

As for the rear end, if it stepped out on you and you were travelling in a straight line, I would make ABSOLUTELY sure that everything is together and where it is supposed to be. That is %100 not a normal or desirable thing. Look at all the control arms. Check all the bushings. Examine your brakes while you are under there.

[Ty]

I'll check the booster tonight when I'm off work, thanks for the heads up. I just did my brakes a few days ago, the arms are good. I'm also going to lift it tomorrow hopefully and check bushings and all that fun stuff. Any idea how to check the bearing without taking it apart? I have a feeling that's where the humming is coming from.

[Ty]

Checked the brake booster. At idle when the brake is pressed, the RPMs dip by about 200-300rpm. I've got another one on the way, goes in tomorrow.