Mitsubishi Repair: 97 gst stalling when stoping, kinda, air fuel ratio, crankcase pressure

Question
I have a 1997 Mitsubishi Eclipse, 2.0L, GS-T, manual and am having some trouble with stalling.  It's hard to describe but I'll do my best.  The trouble is when i shift or go to stop my car wants to stall, most of the time just barely not stalling.  I carefully watch the RPM gauge just to see if it has gotten better / worse after minor things I might try, but it seems to just drop, hit prolly 100 or 200 rpms then raises back up to 600 or 700 and idles fine, uneless it stalls.  I haven't tried anything really since I'm not sure where to start.  The condition of the car is that it has a rebuilt engine (maybe 2k miles on it, if that), aftermarket fuel pump, BOV, full 3" exhaust, K&N air filter. boost gauge and air / fuel ratio gauge.  I kinda tapped the A/F gauge directly to a line in the front air sensor I'm not sure if that has anything to do with it or not.  I honistly can't remember if this was happening when I first got the car or not but any advice would be appreiciated.                                                                   Thanks, -Dan

Answer
Dan,
Some of the upgrades you performed may have affected your idle, or just made it more noticeable.  You should have your timing checked, along with timing belt. If your timing belt is not set correctly, your engine will idle rough and stall at times. Also get a compression check done as well. A quick fix could simply be to replace your PCV valve. A little valve that releases excess crankcase pressure. Very inexpensive and easy to replace.

Check your Idle Air Controller (IAC).  It's sole purpose is to regulate your idle speed.  If your IAC is malfunctioning, then your idle will bounce around in a condition referred to as idle surge.  If you have a shop manual for your car, there will be a procedure to check it.  On top of your throttle body is your Base Idle Setting Screw (BISS).  This is used to set your idle speed.  In a perfect world, this should never need to be adjusted or touched.  But sometimes, the rubber o-rings around the BISS dry out and start leaking air.  That unmeasured air that leaks out can cause rough idle and stalling is severe enough.

There is another  component located just below your throttle body called you Fast Idle Air Valve (FIAV). It's purpose is to provide additional air when the engine is cold to increase the idle speed and therefore warm up the engine faster. The FIAV is the reason (the only reason) that coolant flows through the throttle body. If your rough idle/stalling problem typically happens after you engine has warmed up, you may want to have your FIAV checked out.

Other areas to check are your fuel and ignition systems. Have your fuel pump checked and your fuel filter replaced. A clogged fuel filter will reduce the amount of fuel entering your engine and cause poor idling and possible stalling. Your ignition system is basically made up of your alternator, ignition coil pack, spark plugs and spark plug wires. You'll want those items inspected and replaced if necessary. You spark plugs and plug wires should regularly be replaced every 30,000 miles or so.

As you can see, diagnosing a rough idle and stalling problem is not easy. There can be several causes, and every mechanic and person with some knowledge of cars (like me) might have different views on areas to check. Next mechanic you bring your car to, give them the list of items I've recommended to have checked (unless you perform them yourself), and then you'll know what worked or what can be ruled out.
Good luck!