My name is Matt and I've been stalking this forum as a guest for over a year reading the stories of other 240sx owners and the issues with their cars. It seems one of the majors with these cars is the dreaded hesitation with these KA24DEs. I've owned my 240 for about nine months now and it has been plagued with hesitation between 2-3k RPMs. I've been a car enthusiast ever since I can remember and love working on my cars and more then anything; fixing the problems that bother me the most. I'm in the US Air Force as an F-16 maintainer and turn wrenches on 17$ million aircraft as a daily and could not be happier. Much of my mechanic experience is to be credited to my older brother and my work discipline has trickled to my own personal discipline with my vehicles.
THE PROBLEM
My 1993 240sx has the stock KA24DE ORIGINAL engine with 314k miles. Its my daily driver and more then anything did that hesitation put pain in my heart. Under any sort of load, especially in 1st and 2nd gear between 2-3k RPMs, the tired old motor would hesitate, misfire, buck, and chug until it caught itself around 2800 RPMs. I began diagnosing my 240s sickness about 7 months ago.
THE MAINTENANCE
As it seemed my symptoms were most closely related to a misfire, I began by replacing parts in the ignition system. Plugs, Wires, Rotor, and Cap. "That should do it!", I thought to myself as I fired her back up and took her down the street. I bring her up to 3k and shift to second. [Fart noise] There's that hesitation! So ignition was not my problem. "Next paycheck", I figured. So I find myself stalking these forums trying to dig up some info and I stumbled across another thread on hesitation. Symptoms were similar but not spot on. "What the heck, I'll see". So I walk out to my 240sx and pop the hood. I gaze at the motor and begin inspecting wires. I found the distributor harness chafing on the side of the block! I quickly pulled back the shredded plastic covering to find, the wires were still in tact. "Damn". I tape up the wires and reroute the harness with some zipties to avoid the chafing. I decided maybe I should check the codes on my ECU; Just in case. As luck would know it, I had 2 codes; 33 Oxygen Sensor Circuit and 34 Knock Sensor. I was shocked as my car did not have a CEL. Upon some reading it seems the non-california 240sx will not throw a CEL for either of those codes. Hmmm. I began reading on the knock sensor and its function. The knock sensor is designed to pull timing as it "hears" knock. As I sat there listening to my tired old motor's obnoxious timing chain rattle about, I began thinking again. "False knock?" If the timing chain was causing false knock, it was force the ecu to retard the timing, causing hesitation or sluggishness. "Well unplug it..." is what my brother told me. "It'll throw a code but it won't pull timing anymore". Spot on he was. I unplugged it and holy cow. Up top the tired old KA must have put its vtec sticker on and gained 40 HP [or so it felt] up top! My motor was no longer pulling timing so it was running way better! As I drive back home I stop at the stop sign and drive with my lightfoot up to 3k in first and shift. [Fart noise] AGAIN with the hesitation?! "No way" I thought to myself. Defeated, I put her to bed. I come out to her after a few days and pop the hood. Start the motor and begin pondering again. I read through the FSM and vacuum leaks seem to cause some hefty things. So I pinch the blow by hose and the motor speed DOES NOT increase. So, assuming the FSM [no leaks if the speed doesn't increase] is right, I move onto other things. TPS. I read in the FSM to test the TPS on a hunch that if the TPS is reading an incorrect signal, it would tell the ECU a different load setting to run. I pulled the plug on it and grabbed my multimeter. As per FSM I checked resistance and got a reading OPPOSITE of what the FSM states! "Ah HAH!" I was proud about that one. I read more about the TPS and for giggles googled the TPS reading opposite what the FSM states. Turns out its not uncommon, in fact, the FSM tells you the wrong terminals to read. Once again, defeated. TPS checks good as I measure output voltage and make some adjustments. "Ughhhh..." I tell my 240 good night and sleep it off. I had read several places to look for hesitation on countless forums and I had always absentmindedly skipped over one thing that was mentioned. EGR. "No one likes that thing anyway". Feeling as if I had no where else to go. I began troubleshooting my EGR. I start the 18 year old motor and pop the hood. -Hissssssssssssssss- I heard what sounded like a vacuum leak. I put my ear closer to the back of the motor and it was a CLEAR vacuum leak coming from the EGR components! "Bingo" I was onto a new lead. I drove to the auto parts store and picked up some carb cleaner, some vacuum hose, and a rockstar. So I begin pulling vacuum lines, one by one, and got all RX7 on it [FD joke I guess. Excuse my humor
]. I fire her back up after tearing my hands up replacing all the brittle old cracked vacuum lines behind the motor. My gosh she idled smooth. I revved her and she jumped right back to idle without hunting. "Awesome!" I said outloud. I return to the area where I replaced all the hoses. -Hisssssssss- 
D: I couldn't believe it! It was still leaking! I shut her off and sat down. Frustrated 
I grab the FSM and begin looking at EGR troubleshooting. So I start pulling hoses and blowing and sucking on stuff 
LOL Until I got to it. That damn BPT valve! I pulled it off and couldn't believe it. As per FSM, blocking off both ports or even one should yield a closed situation [you shouldn't be able to suck on it]. No matter what hole I closed and what holes I "Vacuum tested lets say", I was still able to pull vacuum through it! "BAM!" My head started reeling.THE THEORY
The EGR system, aka Exhaust Gas Recirculation, does exactly what it says; it recirculates expelled gases from the engine to the intake. Now it may seem like a silly thing and "Oh its only for emissions so it won't do anything bad" is NOT the way to think about this IMPORTANT system! The purpose of the exhaust gas being recirculated is to lower combustion temps. But only in certain ranges and under certain vacuum as calculated by the ECU and executed through the PAIRC Valve. Now with this BPT valve always letting vacuum through, it was causing my EGR valve to stay open under all circumstances! Exhaust gas was constantly being recycled through my intake, changing the proper air/fuel mixture read by the o2 Sensor [Code 33 remember?] and because the Air and Fuel mixture was incorrect, misfire was occuring creating KNOCK [code 34!].
CONCLUSION AND THE FIX
Replacement of the BPT valve with a new one from Nissan [$80] caused my EGR to properly function and properly recirculate exhaust to lower my combustion temps efficiently.
MORAL OF THE STORY
When you're diagnosing your car, don't underestimate the power of one system and write off its effects as useless. I felt the EGR could never cause my hesitation, and its where I should have started from the very beginning with just a lil bit of sucking and blowing.
~Matt
P.S. If any of the information I may have provided regarding theory is incorrect. Please feel free to comment and I will edit the OP to be correct.