Idle problem, and engine swap......

[project350sx]

Hey guys, i have one quick question here. i recently had to re-weld my intake manifold due to a crack fuel rail mounting towers. (btw, its a '90 240sx with KA24E) but my problem is that after i put the repaired intake manifold back on my car, i've encountered a very low/bouncing idle. at apps 200 to 300 rpm. now the car WILL NOT stall. it wants to, but it won't. it'll just bounce it self to about 900 rpm normal idle again, and in about 15 seconds it will work its way back down to a 200 rpm idle, and kick it self back up again. it's just a repeat process. now theres a couple things to take into consideration when trying to diagnose this problem: 1) the car runs perfect under normal driving conditions, power is up to normal. and 2) i am experiencing normal fuel economy. i'm clueless as to what this could be??? and help would be appreciated.

next quick question. my buddy has a FWD SR20DE. what problems must i overcome to swap this into my 240 if i decide to purchase this engine and turbocharge it?

[[s3]]

vacuum leak

Interesting choice, US spec SR20DE and a turbo kit....not sure how that would bolt on to a RWD 240.