i own a '88 S12 XE with a KA24E swap and i've been having some issues lately. about 2 weeks ago i tried to swap the top part of the intake manfold for a nice powdercoated one i had laying around but quit about halfway though because i couldnt get some of the lines off and i didnt want to have to break or cut anything so i just reassembled everything and i think i got it all right. but since then, the car hasnt idled right. when i start it up when it's cold, it idles at around 300rpm, bearly running unless u give it some gas but after about 15 minutes of driving, the rpms creap up just over 1500rpm and i cant figure out why. also, when i'm at a certain spot on the gas, the car will buck, not real bad but enough that the shifter moves pretty violently and it feels almost as if it would if you were out of gas. i'm gonna guess both of these problems have something to do with something i messed up while i put the manifold back on. anyone have any ideas as to what i could have messed up? any help would be greatly appreciated. thanks in advance.

Check your vacuum lines

yeah sounds like a bad vacuum leak to me to

well i went to check out the issue a little on the KA and after poking around a little bit, i noticed something. as soon as i moved the intake pipe, the RPMs dropped instantly from 1300 to about 500. so i figured since the maf was the only thing that really moves when i picked up the intake that it could be the issue so i swapped that out for my spare and there was no difference, i screwed with the intake temp sensor and there was no difference. i'm kind of stuck.

Did you replace your intake manifold gasket?

check for codes on the ecu

its was never bad enough to bother me too much while driving so i've just been driving on it and as time goes on its slowly gotten better. when i start the car i gotta keep my foot on the gas a little bit to keep the RPMs at about 1000 for maybe 10 seconds before it can sustain a solid idle then after driving for over 20 minutes, it will idle just over 1000 vs the normal 800 under 20 minutes of driving. i dont know, nothing bad has come from it, i'm getting solid gas mileage so it hasnt bothered me too much.

i would definately have to say that you need to replace your intake manifold gasket, even though it seems to be sealed it wont be...it will make your car run like crap because the bad seal form the manifold to the head will act as a big ass vaccum leak, so try getting a new one the part number for the upper plenum is 14032-40F10 and costs like $5, and the part number for the other one is 14035-40F10 and costs like $8 so try that out, btw i know this sounds like a commercial or somthing but its not healthy for your engine to run with big vaccum leaks