90-99cherokee

this jeep i just got has a 90 body and a "99 engine and tranny"

first off how can i really tell what year the engine is? are there any numbers on it that will help me verify? the guy did the swap but doesnt have any papers on it....also if he did indeed put the 99 4.0 in...does he have to use the 99 computer or is it alright to use the existing 90 computer...the reason i would really like to know is because when i attempt to start the jeep it turns over a little longer then normal and idles a little wierd and i noticed on some videos on the internet with other newer engines in older jeeps they do the same thing....should i be looking for a 99 computer?

is it possible to use a programer in the 99 engines? i read people getting way better fuel mileage with a throttle body spacer cold air intake and a programer with a free flowing exaust....makes sense as the engine can now breath a little better.

sorry to spill my life story...just want my beloved 4.0 running like a top

jeep is a 1990 cherokee 4x4 auto 4 door with 4.5 inch lift with a " 1999 4.0 engine and trans"

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