What year is my motor?
 

I bought a 1990 240sx. The guy I bought it from got it out of the junkyard. I know from the vin it is a 1990. Now the weird part is that it has a ka24de in it. I also own a 1991 240sx with the original motor in it. I noticed that the valve covers are different, making me think the motor in the '90 is from a 1994-1999 (s14) 240sx. Does anybody know of a way I can tell for sure what year the motor is.

How are the valve covers different? They shouldn't be.

'91 to '94 Valve cover gasket.
http://i64.photobucket.com/albums/h1..._vc_9194_2.jpg
Notice it has 3 center bolts next to the spark plug holes.

'95 to '99 Valve cover gasket.
http://i64.photobucket.com/albums/h1..._vc_9599_2.jpg
Notice it only has 2 center bolt holes next to the spark plug holes, but the rear of the perimeter gasket has a third hole between the end humps.