My engine blew in 1998 2wd blazer so to replace it I got one our of a 2001 4wd my question is on the back of the block behind the driver side head there is a part that screws into the block it is on both motors but the are different the 98 is bigger and has a round connector to it the 01 is smaller and has a more square connector. My question is first what is it. Second is it needed because they don't swap out I am trying to upload pics. Thanks

 

It is the knock sensor and that's been discussed on here many times. Drill and re-tap it to take the original one.

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