We replaced the leaking rad in my buddy's 01 4-cyl recently and the tranny has been shifting hard ever since. By hard, I mean the car noticeably jerks when shifting in all gears. It sometimes shifts fine, which is weird. The hard shifting is more apparent when it's hotter out and after extended highway driving.

All the tranny cooler and rad hoses are nice and tight with no leaks. Clamps are fine. ATF is at the proper level and drain/filled with DW-1 last year. The tranny shifted perfectly fine before the rad replacement.

 

Did you correctly re-connect the trans cooler lines; inlet and return?

Have you checked ATF level? You probably lost some ATF when trans cooler lines were removed.

Is Check Engine Light On? If yes, get codes.

good luck