Hi,
I have a 2000 nissan maxima. The wheel was hit on the driver's side. Replaced the strut on the driver's side. The used rack and pinion from a junkyard was put on car. However, the steering wheel won't stay straight. The mechanic said he set it straight , but it keeps popping out. When it pops out the steering wheel is vertical instead of horizontal. Do you think I have a bad rack and pinion? Any other suggestions as to why my steering wheel won't stay straight? I want to take the car in for an alignment, but no it's no point until that is fixed. Thanks.

 

There is some bushing in the middle of the rack. Check if they are still there. Without those bushings, the steering wheel won't go back in place (straight) and alignment will never stay straight.
Other things such as, outer tie rods, sway bar end links, Lower Control Arms, etc, that will prevent a straight alignment.