I just replace a fuel injector on a 90 Nissan Maxima, now it is exhaust is smoking. What can possibly be wrong?

P0150 - O2 Sensor Circuit Malfunction Bank 2 Sensor 1 Possible causes:
Faulty Front Heated Oxygen Sensor Bank 2
Front Heated Oxygen Sensor Bank 2 - harness is open or shorted
Front Heated Oxygen Sensor Bank 2 - faulty electrical connection
Fuel pressure not right
Faulty fuel injectors
Intake leaks
Exhaust leaks

Read more: https://www.autocodes.com/p0150.html

There are times when you are working on one fault another totally unrelated fault occurs...Without knowing anymore than what you have expressed It sounds like the PCV valve is plugged or in replacing the injectors you accidentally disconnected the pcv vacuum line...saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

Are you sure it's fuel? Not likely,if you were using excess fuel there
would be alot of black smoke coming from tailpipe. It could be water
as condensation, which is normal. However if you are getting alot of
black smoke, the first thing I would check is fuel pressure to detemine
if the regulator is defective.

of course it won't
black smoke is excessive fuel so the place to fix is the injectors or air filter

black smoke is coming from exhaust possible causes--- excess fuel enters cylinders and does not burn completely. poor fuel milage is also a common complaint, when black smoke comes out tailpipe its running to rich!! This can be caused by anything from a computer malfunction to a leaking fuel injector. A possible fix would possibly be a tune up or carburetor/fuel injection work. run some good injector cleaner in it, it may fix the problem. but if not check it out further. good-day !!

It is possible that the two spark plugs could have become fuel fouled and not respond. A quick way to tell is to remove the suspected spark plugs and switch them with known good spark plugs in the engine and see if the problem follows the spark plugs. If the problem doesn't more from those cylinders then you may need to investigate the injectors (even though they are new) as being on the shelf a long time and operating incorrectly. Not opening enough and doesn't make the cylinder operate therefore cause fuel to enter the exhaust.

probably just your fuel filter. if that doesn't do it , get back to me.

The oxygen sensors in the exhaust pipe are saying the fuel/air ratio is too lean. This commonly happens when the computer in the car reached it's limit putting out the most fuel. Therefore, the mix ratio is still lean.
The possible problem maybe:
The fuel pump not puting out enough pressure.
The fuel filter maybe clogged
The fuel injector maybe clogged or needs to be cleaned.

Were there anymore trouble codes besides the two from Autozone?

Rich fuel=black smoke.
Oil burning=blue smoke
Coolant burning=white smoke

check your injectors.use a ohm meter and check resistance. 1987 maxima reading is somewhere between 10.1-14.2 ohms. if out of spec, replace. Also check your air flow meter and air box, make sure its all clean in there. good luck. post results.