1990 SE Maxima (so the VG30E) Auto

-How many O2 sensors do I have?
-How do I test to see if a Fuel Injector is good?
-How do I test to see if an O2 sensor is good?
-Exhaust studs/gaskets/spark plugs/engine were just replaced 12,000 miles ago, so any other ideas on what is making me get ~18 MPG when it is 80% light freeway driving at 2500rpm?

 

-1 O2 sensor
-if they're on the car and you wanna see if its misfiring, pull one sparkplug out at a time and see if engine gets shaky, if so its good and proceed with next. if engine idles same after pulling wire then thats bad.
if you wanna test injectors while out of you have access to them on, connect a multimeter and make sure they have between 10-14 ohms, no more and no less.
-I dont know how to test O2 but I should test mine some time

I know exactly how you feel on the MPG I'm kind of on the same boat, I'd say make sure you got at least 32PSI on all tires, fresh tune up, make sure timing is correct, coolant temp sensor and thermostat are good, a bottle of injector cleaner or seafoam in gas tank wont hurt, catalytic converter should be good, I wouldn't use cheap gas stations, check for vacuum leaks, I think fuel pressure regulator can affect mpg as well but I'll let someone else chime in with that. Also make sure you dont have a fuel leak or smell gas (which I do sometimes on my car) Its 5:30 am and I havent gone to sleep yet...cant think of anything else..sorry

You know, you got me thinking, everything has pretty much been replaced on my car except O2 sensor...I think I'm gonna work on that next and see if that helps me.

 

Check your timing too - sometimes really bad timing has the same symptoms as one injector malfunctioning

 

replacing the fuel filter wouldn't hurt...