Checking O2 Sensor

[Pacman]

Ok, I don't know if its my fuel gauge, my sending unit, or if its actually the O2 sensor acting up again. Can someone tell me how to check it, or where to look in the FSM cause I'm not seeing how to test it. I notice that the gauge fluctuates on different hill inclines/declines. I also notice that it seems like the first half of the tank drops fast and the second half (or quarter tank cause I fill up at 1/4 tank) gets used up slower (but still fast). There is no visual leaks around the injectors, lines, or tank. The engine is in good condition, newer NGK plugs, year old wires, dist. cap, and rotor, 6 month old O2 job by professional (I screwed in up by cutting the OEM connector; O2 wired right up on harness), 6 month old Cat. Anyone have anything similar happening?

EDIT: The exhaust isn't grounded, can that cause anything bad (like O2 sensor malfunction, cat not working right, higher emssions)?

[idlafie]

UUUHhhhhhh.....what year is your car again??

ID

[Pacman]

Its a 1991. The computer gives me code 55 which means everything is ok. I spent $400 on repairs for smog and to get the O2 sensor working correctly in December so it better not be that...

[idlafie]

So what's the problem? Your gas mileage isn't what it used to be? You think your 02 sensor may have gone bad?

To check your 02 sensor, click on the attached link:
http://www.240sx.org/faq/index.html
After the page loads, scroll down the links to the left till you get to 02 sensor. Click on that link. It will tell you how to put your ECU into test mode so you can check your 02 sensor.

If your 02 Sensor checks out okay, pull your spark plugs. See if they have any excess carbon buildup.
http://www.eric-gorr.com/techarticles/sparkplugs.html
http://www.buckeye-illinois.com/sparkplugs.htm

If your plugs are dirty, then you might want to check your Fuel Pressure Regulator and the vacuum line leading to it, your Engine Coolant Temperature Sensor, your Idle Setting and your wiring harness leading to these sensors. One of them could have gone bad causing your poor gas mileage.

Fuel gauges and sender units seldomly go bad, but you never know....

ID

[Pacman]

NICO FAQ... Frogot about that!

The can't tell the gas mileage, the odometer works intermittetly (sp?). Anyway, the regulateo is good, as well as that vacuum lines running back there, plugs are newer, but old ones looked normal. The coolant sensor shows the engine at normal temperature (half way on the gauge) so I doubt thats the problem. I know that 240's sending units go out/work not 100% on these cars so I was wondering if thats the case. When I get my new cluster in (tomorrow) I'll check the sending unit like it says in the FSM. I'll check the O2 also. Thanks