What sensors determine base a/f ratio?

[DanDrath]

Well i have been having issues with running super rich for a while.I finally got to plug my z into my friends Nissan data scan, everything seemed ok except the base a.f ration was at like the highest menu option, we made it as low as it could go on the options and drove around plugged in. The car still ran rich,but not as bad, but ran 10000 times better, spooled faster and was in general much smoother.
What would cause this problem? All injectors are in spec and are not leaking btw. Could this be an o2 sensor issue? Thanks so much i hope you guys can help!

p.s i posted here for more views, i hope someone can help me! thanks Its a 1990 300zx tt btw
~Dan

[asoomal]

Is the O2 sensor "gauge" sweeping back and forth when running?

[alms24sebring]

Im thinking a bad O2 could be a problem, but hard to say

[OriginalWheelman]

If it's a KA or a VG, it probably needs an 02 sensor. They always seem to.

[asoomal]

Weird, you can pretty much run them without one. I always thought the early computers never really took a look at the O2 sensors :S

[DanDrath]

its a vg30dett the o2s were reading 0 at idle and sometimes reading 0 during throttle and then they would have a reading. Could they be fouled out? but would that cause me to run a 10.9 afr at idle!?

[asoomal]

Hmm, when under throttle, it should be going from .1ish volts to 0.9ish volts.

Sounds like an O2 to me.

[Dittoz7]

O2 or MAF.

[frapjap]

Grab a buddy's MAF.

[alms24sebring]

I mean if the O2 is reading 0.00 most of the time, I would figure It was bad.

[Speedy7_7]

I would say that your o2 is bad. But I would unplug your Maf and see how it runs unplugged. If that doesn't change anything, you have a bad maf.
Sorry, this is really general, but is this obd1 or obd2?

Does it run in closed loop or open loop? Is your o2 sensor hooked to anything other than the gauge i.e. is it linked to the ECU?
I wouldn't peg the cause of the problems on the o2, although the problems may have ruined your o2. It could very well be a Maf problem. I would check there, when you had it on the scanner, was it throwing any codes?
Do you have a boost leak? A post Maf boost leak would cause the car to run very rich.

hopefully you figure it out, I would look for leaks and play with the Maf, the best thing would be to replace it with a known good Maf to determine if that is the problem.

[4cefed]

Speaking from general knowlege on turbo fuel injection systems... O2, MAF/MAP, and coolant temp sensors generally affect A/F ratio. If switching out your MAF has no effect, try a new temp sensor.

[asoomal]

Wouldn't that cause a different temp reading to show up on Nissan Datascan?