98 Nissan Pathfinder, LTFT stuck at 0%

Hey all,
1998 Nissan Pathfinder, 3.3L, 189,000 miles. LTFT stuck at zero for both banks...I've yet to see it move either while parked or during road test. STFT's are at about -20% for both banks at idle and get closer to zero under load at higher RPM's. Any idea what might be locking up the LTFT's? I was wondering if perhaps my scanner is just misnaming the pids and that perhaps Nissan's of this model year just have something like a "Total fuel trim" number that my scanner is translating as STFT with null values for LTFT.

But I don't have any means of knowing that for sure.
Any insight appreciated! Computer seems to be able to control fuel...front O2 is switching well...going rich under load, etc.
Thanks!
Rob

is it in open or closed loop and is there anything else out of place

Closed loop. Fuel control is happening for sure. In regard to other "out of place" stuff, I noticed preliminarily that the downstream O2 looks to be stuck low...and not responding....so I probably have a dead sensor or wiring issue there.. Just haven't looked into it yet because in my assumption has been that these rear o2's are not engaged in fuel control/fuel trims and are simply a catalyst monitor. But I could be wrong...anyone with some service data?
Rob

does it smell like it is rich and right down codes. disconnect the battery and connect a non led test light between the leads for like 10min. see then what the trim dose after a cleared memory.

No smell of fuel. Only code is for a small EVAP leak, which certainly could be messing with my trims (if the leak is in the purge valve), but not locking LTFT at 0%.
I looked up the official Nissan fuel control memory reset and performed it (involved warming up engine, unplugging MAF, setting code, clearing code, then running for 10 minutes). LTFT's still locked at 0% .

Any other insights? No real noticeable driveability symptoms.
Thanks,
Rob

Focus on what you know for now. Don't worry about the LTFT staying at zero.
At idle with your STFT at -20%, what does the O2 signals look like? Are they flat-line rich or moving rich/lean like they should be?

Front o2's look close to normal...though they aren't a nice sine wave....they swoop low and then spike high, swoop low, spike high...those higher voltage spikes (.7-.8v) are momentary. But it's definitely a repetitive pattern.

Both my rear o2's are reading very low....130 on bank 1, .030 (ish) on bank 2. Both of those spike high when I do a snap throttle...so they're definitely not dead. And they're not following the pattern of the upstreams at all.

I recognize there's a chance that all this is entirely acceptable behavior. When I was checking the trims today, the STFT's weren't quite as low...-13 on 1, -10 on the other.

what is your maf/map like, is oil over full and smell like fuel, and is your idle steady

oh and do you have a exhaust leak before the rear o2s

1mouse3 wrote: what is your maf/map like, is oil over full and smell like fuel, and is your idle steady

oh and do you have a exhaust leak before the rear o2s

I always forget about oil burning, thanks for bringing up oil.
As far as exhaust leaks upstream of the O2, this would drive the fuel trims positive and he has a negative (rich exhaust) issue.

Something else to the OP. A headgasket failure can also cause your issue. It contaminated the O2s and they will give a false rich signal and negative trim numbers.
Are you losing coolant?

I not sure on the nissans pre wideband use, I know the the widebands ones only relied on the front sensors. that made me think could a leak between front and rear be a cause of a low reading on the rear o2s but may be off on my thoughts

Not losing coolant as far as I can tell. Rad is full to the top and I haven't touched that since getting my hands on this vehicle.

Re: contaminated o2's...is my thinking correct that if the front o2's WERE contaminated and reading a false rich, that if I could get the car into open loop with the rear sensors warmed up fully, they should jump up and read a more normal voltage?

I'd imagine that running the truck with the MAF unplugged would put it in open loop, correct?

Is my thinking here accurate?

What's the logic with oil burning causing this type of symptom?

Thanks for your help and patience!
Rob