Engine weird a/f ratio, need help.

[DOOMMONKEY777]

hi and thank you for reading, now ive had this problem for a while and i cant figure it out. On my 2010 Z manual.

so the engine been leaning out to ~a/f 15.15 on both banks got highvoltage code for both banks, sensor2,

now typicaly you go check vacuum hoses, that go from bank to bank and the brake line cheked them seems to hold pressure.

but here is the kicker, the car starts and maintains a solid 14.7a/f ratio, i give it gas a bunch of times goes right back to 14.7, if i let it idle for like 5 min, a/f ratio goes rich to ~13.5, if i put load on the car, it starts to lean out to a/f15.15 all this is on both banks.

The map sensor at the intake manifold reads 1.34v @ idle to 2.45v as i rev the engine.

i cant figure out what is causing this weird behavior, cheked vacuum lines any help is appreciated.

[RadioFlyer]

Is the ecu stock or tuned? Also, what intakes are you on? If you're idling for 5 mins, your intake is probably picking up heatsoak from the engine, so you're getting into the different temp compensation fuel trim tables. So if any of your 'learned' fuel trims get applied to the rest of the tune, you end up with 'target of 14.7' but applying the learned fuel trim of something like -10% fuel, so it results in leaning out your closed loop afr to 15.X.

Disclaimer: I'm not super familiar with the architecture of the Nissan ECU, so I'm not sure if this applies. When I tuned my car with the UpRev Tuner cable, I didn't have access to the temp compensation tables. But in general, you want your closed loop corrections minimized or else you could potentially be applying corrections to your whole tune, which is what it sounds like is happening here.

[Wilson2608]

So ill try to help you out as much as i can. The reason you engine idles near 14.7 at first start up is because your car is in open loop( from my understanding) which means it isn't relying on the sensors to calculate the amount of fuel injected into the cylinder. Once warmed up, the vehicle is now in closed loop. This means the vehicles is relying on the sensors, and this is where LTFT (long term fuel trim) and STFT (Short term fuel trim) come into place. Your STFT takes immediate calculations of the AFR, but has no memory that it can store it into therefore it can only change real time AFR. So if I have a rich fuel problem, it will compensate. LTFT has a memory based on the results of STFT and can affect your AFR at all times during closed loop. The LTFT "remembers" the issues of the STFT and will always adjust for it. Anyways, what i'm trying to say is that if you have a intake leak, maf problems, etc your LTFT will compensate for it. Im not the best at explaining things, especially things like this lol so ill try to provide a brief summary of what i just said. The reason( i think) your car hits 13.5 afr after entering closed is because your Long Term Fuel Trim is telling the car "hey man theres a unknown air entering the engine somewhere, and i need to add more gas to compensate for this lean condition (intake leak)". Thus causing a rich condition. As for why the car goes to 15ish under load im not to sure. A vacuum leak becomes less of a issue under load. I hope i've helped, i can't tell you the exact problem but it's possible your tuner doesnt know what hes doing as well. (if the car is tuned) Do you have a aftermarket intake manifold? if both sensors are messed up it obviously has to be a problem with the air/ or gas entering BOTH sides of the car. SO the leak (if any leak) would have to be near the intake mani, not the intake piping itself.
Hope I could help you find the problem.

[DOOMMONKEY777]

thanks for the repy guys, now the car is fully stock, i had my own tune on it before the winter, thats when i started having problems, not that i put the tune at winter time i was running that tune for 3 years w/o problem, it was first bank 1 s2 that gave me problems, so i compensated adjusted my tune, but it kept getting worse as days whent by, its my dd. So i stored it for the winter cuz i cant work on my car outside in the winter. Now am trying to get it going and i cant find the problem, right now i am running stock tune, and Original Post is what i have, i also have the UpRev tuner version cable, after adjusting the tune i realized that it wasnt me messing with the engine thats gave me problems, but there is actually something wrong with it. Thats when i went backto stock tune. Right now i am not sure wich sensor is giving me this problem, i am gonna pick up an MAP sensor, brand new and try it if it isnt it, i dunno then.

why i am changi g the MAP sensor because it gave me 4.45v out of 5v when i turned on everything without starting the engine. Everything else seems to be working fine.

PS if you guys would like me to make a log, so you can take a look, just let me know, what you want to see and how you want it to be done.

again thanks for reply.

[RadioFlyer]

Great input, and important information to know. Just think out loud here... It's happening in your closed loop operation. Closed loop targets AFR off your front O2 sensors and MAF sensors. Have you cleaned your MAF sensors recently? Have you done all the resets (idle speed reset, throttle body reset, etc)? I agree with you that it's not a tune issue if it's doing this with all stock parts/stock tune. A vac leak wouldn't be my first thought if it starts fine, and slowly drifts off. So I'd look at sensors, and start with what's cheapest to change - cleaning the MAFs, running a bottle of injector cleaner, clean your PCVs, doing all the ECU resets... Last thing I would want to try would be the O2 sensors, they're pricey. But how many miles are on your O2 sensors?

[DOOMMONKEY777]

soo i finally got the MAP sensor and as soon as i put it in the engine vibrations became better, sound better, so i logged in to se what is happening, A/F ratio is nice and stable 14.7, BUT both of my O2 second sensors after CATs are both 2.49volts, after i reach engine temp above 180 all ok, until my bank 1 cat heats up and O2 sensor 2 starts working giving ~1.28v while the bank 2 O2 sensor 2 still gives me 2.49v, my A/F ratio goes immediately to 15.09 on both banks.

why do those sensors control my A/F???? arent they only there for EPA reasons??


that bank 2 O2 sensor is new and the car has 44k miles, am sure my cat is done for. il get back when i replace it.

[jchammond]

You mentioned it going over 15 under load; is this WOT ?
Cruising down the interstate,it's not un-common to see 15 or 15.5'ish for fuel milage,
but WOT should go down in the 13's & probably 12's@high rpm's if you're stock.
when my Z was stock,,,AFR went down to 12.2 on the dyno & after bolt-on's/still was in the high 12's WOT....until it came closer to max power & started to drop.
My Tuner has mine set @13.0 across the board & peak HP@7250
These other member's helping you sound like they know their Sheet,,,so keep bumping your thread & you'll figure it out.
It's alway's helpful to put out as much imfo as possible;milage,etc...

[jchammond]

One more idea;
You stated you stored it for the winter? are you using ethanol enriched fuel?
Maybe could try draining fuel and putting a fresh tank/ that stuff is terrible for everything....just a thought