350Rx Oxygen Sensor?

[nagasakihacky]

Hi,

I've owned a 2005 350RX Stagea for almost a year now and I'm finding that it seems to be using more fuel than it used to, and generally feels a little less responsive and less smooth.

I'm somewhat open to the idea that it's all in my head, but while I can get up to around 10-11km/L on the motorway, around town I tend to get more like 6-7km/L and that's going very easy on the loud pedal in both cases. 10-11km/L would be strictly motorway driving. I just did a 1000km trip on the open road, and throwing in a few corners and a handful of 50kph zones through towns and that averaged out to just over 9km/L

I bought an OB-LINK a while back and have been using that to try and track if it was something to do with my driving style, but it seems like it just likes to suck fuel if you're doing anything other than sitting at a constant speed on the motorway.

What I noticed this morning though, was the meter for "A/F correct 1" and "A/F correct 2". I assume these are the two banks, but while A/F correct 1 jumps around a fair bit as you accelerate and decelerate, A/F correct 2, seems to spend 99% of the time at 5%, with very occasional jumps to 10%. I'm not entirely sure what the percentages are supposed to be, and I haven't got any error codes, but I'm wondering if this could point to a problem with the bank 2 O2 sensor? The car has never felt like it's misfired, but there have definitely been times when it's felt like it's running rough. Do these sensors fail in this way or are they either going or not?

Any suggestions?

Cheers,

Tony

[KiwiRS4T]

O2 sensor is only of interest at idle or constant speed so not likely to be part of your problem. What about new plugs and a clean air filter element?

[PN-Mad]

What does the other bank sensor sit on/around?

I don't know how your software displays it, mine is set up so that with O2 correction 100% is no changes 99% is removing fuel, 101% is adding fuel.

It can change to a maximum 25% of the fuel value.

Each bank is individually monitored and adjusted. Only the two upper sensors play an active part. The lower two Are for cat efficiency afaik

[nagasakihacky]

Air filter still looks near new, but I'll try replacing it and I had a look at the spark plugs a while back and they also looked good, though again, I'll probably try and swap them soon to see if there's any difference. I would have thought even if the O2 sensor is only used at constant speed or idle that could have a pretty major influence on fuel consumption, since that would surely set the baseline of the fuel mixture from which any further adjustments would be made. My understanding was that the O2 sensors were really the primary input for adjusting the fuel map since switching to wideband sensors and that MAF was only to find your place in the fuel map, though I'm no mechanic.

The OB-LINK just shows a small bar graph that swings left and right with the current percentage showing above. Based on the readings, I think it's showing the adjustment from 100% since it swings between -10 and +10% usually - I've not seen it go outside +/-15%. A/F correct 1 seems to jump around a bit depending on what you are doing with the throttle, whereas bank 2 reads -5% no matter what. There is also a similar display for A/F Learn 1 and 2, which I assume are probably the post cat sensors. What do your sensors read in your software, and what is your fuel consumption like?

[PN-Mad]

If it's stuck on -5% it sounds like your sensor is borked. However you can check this via swapping them over side to side if you can be bothered. Being stuck on -5 actually means it's taking fuel out all the time, if that's what it is "actually" doing.

A new sensor will only run $160-170 from nissan. I bought one new one, cause I toasted the shit out of the plug on one of mine

To me, your consumption is pretty normal. Freeway for me, is 10L/100km with air con on, and cruising at 110gps (about 120 indicated). The city consumption is about the same too.

[scotty nm35]

Are you sure it isn't your long term and short term fuel trims? If the AFR's are scanning rich then lean many times a minute it sounds like they are doing what they should. Try swapping the sensors over to see if they move banks...

[nagasakihacky]

I will give switching the sensors over a go if they're the same plug for both sides, but unless the software is the problem I think it is probably a bad sensor. I think the A/F learn is probably the long term one, which I imagine is more to do with the downstream sensor, but I can't be sure. Assuming I don't destroy the sensors removing them then swapping banks should answer everything.

Edited October 10, 2014 by nagasakihacky

[Hertz Donut]

I agree with Alex re. fuel consumption, I tend to get around 10km/l on the open road (with the odd "enthusiastic" overtaking manoeuvre) and maybe 7-8km/l around town on short trips where the car barely warms up. Seems to be about the norm, but then I'm lugging around the 4WD system too.

Bear in mind Nissan themselves rated the PM35 at 8.62km/l on the 10/15 fuel cycle (PNM35 is 7.81, gag), so they're not exactly fuel misers.

Edited October 10, 2014 by Hertz Donut

[nagasakihacky]

Interesting to hear - I'm just a bit surprised that they are so thirsty.

Mine's RWD only, yet driven like a granny it seems to use significantly less than my mates RS Four driven fairly hard. Also, a friend of mine had a 350GT skyline which got better mileage than my motorway mileage around town. I realise a skyline is a little more aerodynamic than a stagea, but in terms of weight (which should matter more around town), the stagea is only around 100-150kg heavier. He no longer has the car, so I've not been able to check, but perhaps it had the HR version of the VQ.

Maybe it wouldn't seem so bad if I'd moved from a ford or a holden (although I'm pretty sure my mate gets better mileage from the 3.6L commodore), but coming from a twin turbo subbie, it seems thirsty as hell. I'm not getting rid of it though, so just thought I should be sure that it's not just my car.

[PN-Mad]

Try actually filling up, zero the trip meter, and work out the actual litres per 100kms.

I don't really have much trust in the on board consumption.

Then report back on how much it used, and what kind of driving was done on that tank.

[nagasakihacky]

I realise it probably seems a little analysis retentive, but yeah, I've already been doing that for the past 6 months or so. 10.5L/100km is the best I've got and that was on a long road trip, all open road, not driving hard, mostly just sitting at 110kph (by GPS so close to 120 on the speedo). Around 180km of open road driving plus the rest around town gets 14-15L/100km and all around town is anywhere from 15-18L/100km. That's consistent across multiple full tanks for each case.

I also doubted how accurate the ob-link would be for fuel consumption (I don't have the on board thing since my screen has never worked), but so far it's matched what I put in when I fill up pretty much exactly.

Just realised it's actually +5% that the a/f correct is stuck on, not -5%, so if the sensors reading that and it's not just the ob-link stuffing up, then it probably is running a bit rich on that bank.

To be perfectly honest, I can wear the high fuel consumption, but it just doesn't feel as smooth or responsive as it used to. I remember when I first got it being impressed by just how smooth and responsive it was.

[PN-Mad]

Then Stick to the plan, swap them over, see what happens.

They aren't that expensive to replace, but $160 is a lot of fuel to use to justify it.

[YEE-HAA]

From what I have learnt by going to Bosch training nights many years ago, I recall Bosch saying that oxygen sensors measure the amount of air/fuel in the exhaust gas many times/millisecond.

What happens is they become 'slow' in reading the mixture and this can cause fuel consumption issues as the ecu cannot lean or richen the mixture as needed as accurately as when they are new.

Granted there are other factors which affect fuel comsumption which were also mentioned, plugs, air filters, fuel filters etc but I dont believe that o2 readings only matters when the car is idling or driven at a constant speed. To the contrary, if your driving at a constant speed the fuel/air mixture should be fairly constant and would need little adjustment. The o2 sensors read the values all the time and could be why your car feels less smooth and less responsive.

Thats my thoughts.

[PN-Mad]

Sorry, I don't think you understand the ECU.

On idle and cruise the ecu is looking at the AFR, and adjusting the amount of fuel injected to keep the engine operating at 14.7:1. You can see this if you have a OBD reader by looking at the A/F correction function. When you put your foot down, or the engine receives load, it stops correcting pegging the correction value at "100", and fuel is purely dictated due to the figures in the tables written in the ecu.

There are aftermarket ECUs that can be setup to fuel trim through the rev range to targeted figures, and on individual cylinders, but the stock ECU doesn't do that.

If I had a working lappy, I'd take a log and show you.