Help. O2 Sensor And Fuel Economy.

[d3vic3]

Hello all,

hopefully someone can help me out here.

I only recently had a look at the o2 sensor reading from my Nissan consult and it was peaked out at 2.55v (using ECU_talk to read ECU). off i went and grabbed a new oxygen sensor figuring the old one was stuffed, and when i went to swap them over i found the old one was wired wrong. one of the heater wires was plugged into the sensor wire (it was like that when i got the car).

so put the new one in and tested it out. it was reading in the range of .3v to .6v and moving around that area while driving. i don't know what the value should be exactly but this seemed more normal.

the issue is that my fuel economy has become very poor. i have used half a tank and only just done over 100kms. the consult is reading 22 litres per 100km and i know this is bad.

from everything i have researched im sure it is know set correctly with new sensor but im using soooo much fuel.

before i did the sensor swap i was easily doing 300kms + a tank.

its an rb20.

what have i missed?

im pretty close to putting the old sensor back in the way it was....

[mad082]

the o2 sensor only affects fuel economy at low load (light throttle, so when cruising at a steady speed, etc). the moment you give it the beans the ecu ignores the o2 sensor. so if you spend a lot of time accelerating moderately, as well as do a lot of short trips then your economy will always suck. the other thing to look at is the coolant temp. take not of how long the car takes to come up to temp and what the actual temp is (since you have ecutalk). within about 5 mins of driving the temp should be up around 80 degrees. if it takes much longer to get to that temp, or never does, then you need a new thermostat and replacing it will improve fuel economy.

[d3vic3]

thanks mad

water temp behaviour is normal.

i have put the old sensor back in (wired correctly) and just got back from a 30min run . seems better. recorded fuel usage was around 13 litres per 100km.

seems odd that the old one wired wrong was not so bad on the fuel.

but now an old issue has come back. its throwing code 11. CAS or CAS circuit error. i have only just replaced the CAS and thought i had that sorted. it throws the code after it wont start. i hear a click when i turn the key and all electronics dim but the starter wont turn. after about 3 or 4 key turns it fires right up and then this code 11 will be generated. this is the only time it misbehaves.

it could go weeks in-between faults or it could happen ever time i try start the car.

sorry for the change in topic.

[mad082]

the reason why you probably got ok economy on the old sensor is probably because it was always saying that the o2 reading was rich so it was constantly running at the leaner end of the scale. i may be wrong though as i don't know whether a lower voltage or higher voltage gives the reading of being rich. also with using the new sensor, resetting the ecu may help with that a bit.

now on to the ecu error (better to talk about it in this thread than to start another), i'm not 100% sure, but i think the error can be anywhere in the wiring for the CAS, not just in the unit itself, for it to show the code. however the fact that it is throwing the code after the starting error makes me think that the unit and the wiring isn't the issue. the car not turning over is the cause of the issue. the car not turning over and losing power is probably giving false readings to the ecu. not really sure what the issue of it not starting could be though. could be a dodgy earth, dodgy battery or maybe a starter issue.

[d3vic3]

I reset the ECU everytime i swapped the sensor out. ill do more driving and see if it improves.

Error:

I was leaning towards grounding issues since it only pops up after the large power draw from starting . i have recently changed the battery so i can rule that out. and i have checked all the wiring between cas and ecu for continuity.

I have already cleaned up the neg connection at the body closest the battery, now i have to do the connections on the block and starter etc. see if that helps.

is it possible that the CAS error is casuing the ECU to pull the timing further encriching the mixture.? its does feel sluggish occasionally.... maybe im reading too much into it.

Ive found grounding kits on ebay, are they worth it?. are they anything other than pre lengthened wires with terminals? I can make my own if thats all they are.

[mad082]

i'm not overly up on the grounding kits, but i think they may just be a bunch of wires that you could buy and make up yourself for a lot less.

the error could be causing the issue as you said. you'd really need to take note of the timing, etc when driving normally as well as when the issue is happening

[d3vic3]

ok.

ill take some notes and see where i end up.

[d3vic3]

Alright....

ive narrowed it down a little and i have a theory.

my starting issues and code 11 error started a while back. it was quite sporadic, but eventually got worse and worse. that was when i finally got hold of new CAS, changed they battery, did a service and all was well again. (I changed the battery because it was old and was not taking a charge.)

all this had fixed my issues and the car was running perfectly, i did not really take much though as to what cured my problem.... but then about 3 weeks ago the code 11 cropped up again and i started having starting issues again. but this time i have also noticed that when i applied the brakes the hi-cas light would flash on for a moment, i thought that was odd. and then i also noticed that when i turned the head lights on the hi-cas light would flick on then too.

and now the same situations would see my stereo turn off and on for a moment as well, like it was loosing power as soon as i turn the lights on or the brakes lights came on.....

so i got out the multimeter and had ecutalk running and noticed that it was reporting max 12v at idle and at idle with all the lights on including hi-beams, it would drop under 10v and the car would almost want to stall.

my guess is my alternator is on the way out and is just not supplying the charge the car needs and has also been struggling to charge the battery which is why everything was good when the new battery went in, then over time less charge would go into the battery which is why the issues seemed to get worse over time.

also cause if the CAS is not getting the power it needs (the starter is drawing all the power the battery has left) it wont send the right signal and keep the car from starting and also throw the error code.

i can test my theory by getting my hands on a fresh battery and trying to start it with that.

Let me know if this sound like im on the right track.

It makes sense to me, but i may be missing something.

[mad082]

yep your alternator is shagged. should be charging at around 14V when the car is running. would be best to get that sorted as soon as possible to prevent any more damage to the battery

[d3vic3]

thanks for your help mad.

once i get the new alternator i will report my findings.

[d3vic3]

ive just had a look for a 2nd hand alternator.

not many around for the rb20. (maybe is, ive looked for only about 10mins.)

but there are some that report getting them rebuilt is worth while.?

is this cost effective? and what kind of shop would do this?

[d3vic3]

should probably ask?......

do rb25 alternators fit the rb20?

[mad082]

don't know about the 25 fitting the 20 (it may, i really don't know).

as for being rebuilt, try asking an auto electrician

[d3vic3]

Final Result:

New alternator in.

Have not had the starting issue (yet) (crosses fingers).

Car runs much better. better in low rpm.

better throttle response and generally sharper all though the rev range.

and no ecu errors after a couple of short drives and a few starts.