Celicasaur

Quick one guys, I had my car logs analysed from when it was on the dyno and I was told that long term fuel trim 2 is constantly at 0%.

Am I right in thinking that the O2 sensors have been disabled due to my V6 tune (so a cat delete can be done without fear of a CEL), or is there an issue with one of the sensors, if so does anybody know which one?

Vladds

There are two short terms and two long term fuel trims that I can read in this car.
Out of these 4, only one is zero?

Celicasaur

I wish I could give more detail on the statement tbh, I'm purely going off what i've been told:

"Fuel trims - It is a little strange that long term fuel trim 2 is constantly at 0%. This could indicate some kind of issue with one of the o2 sensors."

I'm guessing it means one of the two LTFT sensors?


Hopefully will be on the dyno tomorrow. I'll ask if the guys can swap the exhaust sensors over to the opposite banks and then re-log to see if the reading moves to the other side...

That being said, if you have a tune, how crucial are the lambda sensors? If their reading is off, does it still affect the AFR? I'm guessing not, because regardless of their health, doesn't the tuner ultimately amend the base map values to satisfy the best suitable AFR at the time of writing the map?

BLKROKT

You need primary O2 sensors. Tunes disable only the irrelevant ones after the secondary cats.

The primary O2 tells the ECU if it's running rich or lean, allowing for adjustment. Generally, if there's no signal, the ECU will default to fully rich.

skratch77

these cars are weird bro my car shows 0.0 volts on one of the 02 sensors and that was with stock tune.

do your short term values adjust on the fly while driving?

what do the voltage reading show for both sensors?

Celicasaur

Something I can work with - thanks man. I'll see if they can work out which one needs changing or if they are both dead and need replacing when I'm at the shop tomorrow.

Question though - if the ecu defaults to fully rich, wouldn't the V6 tune have essentially worked around that? I mean, we could see from the dyno plot that the AFR was a high 12 towards fuel cut etc.

Interesting. I don't know anything else about this issue on my car tbh. I wish I did now. I have the logs, but not sure how to open them.

Lucky for you, your car still made 440whp in spite of the dodgy sensor.

skratch77

yea I'm not sure if it's normal but never looked at it but have a new sensor to try .

I just left it as is since there are no codes and all readiness monitors passed.

what type of dyno are you going to and do you have a baseline? these cars make more power second and third pulls and I used 4th gear.

Jasonoff

It's a V motor trims x2 (one per side)

Was one of the LTFT disabled? If STFT works but LTFT doesn't it's likely disabled.

Your car is all sorts of fkd up man.

BLKROKT

No. Tune only affects closed loop operation and trims, not the absolute fuel tables. When an O2 goes out, the ECU defaults to open loop operation as a safeguard.

https://www.enginebasics.com/EFI%20T...Loop%20o2.html

Jasonoff

The LTFT and STFT get their data from the same sensor. How could it be an 02 sensor problem when the STFT is working?

Vladds

Get a OBD2 reader and log the long term and short term trims.
There are two sets, one per bank.
None should be zero, in stock operation.
The long can't be zero with the short moving around, unless you have PERFECT short trims (short moves exactly +-0.5 by example)
Normal readings, after warm up have the short term constantly going up and down, but with a trend, which the long term summarizes.
At one point the short term will stop trending and stabilize around a value. then that's the reading for the long term.


Like others posted, the main O2 sensors can't be disabled, the rear ones do influence the trims, there may be such a thing as tuning them out (such as the voltage readings get inputed manually resulting in a perfect long fuel trim, of zero). You would never tune out just one bank though.


At one point I was interested in failure modes of these. The ECU expects the readings to be outside a voltage interval before turning on the check engine. But I think I've read something about rear sensors going bad with zero voltage, but rare and could be a wiring fault.

skratch77

any updates?

Celicasaur

That's interesting. I didn't know this... I've only tuned cars without relying on the 02 sensor readings.

that's what I originally thought, because I thought the secondary sensor does cat efficiency, hence why i thought the LTFT was disabled in my tune or something...

I think it'll be good to know if it's one or both of the sensors at this point. Do you know what software (if even available online) i can use to open the log files with and take screenshots of?

I left my car at the workshop yesterday and asked them to price me up on a pair of Bosch primary sensors...might as well fit new primary sensors to rule out any sensor issues. Even if they don't make a difference, at least I'll have done some good preventative maintenance as the car is on 85k miles now anyway.

Updates to come on Monday now hopefully, assuming they can get the work done and have me tuned n logged on the dyno with Jerry etc

Jasonoff

Your primary O2 is the sensor that is used for both STFT and LTFT. The secondary O2 sensor, post cat, is not used at all with a tune. With the OEM tune it can affect the LTFT if there's something wrong with the cat.

Having 0% on LTFT that never changes points to the LTFT being disabled, not the O2 sensor. Was your tuner mucking about trying to sort out your issue and forgot to reenable it?

You have so many issues with your car that never seem to get resolved. What are you doing? Bring your car to someone who knows what they're doing and it will probably be resolved in an hour or less. If you as a customer need to make suggestions to a shop they are likely clueless...

Vladds

What's the termination of the file? (.jpg, .xls, etc)

BLKROKT

Car workshops love customers like you