Welcome to Just Commodores, a site specifically designed for all people who share the same passion as yourself.

New Posts Contact us

Just Commodores Forum Community

It takes just a moment to join our fantastic community

Register

Do these O2 sensor graphs look ok?

PappaYeet

Member
Joined
Apr 19, 2020
Messages
48
Reaction score
10
Points
8
Location
Adelaide Hills
Members Ride
VT Berlina L67
Hello. After getting a random DTC76 code, I've been looking into checking the O2 sensors of my '99 VT L67. Today I cleaned the ground connections for the sensors, reset the PCM and went for a drive.

When I got back I checked for codes (all good) and then logged the voltage of each sensor while holding the engine at 2k rpm. I've read that the sensors should contantly alternate between 100mv and 900mv, which it seems they do. At idle however, they both stay steady at around 800mv.

As I don't know much about these sensors, I was just wondering if someone else could have a look at the graph to see if all is good. I can provide more readings if needed. Thank you for any help.

y1bNJkN.png


XDL log download: here.


Side note: My long term fuel trim is at -11 for both banks. Is this within a normal range? Car feels like it runs fine and I get around 10L/100km.
 

mechanic

Active Member
Joined
Aug 27, 2018
Messages
328
Reaction score
190
Points
43
Location
Australia
Members Ride
Commodore
The single graph in your post looks ok, but constant 800mV at idle is not right. It’s running rich at 800mV.

If your LTFT is -11 your engine is running stupidly rich.

Putting the info together I’d imagine you have an over-fuel situation at idle.

Dripping injector
Fuel pressure regulator diaphragm leak
Fuel pressure regulator stuck
 
Last edited:

Utel67

Active Member
Joined
Jul 15, 2019
Messages
341
Reaction score
146
Points
43
Location
Tasmania
Members Ride
Vs s3 ute
Because you reset the pcm give it time for the fuel trims to adjust.
 
Last edited:

PappaYeet

Member
Joined
Apr 19, 2020
Messages
48
Reaction score
10
Points
8
Location
Adelaide Hills
Members Ride
VT Berlina L67
The single graph in your post looks ok, but constant 800mV at idle is not right. It’s running rough at 800mV.

If your LTFT is -11 your engine is running stupidly rich.

Putting the info together I’d imagine you have an over-fuel situation at idle.

That's what I was thinking, but the idle feels smooth and exhaust doesn't smell/look rich. Are there any other numbers I could be looking at to find what's causing this?

Another issue I have is a small stutter when accelerating while the engine is cold. Issue goes away after around 5 minutes of running. I guess this could be any sort of fuel/ignition problem, but strange that it's only while cold. I guess I'll do a full fuel system check this weekend.

Because you reset the pcm give it time for the fuel trims to adjust.

Probably correct, but I remember when I had a look at some codes a while ago the LTFT was still around -9 which is probably still a bit high.
 

07GTS

Well-Known Member
Joined
Sep 8, 2013
Messages
4,953
Reaction score
6,509
Points
113
Location
Australia
Members Ride
VEGTS BUILT BLOWN E85
ive seen almost new cars running 10% errors on the trims they are not dialled in from factory so it can be totally normal that's why getting a tune helps as it cleans it all up to closer to zero to suit your engine, -11 at least is safe as its removing 11% fuel to reach stoich
 

mechanic

Active Member
Joined
Aug 27, 2018
Messages
328
Reaction score
190
Points
43
Location
Australia
Members Ride
Commodore
ive seen almost new cars running 10% errors on the trims they are not dialled in from factory so it can be totally normal
That’s not how fuel trims work. LTFT adjusts to engine conditions every few minutes. Other than initial emissions programming the factory has nothing to do with it. Nothing is dialed in.
 
Top