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Changed upstream O2 sensors because my LTFT was shooting high and O2 values stuck lean/rich for many seconds. Do my O2 values seem good now?
What about my shortterm and long term fuel trim? They still seem off trying to compensate for a lean condition but much better than with older O2 sensors. I did the reset LTFT after changing them
Still having misfires on cyl5 mainly (and a few more on cyl3 and cyl7, could this "contamination" come from a bad lifter/spring?). New plugs/injectors didn't help. Switched wires didn't help. Still misfires on cyl5. Engine is rough on idle and rev
This is my scan afterwards if anyone would like to have a look:
Last edited by mjollnir51; Jul 8, 2025 at 07:44 AM.
If this sensor was “stuck” lean it may be because the sensor signal wire is intermittently being shorted to ground…if you have that bank actually lean your downstream sensor should also be lean not the normal 600-800 mv’s where it’s normally at..I’d disconnect that O2 sensor and you should see 450mv ignition on…then start shaking the wiring down until that 450mv’s goes to ground…the video is a sensor “stuck” rich.
You need to do a screen capture when lean…this doesn’t tell me anything…is this both banks ??
Screen capture is on lean, see S1B1 @ 87 mV and B2S1 @ 98 mV. Not sure what you mean. Do you want me to plug back the old O2 sensors to capture the previous issue?
I hate looking at HP Tuners but both upsteam and downsteam sensors are lean…when you look at MAF you read in grams per second and not pounds per minute…your MAF is 11.5 GPS…you have either a fuel delivery issue or a MAF one…what does your MAF read at WOT ??…you are a little off idle at 1500 RPM….are the O2’s stuck lean at idle ??…if so introduce something like Brake Kleen into the intake manifold and see if they respond and go rich.
Forgot to mention but as you did I also suspected the MAF and it's brand new, installed on the same day before this scan.
You can see O2 upstream sensors switching from lean to rich every second on the graph. While O2 downstream sensors stay around 600-700 mV. That's not expected?
Alright I'll do a brake cleaner spray next time and post results! And include fuel pressure graph + MAF to lbs/min
Did not try WOT because engine not running very good, I'm a bit concerned I'd ruin something
Where did you buy your MAF sensor and is it aftermarket ??…upstreams normally switch 200-800 mv’s and downstream remain fairly steady at 600-800mv’s…I’d leave HP Tuners for “tuning” and not to diagnose your car…you can get a decent aftermarket bidirectional scan tool for less than $500.00.
upstreams normally switch 200-800 mv’s and downstream remain fairly steady at 600-800mv’s
that's exactly what they do:
Bank 1: cyan is B1S1
dark blue is B1S2
graph range is 0 to 1000 mV
Bank2:
I have GDS2, what kind of test or procedure could I run with?
My air filter is removed, not sure it really matters but I'll install it back for next scan and reset the LTFT tables.
There is really no test you’d have to do but you just need to interpret the data you’re looking at and how they all interrelate…a scan tool doesn’t tell you what’s wrong…and remember the fuel trims will show 0% at WOT because the ECM goes into “open loop”…you will need to look at the data when the O2’s are stuck lean…not much else I can help you with…for an issue like yours I’d “graph” the O2’s and be looking at RPM, MAP, MAF (grams per second), LOAD, ECT, TPS.
Again, the O2 are not stuck lean. That was the issue before I replaced both upstream O2. The only thing here I could interpret as a lean situation is the STFT and LTFT are all positive but I'm not sure it's coming from the O2
Ok, if your fuel trims are plus or minus 10% you’re fine !!…friend of mine did a great 2 part video on diagnostics…you should watch it…if way over 10% at idle and decreases at higher RPM that denotes a vacuum leak.
Again, the O2 are not stuck lean. That was the issue before I replaced both upstream O2. The only thing here I could interpret as a lean situation is the STFT and LTFT are all positive but I'm not sure it's coming from the O2
Ok, if your fuel trims are plus or minus 10% you’re fine !!…friend of mine did a great 2 part video on diagnostics…you should watch it…if way over 10% at idle and decreases at higher RPM that denotes a vacuum leak.
Good to know. Fuel trims are now below 10% (was above 10% before O2 change)! I'll watch your friend vid ty
Next step is to install my BTR 220 cam + lifters,springs and DOD delete kit. Hopefully it will solve my cyl 5 misfires and roughness
That won’t solve your cylinder 5 misfire as the AFM cylinders are 1,7,4 and 6 !!…the best a DIY’er can do is change plugs and wires…after that you’ll need a pro diagnostian !!…a cam with a lot of valve overlap will make the car run even rougher.
So min value is 203 psi, max value is 232 psi.
I couldn't find expected values for a healthy LT1 but that's a 13% variance
C5 Diag, valve springs look good but I'm no pro! I also released and torqued the rocker arms to spec
As far as compression you look OK…now as far as a compression test that lets you know the cylinders ability to seal…doesn’t tell you the cylinders ability to breathe…you can have an issue like say an intake valve that doesn’t open or a wiped cam lobe that can cause a misfire…I do advanced diagnostics with a pressure transducer in the misfiring cylinder where I can know in a few minutes if you have an internal engine issue without engine disassembly…unfortunately maybe 1 in a 1000 shops do this…and dealerships don’t do this testing either.
Changed upstream O2 sensors
