Long Term Fuel Trim = 0.0%, always
05-11-2015, 08:03 PM
#1
Long Term Fuel Trim = 0.0%, always I have a 2005 LS6 CTS-V with H/C/I, long tube headers, front and back 02 sensors. Even though it has been dynotuned (twice) I've been having trouble with an idle stumble when letting off the throttle plus signs things are running rich (gas smell, sooty exhaust tips).
A tuner friend of mine hooked up HPT and said that I had short term fuel trim values between -0.8 and -3.0 and my upstream 02 sensors were putting out voltage that oscillated between 700-900mV (rich) BUT that my long term fuel trim values were 0.0% at all times. Since I am getting oscillating values, I'd think the 02 sensors are good and it must be the 02 wiring, right?
I got under the car and, with the ignition on, was able to read 12v across the heater (+) and (-) wires on both sides. I was also able to read 390mv across the signal (+) and (-) wires on both sides. Doesn't that tell me the wiring is good?
What else (and how) can I test?
Thanks in advance!
A tuner friend of mine hooked up HPT and said that I had short term fuel trim values between -0.8 and -3.0 and my upstream 02 sensors were putting out voltage that oscillated between 700-900mV (rich) BUT that my long term fuel trim values were 0.0% at all times. Since I am getting oscillating values, I'd think the 02 sensors are good and it must be the 02 wiring, right?
I got under the car and, with the ignition on, was able to read 12v across the heater (+) and (-) wires on both sides. I was also able to read 390mv across the signal (+) and (-) wires on both sides. Doesn't that tell me the wiring is good?
What else (and how) can I test?
Thanks in advance!
05-11-2015, 10:51 PM
#4
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Faulted sensors will get pinned to 490mV or so by the
PCM. But if STFTs are active then there are not active
sensor codes.
I expect your idle stumble is really to do with over-rich
at the low end. Cam causes this, and so can LTs via
false trimming (and shutting off LTFTs is a common way
that people try to "fix" the false trimming). Nobody can
dyno-tune your VE table's 400RPM column, which is the
place to concentrate for idle oscillation / mixture-stumble.
You have to feather that in by eye, based on a decently
profiled VE table at idle & above. Although you might be
able to work it into shape by deliberately idling it down,
a lot of the time you're starting from a place where it
wants to just die. So take a French Curve guess at it
first.
PCM. But if STFTs are active then there are not active
sensor codes.
I expect your idle stumble is really to do with over-rich
at the low end. Cam causes this, and so can LTs via
false trimming (and shutting off LTFTs is a common way
that people try to "fix" the false trimming). Nobody can
dyno-tune your VE table's 400RPM column, which is the
place to concentrate for idle oscillation / mixture-stumble.
You have to feather that in by eye, based on a decently
profiled VE table at idle & above. Although you might be
able to work it into shape by deliberately idling it down,
a lot of the time you're starting from a place where it
wants to just die. So take a French Curve guess at it
first.
05-12-2015, 08:26 AM
#7
Thanks for the replies. I'll see if the ltft is just turned off - a tuner could have done that trying to track down an issue.
As far as the mV of the oscillation - I thought they could range between like 100 to 900mV and if they stayed at the high end, it meant things were really rich...
What I thought was odd was that the signal wire was reading 390 while the sensors were pretty well cold - I didn't think they are supposed to read anything until warmed up.
I've got new 02s on the way just in case but because of this plus some issue with my alternator F signal, I'm beginning to think ie got a PCM issue or PCM grounding issue
As far as the mV of the oscillation - I thought they could range between like 100 to 900mV and if they stayed at the high end, it meant things were really rich...
What I thought was odd was that the signal wire was reading 390 while the sensors were pretty well cold - I didn't think they are supposed to read anything until warmed up.
I've got new 02s on the way just in case but because of this plus some issue with my alternator F signal, I'm beginning to think ie got a PCM issue or PCM grounding issue
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05-12-2015, 01:09 PM
#9
just confirmed my tuner says LTFT was turned off but he turned them back on - and still got 0s. He says it's likely something in the wiring to the 02s but, as I say, I got 12v by connecting leads to the + and - of the heater circuit and got 390mv by connecting to the + and - of the signal wire. Doesn't that confirm the wiring is good?
05-13-2015, 08:22 AM
#10
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There are several ways that the PCM verifies the operation of the O2s. The trouble with relying on this is that the tuner could have disabled the codes. For me I a skeptical of bad oxygen sensors, as two at the same time is very unusual. I would suspect an issue with the tune. The oxygen sensor puts out a 0 to 1V signal, based on being rich or lean. The factory ones are narrow bands sensors.
Most OBDII handheld scanners will let you view parameters. View the ones for the O2s. They should be oscillating rich and lean.
Most OBDII handheld scanners will let you view parameters. View the ones for the O2s. They should be oscillating rich and lean.
05-13-2015, 11:02 AM
#11
yeah, the check engine codes are definitely disabled and the 02 sensors are definitely oscillating so I think the 02s themselves are good. The reason I started looking at the 02s wasn't because I threw a code but it was b/c I'm running rich, stumbling on decel and then I saw I didn't have LTFT numbers coming up as I drove... I'm thinking I either have a problem in the wiring to the 02s or to the PCM, where the 02s get their "grounds"...
05-14-2015, 12:37 AM
#13
If you force the car into closed loop via the live hpt controls, you will see them change. They are likely not registering because the car is somewhat smart enough to know when you have insufficient switching and stay on open loop. change the spark plugs and O2s with new and start over.
05-16-2015, 07:11 PM
#17
Jesus why are people replying with "answers" that are completely incorrect.
LTFTs are commonly turned off so you don't get any rogue locked fueling adjustments at WOT and it keeps it more consistent that way. If you have STFTs that are updating but no LTFTs then that is more than likely the case. I have never seen nor heard of anybody turning off LTFTs to prevent "false trimming" of rich idle conditions, especially since your STFTs will also adjust fueling. Also LTFTs update just about immediately when in closed loop unless they are turned off, no time or mileage restrictions.
If your STFTs are moving around and close to 0 then your problem isn't fueling related more than likely, other parts of the tune would definitely be involved. You didn't say what cam you have so I'm going to assume it's a tune issue but not because of the fuel trims.
Having a gas smell and sooty tips is a given no matter what if you don't have cats and have an aftermarket camshaft of any consequence. Hell you can make a stock car smell gas like by just removing the cats and high flow cats don't filter for ****.
If you want a real educated guess post up more information like the cam specs and the tune.
LTFTs are commonly turned off so you don't get any rogue locked fueling adjustments at WOT and it keeps it more consistent that way. If you have STFTs that are updating but no LTFTs then that is more than likely the case. I have never seen nor heard of anybody turning off LTFTs to prevent "false trimming" of rich idle conditions, especially since your STFTs will also adjust fueling. Also LTFTs update just about immediately when in closed loop unless they are turned off, no time or mileage restrictions.
If your STFTs are moving around and close to 0 then your problem isn't fueling related more than likely, other parts of the tune would definitely be involved. You didn't say what cam you have so I'm going to assume it's a tune issue but not because of the fuel trims.
Having a gas smell and sooty tips is a given no matter what if you don't have cats and have an aftermarket camshaft of any consequence. Hell you can make a stock car smell gas like by just removing the cats and high flow cats don't filter for ****.
If you want a real educated guess post up more information like the cam specs and the tune.
05-16-2015, 07:25 PM
#19
When I say rogue I mean a couple percent difference since fuel trims will move that much from morning to mid day. I myself prefer to have the most consistent fueling at WOT by not having any interference from the fuel trims locking onto a couple percent difference. I didn't see anybody mention anything about tuning WOT before idle and driveability.
05-16-2015, 09:20 PM
#20
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Jesus why are people replying with "answers" that are completely incorrect.
LTFTs are commonly turned off so you don't get any rogue locked fueling adjustments at WOT and it keeps it more consistent that way. If you have STFTs that are updating but no LTFTs then that is more than likely the case. I have never seen nor heard of anybody turning off LTFTs to prevent "false trimming" of rich idle conditions, especially since your STFTs will also adjust fueling. Also LTFTs update just about immediately when in closed loop unless they are turned off, no time or mileage restrictions..
LTFTs are commonly turned off so you don't get any rogue locked fueling adjustments at WOT and it keeps it more consistent that way. If you have STFTs that are updating but no LTFTs then that is more than likely the case. I have never seen nor heard of anybody turning off LTFTs to prevent "false trimming" of rich idle conditions, especially since your STFTs will also adjust fueling. Also LTFTs update just about immediately when in closed loop unless they are turned off, no time or mileage restrictions..
when I had long tubes and couldn't get consistent fueling
at the low end. I don't go WOT from idle, that's no good
for anything, and WOT LTFT residue was not a concern
since the cells I'd enter WOT from, did not have the same
sensor heat issue (my FTCs are set up more sensibly than
stock).
So now you've heard of one.
With the shorty headers I don't have the sensor problems
and went back to LTFTs enabled.