My current fuel trims - idle, 2000rpm, 3000rpm all no load

[Fless]

@mudsport may not see your prior post (now that I tagged him he will be notified). In your post use the "@" symbol and start typing his username, then pick it from the list; that'll notify him that he's been mentioned. Either that or use the "Reply" button to quote his prior post in your new one. Keeps the communication connection.

 

[2006Tahoe2WD]

@mudsport may not see your prior post (now that I tagged him he will be notified). In your post use the "@" symbol and start typing his username, then pick it from the list; that'll notify him that he's been mentioned. Either that or use the "Reply" button to quote his prior post in your new one. Keeps the communication connection.

ah, okay will do.

 

[2006Tahoe2WD]

I'm not trying to cause controversy. Tempting to reverse engineer the algorithm to some extent. E.g. When and what sort of data ends up affecting the LTFT? There are a thousand nebulous opinions on the web. Maybe we can make some headway to a better understanding.

 

[Scottydoggs]

the ltft and stft are the engines fueling. many things can affect the fueling. exhaust leak, vac leak, needing a tune up real bad, dirty injectors, stuck open injector, clogged cats, to even bad high mile o2 sensors that have yet to be bad enough to set a code. truth is, your trims look fine to me. id just carry on.

 

[2006Tahoe2WD]

the ltft and stft are the engines fueling. many things can affect the fueling. exhaust leak, vac leak, needing a tune up real bad, dirty injectors, stuck open injector, clogged cats, to even bad high mile o2 sensors that have yet to be bad enough to set a code. truth is, your trims look fine to me. id just carry on.

Agree but I may have figured it out. Something is going on because it is not zero.
@Fless mentioned something that came back to mind. I see a very consistent 9-12% LTFT values across all the driving conditions I use when on the throttle. I wonder if the base fuel map was originally set for 0% ethanol? Considering 2006 or our range of vehicles I would guess the base map would be assuming 0% ethanol. Most all the country including CA use 10% ethanol now - some states have E85. THIS would account for 4.7% of the positive values of LTFTs I see. Ethanol makes the engine run lean without a correction. Also I read that when resetting LTFTs it is a good idea to unplug or turn off the purge valve. The purge valve runs much of the time when off idle (air coming in = lean). So at this point I'm thinking the root cause(s) of "high" LTFTs:
- 10% ethanol
- purge valve active
- misc. small vacuum leaks

GM knows this and CELs are not set until the LTFT go way above 10%, exactly what that number is I don't know.

What do you think?

 

[Fless]

Having an open purge valve brings in fuel vapors, so the mixture would get richer. The O2 sensors would read that and the PCM would adjust. I forget -- how old are the O2 sensors?

I'm curious if you have a physical alcohol sensor. Either way, it would be interesting to:

1) reset the LTFTs and monitor them over time
2) read the current fuel alcohol content as determined by the vehicle, using an advanced scanner
3) compare that to the fuel you've been buying
3a) if they don't compare favorably, reset the alcohol content
4) sometimes there are revised PCM "calibrations" that can be updated; a shop with access to GM programming could read yours and advise if an update is available. There may be one that updates the alcohol content algorithm.

You could also consider using ethanol-free fuel for a few tankfuls, as a test, and if it's available in your area. Monitor the trims using that fuel.

 

[Scottydoggs]

you will never see a zero trim. ever....... they are always moving.

install a wideband sensor. not cheap but you'll for damn sure know if your afr is right or not.

 

[2006Tahoe2WD]

Having an open purge valve brings in fuel vapors, so the mixture would get richer. The O2 sensors would read that and the PCM would adjust. I forget -- how old are the O2 sensors?

I'm curious if you have a physical alcohol sensor. Either way, it would be interesting to:

1) reset the LTFTs and monitor them over time
2) read the current fuel alcohol content as determined by the vehicle, using an advanced scanner
3) compare that to the fuel you've been buying
3a) if they don't compare favorably, reset the alcohol content
4) sometimes there are revised PCM "calibrations" that can be updated; a shop with access to GM programming could read yours and advise if an update is available. There may be one that updates the alcohol content algorithm.

You could also consider using ethanol-free fuel for a few tankfuls, as a test, and if it's available in your area. Monitor the trims using that fuel.

The purge valve will pull vapors initially but soon should be almost "clean" air as the charcoal "dries" out. That's my understanding but I haven't seen any data on that.

My 4 O2 sensors are new, MAF is cleaned but not assumed to read correctly but does show expected data. The air filter was only slightly off color, no debris. Connections have been triple checked but there may be a crack somewhere.

There was an explanation regarding alcohol sensors. Apparently there used to be a sensor and then they did away with the sensor and used the O2 sensors and a few tricks to estimate the alcohol content. I don't know if this was for flex fuel or the "regular" fuel versions. I think there is also an alcohol reset but I don't know if my scan tool can do that - after I get it running again.

Additional info that I failed to post. During idle - I only have about 4-5% LTFTs - which supports my hypothesis. I would need to double check or unplug my purge valve to confirm to some extent.

A theory (matches what I see on my Tahoe):
Idling - only the alcohol adjustment is in play (LTFTs are + 4 to 6%)
Normal Loads - 10% alcohol offset and purge valve are in play (LTFTs are + 9 to 12%)

 

[2006Tahoe2WD]

you will never see a zero trim. ever....... they are always moving.

install a wideband sensor. not cheap but you'll for damn sure know if your afr is right or not.

I expect the O2 voltage to swing back and forth across zero (~14.7 or whatever it is going for). The controller must be also swinging the injector timing - I would expect to see the wideband also swing but it might look like a ripple and not 0.1-0.7-0.1 back and forth.

 

[2006Tahoe2WD]

I expect the O2 voltage to swing back and forth across zero (~14.7 or whatever it is going for). The controller must be also swinging the injector timing - I would expect to see the wideband also swing but it might look like a ripple and not 0.1-0.7-0.1 back and forth.

In other words - Yes, you will never see the STFTs being steady - they will always swing along with the O2 sensor outputs.