- Dec 12, 2011
- 4,656
So this is pretty much opposite of some threads here, I've been dealing with a slowly increasing negative LTFT at idle. This has actually been going on for quite some time, but never really was out of hand. At times it would make it to -10 and I could change out a FPR, plugs, some fuel additives and while that seemed to help bring it down around -7 at times, but the LTFT would slowly tend to creep more towards the negative.
Weather could play a part, cold air being more dense would need more fuel and that could render a slight shift back towards 0. Either way, nothing to really be overly worried about but I try to keep my eye on these things...never threw a code.
Recently I replaced a lot of fuel and emissions items for the sake of the age of the components, all original EVAP components and injectors. I changed out the injectors, all 6 ign coils, FPR, purge solenoid, fuel filter, charcoal canister, and vent valve...fuel pump was replaced about 6 years ago but still has good pressure.
About a month after I replaced all those fuel components, the engine ran very well and the LTFT at idle started moving from about -7 to -10, then at times almost -12.. still no codes and runs fine.
Recently, about 2 weeks ago, I let the engine idle for a few min as I always do before driving away cold, I got a CEL and it was a P0440. I cleared the code and all was good for another week. During that week, with the engine hot, in gear, A/C on, my LTFT was at -13, the next day it was at -16...driving it was around -3.
Leaving work again the CEL came back and was the P0440. I thought maybe a vacuum line is bad as I didn't change those out, or perhaps a faulty component despite being all AC Delco. The only thing I didn't change was the gas cap. So a trip to the dealer and I picked up a new cap as my old one was the original.
I installed the cap at the dealer and by the time I reached the house, the LTFT was down from -16 to -13. I pulled fuses 10 & 28 for 15 min and tried this again. Doesn't look like much of a seal left.....surprised I've gone this long without any codes.
I have about 12 drive cycles now so anything is still possible, but I haven't had these numbers in years. Generally -7 at idle (when conditions are the best) when hot which is fine.
@xavierny25 , I changed my STFT display to the needle and it's much easier to see the back and forth swing. Intersting how it will swing back and forth between -10 & -6 when pulling to a stop, then slowly the swinging action tends to center itself around "0" as it corrects.
This is what I was seeing with the LTFT the couple days before replacing the gas cap. This is stopped, in gear, A/C on. This would be the same even if I cleared the codes, these high negative LTFT numbers would be back within the day.
This is 4 days after the new gas cap, same situation...2 pics showing the STFT swing.
This is stopped in PARK, A/C off, note higher vacuum but PCM is still learning all these LTFT's and still have more drive cycles to go before all settles down.
This could completely go back to the drawing boards but so far it's made a huge difference.
Just something to check if your LTFT numbers drift far into the negative range.
Weather could play a part, cold air being more dense would need more fuel and that could render a slight shift back towards 0. Either way, nothing to really be overly worried about but I try to keep my eye on these things...never threw a code.
Recently I replaced a lot of fuel and emissions items for the sake of the age of the components, all original EVAP components and injectors. I changed out the injectors, all 6 ign coils, FPR, purge solenoid, fuel filter, charcoal canister, and vent valve...fuel pump was replaced about 6 years ago but still has good pressure.
About a month after I replaced all those fuel components, the engine ran very well and the LTFT at idle started moving from about -7 to -10, then at times almost -12.. still no codes and runs fine.
Recently, about 2 weeks ago, I let the engine idle for a few min as I always do before driving away cold, I got a CEL and it was a P0440. I cleared the code and all was good for another week. During that week, with the engine hot, in gear, A/C on, my LTFT was at -13, the next day it was at -16...driving it was around -3.
Leaving work again the CEL came back and was the P0440. I thought maybe a vacuum line is bad as I didn't change those out, or perhaps a faulty component despite being all AC Delco. The only thing I didn't change was the gas cap. So a trip to the dealer and I picked up a new cap as my old one was the original.
I installed the cap at the dealer and by the time I reached the house, the LTFT was down from -16 to -13. I pulled fuses 10 & 28 for 15 min and tried this again. Doesn't look like much of a seal left.....surprised I've gone this long without any codes.
I have about 12 drive cycles now so anything is still possible, but I haven't had these numbers in years. Generally -7 at idle (when conditions are the best) when hot which is fine.
@xavierny25 , I changed my STFT display to the needle and it's much easier to see the back and forth swing. Intersting how it will swing back and forth between -10 & -6 when pulling to a stop, then slowly the swinging action tends to center itself around "0" as it corrects.
This is what I was seeing with the LTFT the couple days before replacing the gas cap. This is stopped, in gear, A/C on. This would be the same even if I cleared the codes, these high negative LTFT numbers would be back within the day.
This is 4 days after the new gas cap, same situation...2 pics showing the STFT swing.
This is stopped in PARK, A/C off, note higher vacuum but PCM is still learning all these LTFT's and still have more drive cycles to go before all settles down.
This could completely go back to the drawing boards but so far it's made a huge difference.
Just something to check if your LTFT numbers drift far into the negative range.
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